output/csv: use intermediate time_t var, silence compiler warning

[libsigrok.git] / src / serial_hid.c

/*
 * This file is part of the libsigrok project.
 *
 * Copyright (C) 2017-2019 Gerhard Sittig <gerhard.sittig@gmx.net>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include "config.h"
#include <glib.h>
#ifdef HAVE_LIBHIDAPI
#include <hidapi.h>
#endif
#include <libsigrok/libsigrok.h>
#include "libsigrok-internal.h"
#include "serial_hid.h"
#include <stdlib.h>
#include <string.h>
#ifdef G_OS_WIN32
#include <windows.h> /* for HANDLE */
#endif

#define LOG_PREFIX "serial-hid"

#ifdef HAVE_SERIAL_COMM

/**
 * @file
 *
 * Serial port handling, HIDAPI library specific support code.
 */

/**
 * @defgroup grp_serial_hid Serial port handling, HID group
 *
 * Make serial-over-HID communication appear like a regular serial port.
 *
 * @{
 */

#ifdef HAVE_LIBHIDAPI
/* {{{ helper routines */

/* Strip off parity bits for "odd" data bit counts like in 7e1 frames. */
static void ser_hid_mask_databits(struct sr_serial_dev_inst *serial,
        uint8_t *data, size_t len)
{
        uint32_t mask32;
        uint8_t mask;
        size_t idx;

        if ((serial->comm_params.data_bits % 8) == 0)
                return;

        mask32 = (1UL << serial->comm_params.data_bits) - 1;
        mask = mask32 & 0xff;
        for (idx = 0; idx < len; idx++)
                data[idx] &= mask;
}

/* }}} */
/* {{{ open/close/list/find HIDAPI connection, exchange HID requests and data */

#define IOKIT_PATH_PREFIX       "IOService:"

/*
 * Convert a HIDAPI path (which depends on the target platform, and may
 * depend on one of several available API variants on that platform) to
 * something that is usable as a "port name" in conn= specs.
 *
 * Since conn= is passed with -d where multiple options (among them conn=)
 * are separated by colons, port names themselves cannot contain colons.
 *
 * Just replace colons by a period in the simple case (Linux platform,
 * hidapi-libusb implementation, bus/address/interface). Prefix the
 * HIDAPI path in the complex cases (Linux hidapi-hidraw, Windows, Mac).
 * Paths with colons outside of libusb based implementations are unhandled
 * here, but were not yet seen on any sigrok supported platform either.
 * So just reject them.
 */
static char *get_hidapi_path_copy(const char *path)
{
        static const char *accept = "0123456789abcdefABCDEF:";
        static const char *keep = "0123456789abcdefABCDEF";

        int has_colon;
        int is_hex_colon;
        const char *parse, *remain;
        char *copy;

        parse = path;
        has_colon = strchr(parse, ':') != NULL;
        is_hex_colon = strspn(parse, accept) == strlen(parse);
        if (is_hex_colon) {
                /* All hex digits and colon only. Simple substitution. */
                copy = g_strdup_printf("%s%s", SER_HID_USB_PREFIX, parse);
                g_strcanon(copy + strlen(SER_HID_USB_PREFIX), keep, '.');
                return copy;
        }
        if (!has_colon) {
                /* "Something raw" and no colon. Add raw= prefix. */
                copy = g_strdup_printf("%s%s", SER_HID_RAW_PREFIX, parse);
                return copy;
        }
        if (g_str_has_prefix(parse, IOKIT_PATH_PREFIX)) do {
                /*
                 * Path starts with Mac IOKit literal which contains the
                 * colon. Drop that literal from the start of the path,
                 * and check whether any colon remains which we cannot
                 * deal with. Fall though to other approaches which could
                 * be more generic, or to the error path.
                 */
                remain = &parse[strlen(IOKIT_PATH_PREFIX)];
                if (strchr(remain, ':'))
                        break;
                copy = g_strdup_printf("%s%s", SER_HID_IOKIT_PREFIX, remain);
                return copy;
        } while (0);

        /* TODO
         * Consider adding support for more of the currently unhandled
         * cases. When we get here, the HIDAPI path could be arbitrarily
         * complex, none of the above "straight" approaches took effect.
         * Proper escaping or other transformations could get applied,
         * though they decrease usability the more they obfuscate the
         * resulting port name. Ideally users remain able to recognize
         * their device or cable or port after the manipulation.
         */
        sr_err("Unsupported HIDAPI path format: %s", path);
        return NULL;
}

/*
 * Undo the port name construction that was done during scan. Extract
 * the HIDAPI path from a conn= input spec (the part after the hid/
 * prefix and chip type).
 *
 * Strip off the "raw" prefix, or undo colon substitution. See @ref
 * get_hidapi_path_copy() for details.
 */
static char *extract_hidapi_path(const char *copy)
{
        static const char *keep = "0123456789abcdefABCDEF:";

        const char *p;
        char *path;

        p = copy;
        if (!p || !*p)
                return NULL;

        if (g_str_has_prefix(p, SER_HID_IOKIT_PREFIX)) {
                p += strlen(SER_HID_IOKIT_PREFIX);
                path = g_strdup_printf("%s%s", IOKIT_PATH_PREFIX, p);
                return path;
        }
        if (g_str_has_prefix(p, SER_HID_RAW_PREFIX)) {
                p += strlen(SER_HID_RAW_PREFIX);
                path = g_strdup(p);
                return path;
        }
        if (g_str_has_prefix(p, SER_HID_USB_PREFIX)) {
                p += strlen(SER_HID_USB_PREFIX);
                path = g_strdup(p);
                g_strcanon(path, keep, ':');
                return path;
        }

        return NULL;
}

/*
 * Enumerate all devices (no VID:PID is involved). Invoke an 'append'
 * callback with "path" and "name". Exclusively list connections that
 * involve supported chip types, because mice and keyboards etc are not
 * too useful to communicate to measurement equipment.
 */
static GSList *ser_hid_hidapi_list(GSList *list, sr_ser_list_append_t append)
{
        struct hid_device_info *devs, *curdev;
        const char *chipname;
        char *path, *name;
        wchar_t *manuf, *prod, *serno;
        uint16_t vid, pid;
        GString *desc;

        devs = hid_enumerate(0x0000, 0x0000);
        for (curdev = devs; curdev; curdev = curdev->next) {
                /*
                 * Determine the chip name from VID:PID. Exlusively list
                 * supported connection types (known chips).
                 */
                vid = curdev->vendor_id;
                pid = curdev->product_id;
                chipname = ser_hid_chip_find_name_vid_pid(vid, pid);
                if (!chipname)
                        continue;

                /*
                 * Prefix port names such that open() calls with this
                 * conn= spec will end up here and contain all details
                 * that are essential for processing.
                 */
                path = get_hidapi_path_copy(curdev->path);
                if (!path)
                        continue;
                name = g_strdup_printf("%s/%s/%s",
                        SER_HID_CONN_PREFIX, chipname, path);
                g_free(path);

                /*
                 * Print whatever information was available. Construct
                 * the description text from pieces. Absence of fields
                 * is not fatal, we have seen perfectly usable cables
                 * that only had a VID and PID (permissions were not an
                 * issue).
                 */
                manuf = curdev->manufacturer_string;
                prod = curdev->product_string;
                serno = curdev->serial_number;
                vid = curdev->vendor_id;
                pid = curdev->product_id;
                desc = g_string_sized_new(128);
                g_string_append_printf(desc, "HID");
                if (manuf && wcslen(manuf) != 0)
                        g_string_append_printf(desc, " %ls", manuf);
                if (prod && wcslen(prod) != 0)
                        g_string_append_printf(desc, " %ls", prod);
                if (serno && wcslen(serno) != 0)
                        g_string_append_printf(desc, " %ls", serno);
                if (vid && pid)
                        g_string_append_printf(desc, " [%04hx.%04hx]", vid, pid);
                list = append(list, name, desc->str);
                g_string_free(desc, TRUE);
                g_free(name);
        }
        hid_free_enumeration(devs);

        return list;
}

/*
 * Enumerate devices for the specified VID:PID pair. Invoke an "append"
 * callback with 'path' for found devices. Exclusively finds supported
 * chip types, skips unknown VID:PID pairs (even if caller specified).
 */
static GSList *ser_hid_hidapi_find_usb(GSList *list, sr_ser_find_append_t append,
                uint16_t vendor_id, uint16_t product_id)
{
        const char *caller_chip;
        const char *dev_chip;
        struct hid_device_info *devs, *curdev;
        const char *name;
        char *path;

        caller_chip = ser_hid_chip_find_name_vid_pid(vendor_id, product_id);

        devs = hid_enumerate(vendor_id, product_id);
        for (curdev = devs; curdev; curdev = curdev->next) {
                dev_chip = caller_chip;
                if (!dev_chip) {
                        dev_chip = ser_hid_chip_find_name_vid_pid(
                                curdev->vendor_id, curdev->product_id);
                }
                if (!dev_chip)
                        continue;
                path = get_hidapi_path_copy(curdev->path);
                if (!path)
                        continue;
                name = g_strdup_printf("%s/%s/%s",
                        SER_HID_CONN_PREFIX, dev_chip, path);
                g_free(path);
                list = append(list, name);
        }
        hid_free_enumeration(devs);

        return list;
}

/* Get the serial number of a device specified by path. */
static int ser_hid_hidapi_get_serno(const char *path, char *buf, size_t blen)
{
        char *hidpath;
        hid_device *dev;
        wchar_t *serno_wch;
        int rc;

        if (!path || !*path)
                return SR_ERR_ARG;
        hidpath = extract_hidapi_path(path);
        dev = hidpath ? hid_open_path(hidpath) : NULL;
        g_free(hidpath);
        if (!dev)
                return SR_ERR_IO;

        serno_wch = g_malloc0(blen * sizeof(*serno_wch));
        rc = hid_get_serial_number_string(dev, serno_wch, blen - 1);
        hid_close(dev);
        if (rc != 0) {
                g_free(serno_wch);
                return SR_ERR_IO;
        }

        snprintf(buf, blen, "%ls", serno_wch);
        g_free(serno_wch);

        return SR_OK;
}

/* Get the VID and PID of a device specified by path. */
static int ser_hid_hidapi_get_vid_pid(const char *path,
        uint16_t *vid, uint16_t *pid)
{
#if 0
        /*
         * Bummer! It would have been most reliable to just open the
         * device by the specified path, and grab its VID:PID. But
         * there is no way to get these parameters, neither in the
         * HIDAPI itself, nor when cheating and reaching behind the API
         * and accessing the libusb handle in dirty ways. :(
         */
        hid_device *dev;

        if (!path || !*path)
                return SR_ERR_ARG;
        dev = hid_open_path(path);
        if (!dev)
                return SR_ERR_IO;
        if (vid)
                *vid = dev->vendor_id;
        if (pid)
                *pid = dev->product_id;
        hid_close(dev);

        return SR_OK;
#else
        /*
         * The fallback approach. Enumerate all devices, compare the
         * enumerated USB path, and grab the VID:PID. Unfortunately the
         * caller can provide path specs that differ from enumerated
         * paths yet mean the same (address the same device). This needs
         * more attention. Though the specific format of the path and
         * its meaning are said to be OS specific, which is why we may
         * not assume anything about it...
         */
        char *hidpath;
        struct hid_device_info *devs, *dev;
        int found;

        hidpath = extract_hidapi_path(path);
        if (!hidpath)
                return SR_ERR_NA;

        devs = hid_enumerate(0x0000, 0x0000);
        found = 0;
        for (dev = devs; dev; dev = dev->next) {
                if (strcmp(dev->path, hidpath) != 0)
                        continue;
                if (vid)
                        *vid = dev->vendor_id;
                if (pid)
                        *pid = dev->product_id;
                found = 1;
                break;
        }
        hid_free_enumeration(devs);
        g_free(hidpath);

        return found ? SR_OK : SR_ERR_NA;
#endif
}

static int ser_hid_hidapi_open_dev(struct sr_serial_dev_inst *serial)
{
        hid_device *hid_dev;

        if (!serial->usb_path || !*serial->usb_path)
                return SR_ERR_ARG;

        /*
         * A path is available, assume that either a GUI or a
         * user has copied what a previous listing has provided.
         * Or a scan determined a matching device's USB path.
         */
        if (!serial->hid_path)
                serial->hid_path = extract_hidapi_path(serial->usb_path);
        hid_dev = hid_open_path(serial->hid_path);
        if (!hid_dev) {
                g_free((void *)serial->hid_path);
                serial->hid_path = NULL;
                return SR_ERR_IO;
        }

        serial->hid_dev = hid_dev;
        hid_set_nonblocking(hid_dev, 1);

        return SR_OK;
}

static void ser_hid_hidapi_close_dev(struct sr_serial_dev_inst *serial)
{
        if (serial->hid_dev) {
                hid_close(serial->hid_dev);
                serial->hid_dev = NULL;
                g_free((void *)serial->hid_path);
                serial->hid_path = NULL;
        }
        g_slist_free_full(serial->hid_source_args, g_free);
        serial->hid_source_args = NULL;
}

struct hidapi_source_args_t {
        /* Application callback. */
        sr_receive_data_callback cb;
        void *cb_data;
        /* The serial device, to store RX data. */
        struct sr_serial_dev_inst *serial;
};

/*
 * Gets periodically invoked by the glib main loop. "Drives" (checks)
 * progress of USB communication, and invokes the application's callback
 * which processes RX data (when some has become available), as well as
 * handles application level timeouts.
 */
static int hidapi_source_cb(int fd, int revents, void *cb_data)
{
        struct hidapi_source_args_t *args;
        uint8_t rx_buf[SER_HID_CHUNK_SIZE];
        int rc;

        args = cb_data;

        /*
         * Drain receive data which the chip might have pending. This is
         * "a copy" of the "background part" of ser_hid_read(), without
         * the timeout support code, and not knowing how much data the
         * application is expecting.
         */
        do {
                rc = args->serial->hid_chip_funcs->read_bytes(args->serial,
                                rx_buf, sizeof(rx_buf), 0);
                if (rc > 0) {
                        ser_hid_mask_databits(args->serial, rx_buf, rc);
                        sr_ser_queue_rx_data(args->serial, rx_buf, rc);
                }
        } while (rc > 0);

        /*
         * When RX data became available (now or earlier), pass this
         * condition to the application callback. Always periodically
         * run the application callback, since it handles timeouts and
         * might carry out other tasks as well like signalling progress.
         */
        if (sr_ser_has_queued_data(args->serial))
                revents |= G_IO_IN;
        rc = args->cb(fd, revents, args->cb_data);

        return rc;
}

#define WITH_MAXIMUM_TIMEOUT_VALUE      10
static int ser_hid_hidapi_setup_source_add(struct sr_session *session,
        struct sr_serial_dev_inst *serial, int events, int timeout,
        sr_receive_data_callback cb, void *cb_data)
{
        struct hidapi_source_args_t *args;
        int rc;

        (void)events;

        /* Optionally enforce a minimum poll period. */
        if (WITH_MAXIMUM_TIMEOUT_VALUE && timeout > WITH_MAXIMUM_TIMEOUT_VALUE)
                timeout = WITH_MAXIMUM_TIMEOUT_VALUE;

        /* Allocate status container for background data reception. */
        args = g_malloc0(sizeof(*args));
        args->cb = cb;
        args->cb_data = cb_data;
        args->serial = serial;

        /*
         * Have a periodic timer installed. Register the allocated block
         * with the serial device, since the GSource's finalizer won't
         * free the memory, and we haven't bothered to create a custom
         * HIDAPI specific GSource.
         */
        rc = sr_session_source_add(session, -1, events, timeout,
                        hidapi_source_cb, args);
        if (rc != SR_OK) {
                g_free(args);
                return rc;
        }
        serial->hid_source_args = g_slist_append(serial->hid_source_args, args);

        return SR_OK;
}

static int ser_hid_hidapi_setup_source_remove(struct sr_session *session,
        struct sr_serial_dev_inst *serial)
{
        (void)serial;

        (void)sr_session_source_remove(session, -1);
        /*
         * Release callback args here already? Can there be more than
         * one source registered at any time, given that we pass fd -1
         * which is used as the key for the session?
         */

        return SR_OK;
}

SR_PRIV int ser_hid_hidapi_get_report(struct sr_serial_dev_inst *serial,
        uint8_t *data, size_t len)
{
        int rc;

        rc = hid_get_feature_report(serial->hid_dev, data, len);
        if (rc < 0)
                return SR_ERR_IO;

        return rc;
}

SR_PRIV int ser_hid_hidapi_set_report(struct sr_serial_dev_inst *serial,
        const uint8_t *data, size_t len)
{
        int rc;
        const wchar_t *err_text;

        rc = hid_send_feature_report(serial->hid_dev, data, len);
        if (rc < 0) {
                err_text = hid_error(serial->hid_dev);
                sr_dbg("%s() hidapi error: %ls", __func__, err_text);
                return SR_ERR_IO;
        }

        return rc;
}

SR_PRIV int ser_hid_hidapi_get_data(struct sr_serial_dev_inst *serial,
        uint8_t ep, uint8_t *data, size_t len, int timeout)
{
        int rc;

        (void)ep;

        if (timeout)
                rc = hid_read_timeout(serial->hid_dev, data, len, timeout);
        else
                rc = hid_read(serial->hid_dev, data, len);
        if (rc < 0)
                return SR_ERR_IO;
        if (rc == 0)
                return 0;

        return rc;
}

SR_PRIV int ser_hid_hidapi_set_data(struct sr_serial_dev_inst *serial,
        uint8_t ep, const uint8_t *data, size_t len, int timeout)
{
        int rc;

        (void)ep;
        (void)timeout;

        rc = hid_write(serial->hid_dev, data, len);
        if (rc < 0)
                return SR_ERR_IO;

        return rc;
}

/* }}} */
/* {{{ support for serial-over-HID chips */

static struct ser_hid_chip_functions **chips[SER_HID_CHIP_LAST] = {
        [SER_HID_CHIP_UNKNOWN] = NULL,
        [SER_HID_CHIP_BTC_BU86X] = &ser_hid_chip_funcs_bu86x,
        [SER_HID_CHIP_SIL_CP2110] = &ser_hid_chip_funcs_cp2110,
        [SER_HID_CHIP_VICTOR_DMM] = &ser_hid_chip_funcs_victor,
        [SER_HID_CHIP_WCH_CH9325] = &ser_hid_chip_funcs_ch9325,
};

static struct ser_hid_chip_functions *get_hid_chip_funcs(enum ser_hid_chip_t chip)
{
        struct ser_hid_chip_functions *funcs;

        if (chip >= ARRAY_SIZE(chips))
                return NULL;
        if (!chips[chip])
                return NULL;
        funcs = *chips[chip];
        if (!funcs)
                return NULL;

        return funcs;
}

static int ser_hid_setup_funcs(struct sr_serial_dev_inst *serial)
{

        if (!serial)
                return -1;

        if (serial->hid_chip && !serial->hid_chip_funcs) {
                serial->hid_chip_funcs = get_hid_chip_funcs(serial->hid_chip);
                if (!serial->hid_chip_funcs)
                        return -1;
        }

        return 0;
}

/*
 * Takes a pointer to the chip spec with potentially trailing data,
 * returns the chip index and advances the spec pointer upon match,
 * returns SER_HID_CHIP_UNKNOWN upon mismatch.
 */
static enum ser_hid_chip_t ser_hid_chip_find_enum(const char **spec_p)
{
        const gchar *spec;
        enum ser_hid_chip_t idx;
        struct ser_hid_chip_functions *desc;

        if (!spec_p || !*spec_p)
                return SER_HID_CHIP_UNKNOWN;
        spec = *spec_p;
        if (!*spec)
                return SER_HID_CHIP_UNKNOWN;
        for (idx = 0; idx < SER_HID_CHIP_LAST; idx++) {
                desc = get_hid_chip_funcs(idx);
                if (!desc)
                        continue;
                if (!desc->chipname)
                        continue;
                if (!g_str_has_prefix(spec, desc->chipname))
                        continue;
                spec += strlen(desc->chipname);
                *spec_p = spec;
                return idx;
        }

        return SER_HID_CHIP_UNKNOWN;
}

/* See if we can find a chip name for a VID:PID spec. */
SR_PRIV const char *ser_hid_chip_find_name_vid_pid(uint16_t vid, uint16_t pid)
{
        size_t chip_idx;
        struct ser_hid_chip_functions *desc;
        const struct vid_pid_item *vid_pids;

        for (chip_idx = 0; chip_idx < SER_HID_CHIP_LAST; chip_idx++) {
                desc = get_hid_chip_funcs(chip_idx);
                if (!desc)
                        continue;
                if (!desc->chipname)
                        continue;
                vid_pids = desc->vid_pid_items;
                if (!vid_pids)
                        continue;
                while (vid_pids->vid) {
                        if (vid_pids->vid == vid && vid_pids->pid == pid)
                                return desc->chipname;
                        vid_pids++;
                }
        }

        return NULL;
}

/**
 * See if a text string is a valid USB path for a HID device.
 * @param[in] serial The serial port that is about to get opened.
 * @param[in] path The (assumed) USB path specification.
 * @return SR_OK upon success, SR_ERR* upon failure.
 */
static int try_open_path(struct sr_serial_dev_inst *serial, const char *path)
{
        int rc;

        serial->usb_path = g_strdup(path);
        rc = ser_hid_hidapi_open_dev(serial);
        ser_hid_hidapi_close_dev(serial);
        g_free(serial->usb_path);
        serial->usb_path = NULL;

        return rc;
}

/**
 * Parse conn= specs for serial over HID communication.
 *
 * @param[in] serial The serial port that is about to get opened.
 * @param[in] spec The caller provided conn= specification.
 * @param[out] chip_ref Pointer to a chip type (enum).
 * @param[out] path_ref Pointer to a USB path (text string).
 * @param[out] serno_ref Pointer to a serial number (text string).
 *
 * @return 0 upon success, non-zero upon failure. Fills the *_ref output
 * values.
 *
 * Summary of parsing rules as they are implemented:
 * - Insist on the "hid" prefix. Accept "hid" alone without any other
 *   additional field.
 * - The first field that follows can be a chip spec, yet is optional.
 * - Any other field is assumed to be either a USB path or a serial
 *   number. There is no point in specifying both of these, as either
 *   of them uniquely identifies a device.
 *
 * Supported formats resulting from these rules:
 *   hid[/<chip>]
 *   hid[/<chip>]/usb=<bus>.<dev>[.<if>]
 *   hid[/<chip>]/raw=<path>    (may contain slashes!)
 *   hid[/<chip>]/sn=serno
 *
 * This routine just parses the conn= spec, which either was provided by
 * a user, or may reflect (cite) an item of a previously gathered listing
 * (clipboard provided by CLI clients, or selected from a GUI form).
 * Another routine will fill in the blanks, and do the cable selection
 * when a filter was specified.
 *
 * Users will want to use short forms when they need to come up with the
 * specs by themselves. The "verbose" or seemingly redundant forms (chip
 * _and_ path/serno spec) are useful when the cable uses non-standard or
 * not-yet-supported VID:PID items when automatic chip detection fails.
 */
static int ser_hid_parse_conn_spec(
        struct sr_serial_dev_inst *serial, const char *spec,
        enum ser_hid_chip_t *chip_ref, char **path_ref, char **serno_ref)
{
        const char *p;
        enum ser_hid_chip_t chip;
        char *path, *serno;
        int rc;

        if (chip_ref)
                *chip_ref = SER_HID_CHIP_UNKNOWN;
        if (path_ref)
                *path_ref = NULL;
        if (serno_ref)
                *serno_ref = NULL;
        chip = SER_HID_CHIP_UNKNOWN;
        path = serno = NULL;

        if (!serial || !spec || !*spec)
                return SR_ERR_ARG;
        p = spec;

        /* The "hid" prefix is mandatory. */
        if (!g_str_has_prefix(p, SER_HID_CONN_PREFIX))
                return SR_ERR_ARG;
        p += strlen(SER_HID_CONN_PREFIX);

        /*
         * Check for prefixed fields, assume chip type spec otherwise.
         * Paths and serial numbers "are greedy" (span to the end of
         * the input spec). Chip types are optional, and cannot repeat
         * multiple times.
         */
        while (*p) {
                if (*p == '/')
                        p++;
                if (!*p)
                        break;
                if (g_str_has_prefix(p, SER_HID_USB_PREFIX)) {
                        rc = try_open_path(serial, p);
                        if (rc != SR_OK)
                                return rc;
                        path = g_strdup(p);
                        p += strlen(p);
                } else if (g_str_has_prefix(p, SER_HID_IOKIT_PREFIX)) {
                        rc = try_open_path(serial, p);
                        if (rc != SR_OK)
                                return rc;
                        path = g_strdup(p);
                        p += strlen(p);
                } else if (g_str_has_prefix(p, SER_HID_RAW_PREFIX)) {
                        rc = try_open_path(serial, p);
                        if (rc != SR_OK)
                                return rc;
                        path = g_strdup(p);
                        p += strlen(p);
                } else if (g_str_has_prefix(p, SER_HID_SNR_PREFIX)) {
                        p += strlen(SER_HID_SNR_PREFIX);
                        serno = g_strdup(p);
                        p += strlen(p);
                } else if (!chip) {
                        char *copy;
                        const char *endptr;
                        copy = g_strdup(p);
                        endptr = copy;
                        chip = ser_hid_chip_find_enum(&endptr);
                        if (!chip) {
                                g_free(copy);
                                return SR_ERR_ARG;
                        }
                        p += endptr - copy;
                        g_free(copy);
                } else {
                        sr_err("unsupported conn= spec %s, error at %s", spec, p);
                        return SR_ERR_ARG;
                }
                if (*p == '/')
                        p++;
                if (path || serno)
                        break;
        }

        if (chip_ref)
                *chip_ref = chip;
        if (path_ref && path)
                *path_ref = path;
        if (serno_ref && serno)
                *serno_ref = serno;

        return SR_OK;
}

/* Get and compare serial number. Boolean return value. */
static int check_serno(const char *path, const char *serno_want)
{
        char *hid_path;
        char serno_got[128];
        int rc;

        hid_path = extract_hidapi_path(path);
        rc = ser_hid_hidapi_get_serno(hid_path, serno_got, sizeof(serno_got));
        g_free(hid_path);
        if (rc) {
                sr_dbg("DBG: %s(), could not get serial number", __func__);
                return 0;
        }

        return strcmp(serno_got, serno_want) == 0;
}

static GSList *append_find(GSList *devs, const char *path)
{
        char *copy;

        if (!path || !*path)
                return devs;

        copy = g_strdup(path);
        devs = g_slist_append(devs, copy);

        return devs;
}

static GSList *list_paths_for_vids_pids(const struct vid_pid_item *vid_pids)
{
        GSList *list;
        size_t idx;
        uint16_t vid, pid;

        list = NULL;
        for (idx = 0; /* EMPTY */; idx++) {
                if (!vid_pids) {
                        vid = pid = 0;
                } else if (!vid_pids[idx].vid) {
                        break;
                } else {
                        vid = vid_pids[idx].vid;
                        pid = vid_pids[idx].pid;
                }
                list = ser_hid_hidapi_find_usb(list, append_find, vid, pid);
                if (!vid_pids)
                        break;
        }

        return list;
}

/**
 * Search for a matching USB device for HID communication.
 *
 * @param[inout] chip The HID chip type (enum).
 * @param[inout] usbpath The USB path for the device (string).
 * @param[in] serno The serial number to search for.
 *
 * @retval SR_OK upon success
 * @retval SR_ERR_* upon failure.
 *
 * This routine fills in blanks which the conn= spec parser left open.
 * When not specified yet, the HID chip type gets determined. When a
 * serial number was specified, then search the corresponding device.
 * Upon completion, the chip type and USB path for the device shall be
 * known, as these are essential for subsequent operation.
 */
static int ser_hid_chip_search(enum ser_hid_chip_t *chip_ref,
        char **path_ref, const char *serno)
{
        enum ser_hid_chip_t chip;
        char *path;
        int have_chip, have_path, have_serno;
        struct ser_hid_chip_functions *chip_funcs;
        int rc;
        int serno_matched;
        uint16_t vid, pid;
        const char *name;
        const struct vid_pid_item *vid_pids;
        GSList *list, *matched, *matched2, *tmplist;

        if (!chip_ref)
                return SR_ERR_ARG;
        chip = *chip_ref;
        if (!path_ref)
                return SR_ERR_ARG;
        path = *path_ref;

        /*
         * Simplify the more complex conditions somewhat by assigning
         * to local variables. Handle the easiest conditions first.
         * - Either path or serial number can be specified, but not both
         *   at the same time.
         * - When a USB path is given, immediately see which HID chip
         *   the device has, without the need for enumeration.
         * - When a serial number is given, enumerate the devices and
         *   search for that number. Either enumerate all devices of the
         *   specified HID chip type (try the VID:PID pairs that we are
         *   aware of), or try all HID devices for unknown chip types.
         *   Not finding the serial number is fatal.
         * - When no path was found yet, enumerate the devices and pick
         *   one of them. Try known VID:PID pairs for a HID chip, or all
         *   devices for unknown chips. Make sure to pick a device of a
         *   supported chip type if the chip was not specified.
         * - Determine the chip type if not yet known. There should be
         *   a USB path by now, determined in one of the above blocks.
         */
        have_chip = (chip != SER_HID_CHIP_UNKNOWN) ? 1 : 0;
        have_path = (path && *path) ? 1 : 0;
        have_serno = (serno && *serno) ? 1 : 0;
        if (have_path && have_serno) {
                sr_err("Unsupported combination of USB path and serno");
                return SR_ERR_ARG;
        }
        chip_funcs = have_chip ? get_hid_chip_funcs(chip) : NULL;
        if (have_chip && !chip_funcs)
                return SR_ERR_NA;
        if (have_chip && !chip_funcs->vid_pid_items)
                return SR_ERR_NA;
        if (have_path && !have_chip) {
                vid = pid = 0;
                rc = ser_hid_hidapi_get_vid_pid(path, &vid, &pid);
                if (rc != SR_OK)
                        return rc;
                name = ser_hid_chip_find_name_vid_pid(vid, pid);
                if (!name || !*name)
                        return SR_ERR_NA;
                chip = ser_hid_chip_find_enum(&name);
                if (chip == SER_HID_CHIP_UNKNOWN)
                        return SR_ERR_NA;
                have_chip = 1;
        }
        if (have_serno) {
                vid_pids = have_chip ? chip_funcs->vid_pid_items : NULL;
                list = list_paths_for_vids_pids(vid_pids);
                if (!list)
                        return SR_ERR_NA;
                matched = NULL;
                for (tmplist = list; tmplist; tmplist = tmplist->next) {
                        path = get_hidapi_path_copy(tmplist->data);
                        serno_matched = check_serno(path, serno);
                        g_free(path);
                        if (!serno_matched)
                                continue;
                        matched = tmplist;
                        break;
                }
                if (!matched)
                        return SR_ERR_NA;
                path = g_strdup(matched->data);
                have_path = 1;
                g_slist_free_full(list, g_free);
        }
        if (!have_path) {
                vid_pids = have_chip ? chip_funcs->vid_pid_items : NULL;
                list = list_paths_for_vids_pids(vid_pids);
                if (!list)
                        return SR_ERR_NA;
                matched = matched2 = NULL;
                if (have_chip) {
                        /* List already only contains specified chip. */
                        matched = list;
                        matched2 = list->next;
                }
                /* Works for lists with one or multiple chips. Saves indentation. */
                for (tmplist = list; tmplist; tmplist = tmplist->next) {
                        if (have_chip)
                                break;
                        path = tmplist->data;
                        rc = ser_hid_hidapi_get_vid_pid(path, &vid, &pid);
                        if (rc || !ser_hid_chip_find_name_vid_pid(vid, pid))
                                continue;
                        if (!matched) {
                                matched = tmplist;
                                continue;
                        }
                        if (!matched2) {
                                matched2 = tmplist;
                                break;
                        }
                }
                if (!matched) {
                        g_slist_free_full(list, g_free);
                        return SR_ERR_NA;
                }
                /*
                 * TODO Optionally fail harder, expect users to provide
                 * unambiguous cable specs.
                 */
                if (matched2)
                        sr_info("More than one cable matches, random pick.");
                path = get_hidapi_path_copy(matched->data);
                have_path = 1;
                g_slist_free_full(list, g_free);
        }
        if (have_path && !have_chip) {
                vid = pid = 0;
                rc = ser_hid_hidapi_get_vid_pid(path, &vid, &pid);
                if (rc != SR_OK)
                        return rc;
                name = ser_hid_chip_find_name_vid_pid(vid, pid);
                if (!name || !*name)
                        return SR_ERR_NA;
                chip = ser_hid_chip_find_enum(&name);
                if (chip == SER_HID_CHIP_UNKNOWN)
                        return SR_ERR_NA;
                have_chip = 1;
        }
        (void)have_chip;

        if (chip_ref)
                *chip_ref = chip;
        if (path_ref)
                *path_ref = path;

        return SR_OK;
}

/* }}} */
/* {{{ transport methods called by the common serial.c code */

/* See if a serial port's name refers to an HID type. */
SR_PRIV int ser_name_is_hid(struct sr_serial_dev_inst *serial)
{
        size_t off;
        char sep;

        if (!serial)
                return 0;
        if (!serial->port || !*serial->port)
                return 0;

        /* Accept either "hid" alone, or "hid/" as a prefix. */
        if (!g_str_has_prefix(serial->port, SER_HID_CONN_PREFIX))
                return 0;
        off = strlen(SER_HID_CONN_PREFIX);
        sep = serial->port[off];
        if (sep != '\0' && sep != '/')
                return 0;

        return 1;
}

static int ser_hid_open(struct sr_serial_dev_inst *serial, int flags)
{
        enum ser_hid_chip_t chip;
        char *usbpath, *serno;
        int rc;

        (void)flags;

        if (ser_hid_setup_funcs(serial) != 0) {
                sr_err("Cannot determine HID communication library.");
                return SR_ERR_NA;
        }

        rc = ser_hid_parse_conn_spec(serial, serial->port,
                        &chip, &usbpath, &serno);
        if (rc != SR_OK)
                return SR_ERR_ARG;

        /*
         * When a serial number was specified, or when the chip type or
         * the USB path were not specified, do a search to determine the
         * device's USB path.
         */
        if (!chip || !usbpath || serno) {
                rc = ser_hid_chip_search(&chip, &usbpath, serno);
                if (rc != 0)
                        return SR_ERR_NA;
        }

        /*
         * Open the HID device. Only store chip type and device handle
         * when open completes successfully.
         */
        serial->hid_chip = chip;
        if (ser_hid_setup_funcs(serial) != 0) {
                sr_err("Cannot determine HID chip specific routines.");
                return SR_ERR_NA;
        }
        if (usbpath && *usbpath)
                serial->usb_path = usbpath;
        if (serno && *serno)
                serial->usb_serno = serno;

        rc = ser_hid_hidapi_open_dev(serial);
        if (rc) {
                sr_err("Failed to open HID device.");
                serial->hid_chip = 0;
                g_free(serial->usb_path);
                serial->usb_path = NULL;
                g_free(serial->usb_serno);
                serial->usb_serno = NULL;
                return SR_ERR_IO;
        }

        if (!serial->rcv_buffer)
                serial->rcv_buffer = g_string_sized_new(SER_HID_CHUNK_SIZE);

        return SR_OK;
}

static int ser_hid_close(struct sr_serial_dev_inst *serial)
{
        ser_hid_hidapi_close_dev(serial);

        return SR_OK;
}

static int ser_hid_set_params(struct sr_serial_dev_inst *serial,
        int baudrate, int bits, int parity, int stopbits,
        int flowcontrol, int rts, int dtr)
{
        if (ser_hid_setup_funcs(serial) != 0)
                return SR_ERR_NA;
        if (!serial->hid_chip_funcs || !serial->hid_chip_funcs->set_params)
                return SR_ERR_NA;

        return serial->hid_chip_funcs->set_params(serial,
                baudrate, bits, parity, stopbits,
                flowcontrol, rts, dtr);
}

static int ser_hid_setup_source_add(struct sr_session *session,
        struct sr_serial_dev_inst *serial, int events, int timeout,
        sr_receive_data_callback cb, void *cb_data)
{
        return ser_hid_hidapi_setup_source_add(session, serial,
                events, timeout, cb, cb_data);
}

static int ser_hid_setup_source_remove(struct sr_session *session,
        struct sr_serial_dev_inst *serial)
{
        return ser_hid_hidapi_setup_source_remove(session, serial);
}

static GSList *ser_hid_list(GSList *list, sr_ser_list_append_t append)
{
        return ser_hid_hidapi_list(list, append);
}

static GSList *ser_hid_find_usb(GSList *list, sr_ser_find_append_t append,
        uint16_t vendor_id, uint16_t product_id)
{
        return ser_hid_hidapi_find_usb(list, append, vendor_id, product_id);
}

static int ser_hid_flush(struct sr_serial_dev_inst *serial)
{
        if (!serial->hid_chip_funcs || !serial->hid_chip_funcs->flush)
                return SR_ERR_NA;

        return serial->hid_chip_funcs->flush(serial);
}

static int ser_hid_drain(struct sr_serial_dev_inst *serial)
{
        if (!serial->hid_chip_funcs || !serial->hid_chip_funcs->drain)
                return SR_ERR_NA;

        return serial->hid_chip_funcs->drain(serial);
}

static int ser_hid_write(struct sr_serial_dev_inst *serial,
        const void *buf, size_t count,
        int nonblocking, unsigned int timeout_ms)
{
        int total, max_chunk, chunk_len;
        int rc;

        if (!serial->hid_chip_funcs || !serial->hid_chip_funcs->write_bytes)
                return SR_ERR_NA;
        if (!serial->hid_chip_funcs->max_bytes_per_request)
                return SR_ERR_NA;

        total = 0;
        max_chunk = serial->hid_chip_funcs->max_bytes_per_request;
        while (count > 0) {
                chunk_len = count;
                if (max_chunk && chunk_len > max_chunk)
                        chunk_len = max_chunk;
                rc = serial->hid_chip_funcs->write_bytes(serial, buf, chunk_len);
                if (rc < 0) {
                        sr_err("Error sending transmit data to HID device.");
                        return total;
                }
                if (rc != chunk_len) {
                        sr_warn("Short transmission to HID device (%d/%d bytes)?",
                                        rc, chunk_len);
                        return total;
                }
                buf += chunk_len;
                count -= chunk_len;
                total += chunk_len;
                /* TODO
                 * Need we wait here? For data to drain through the slow
                 * UART. Not all UART-over-HID chips will have FIFOs.
                 */
                if (!nonblocking) {
                        (void)timeout_ms;
                        /* TODO */
                }
        }

        return total;
}

static int ser_hid_read(struct sr_serial_dev_inst *serial,
        void *buf, size_t count,
        int nonblocking, unsigned int timeout_ms)
{
        gint64 deadline_us, now_us;
        uint8_t buffer[SER_HID_CHUNK_SIZE];
        int rc;
        unsigned int got;

        if (!serial->hid_chip_funcs || !serial->hid_chip_funcs->read_bytes)
                return SR_ERR_NA;
        if (!serial->hid_chip_funcs->max_bytes_per_request)
                return SR_ERR_NA;

        /*
         * Immediately satisfy the caller's request from the RX buffer
         * if the requested amount of data is available already.
         */
        if (sr_ser_has_queued_data(serial) >= count)
                return sr_ser_unqueue_rx_data(serial, buf, count);

        /*
         * When a timeout was specified, then determine the deadline
         * where to stop reception.
         */
        deadline_us = 0;
        now_us = 0;             /* Silence a (false) compiler warning. */
        if (timeout_ms) {
                now_us = g_get_monotonic_time();
                deadline_us = now_us + timeout_ms * 1000;
        }

        /*
         * Keep receiving from the port until the caller's requested
         * amount of data has become available, or the timeout has
         * expired. In the absence of a timeout, stop reading when an
         * attempt no longer yields receive data.
         *
         * This implementation assumes that applications will call the
         * read routine often enough, or that reception continues in
         * background, such that data is not lost and hardware and
         * software buffers won't overrun.
         */
        while (TRUE) {
                /*
                 * Determine the timeout (in milliseconds) for this
                 * iteration. The 'now_us' timestamp was initially
                 * determined above, and gets updated at the bottom of
                 * the loop.
                 */
                if (deadline_us) {
                        timeout_ms = (deadline_us - now_us) / 1000;
                        if (!timeout_ms)
                                timeout_ms = 1;
                } else if (nonblocking) {
                        timeout_ms = 10;
                } else {
                        timeout_ms = 0;
                }

                /*
                 * Check the HID transport for the availability of more
                 * receive data.
                 */
                rc = serial->hid_chip_funcs->read_bytes(serial,
                                buffer, sizeof(buffer), timeout_ms);
                if (rc < 0) {
                        sr_dbg("DBG: %s() read error %d.", __func__, rc);
                        return SR_ERR;
                }
                if (rc) {
                        ser_hid_mask_databits(serial, buffer, rc);
                        sr_ser_queue_rx_data(serial, buffer, rc);
                }
                got = sr_ser_has_queued_data(serial);

                /*
                 * Stop reading when the requested amount is available,
                 * or when the timeout has expired.
                 *
                 * TODO Consider whether grabbing all RX data is more
                 * desirable. Implementing this approach requires a cheap
                 * check for the availability of more data on the USB level.
                 */
                if (got >= count)
                        break;
                if (nonblocking && !rc)
                        break;
                if (deadline_us) {
                        now_us = g_get_monotonic_time();
                        if (now_us >= deadline_us) {
                                sr_dbg("DBG: %s() read loop timeout.", __func__);
                                break;
                        }
                }
        }

        /*
         * Satisfy the caller's demand for receive data from previously
         * queued incoming data.
         */
        if (got > count)
                got = count;

        return sr_ser_unqueue_rx_data(serial, buf, count);
}

static struct ser_lib_functions serlib_hid = {
        .open = ser_hid_open,
        .close = ser_hid_close,
        .flush = ser_hid_flush,
        .drain = ser_hid_drain,
        .write = ser_hid_write,
        .read = ser_hid_read,
        .set_params = ser_hid_set_params,
        .set_handshake = std_dummy_set_handshake,
        .setup_source_add = ser_hid_setup_source_add,
        .setup_source_remove = ser_hid_setup_source_remove,
        .list = ser_hid_list,
        .find_usb = ser_hid_find_usb,
        .get_frame_format = NULL,
};
SR_PRIV struct ser_lib_functions *ser_lib_funcs_hid = &serlib_hid;

/* }}} */
#else

SR_PRIV int ser_name_is_hid(struct sr_serial_dev_inst *serial)
{
        (void)serial;

        return 0;
}

SR_PRIV struct ser_lib_functions *ser_lib_funcs_hid = NULL;

#endif
#endif
/** @} */