asix-sigma: comment on trigger/stop position, silence warning

Add a comment on the logic which skips the upper 64 bytes of a 512 bytes
chunk in the Asix Sigma's sample memory. Move the initial assignment and
the subsequent update from a value which was retrieved from a hardware
register closer together for awareness during maintenance. Pre-setting a
high position value that will never match when the feature is not in use
is very appropriate.

Adjust the sigma_read_pos() routine to handle triggerpos identically to
stoppos. The test condition's intention is to check whether a decrement
of the position ends up in the meta data section of a chunk. The previous
implementation tested whether a pointer to the position variable ended in
0x1ff when decremented -- which is unrelated to the driver's operation.
It's assumed that no harm was done because the trigger feature is
unsupported (see bug #359).

This silences the compiler warning reported in bug #1411.

diff --git a/src/hardware/asix-sigma/protocol.c b/src/hardware/asix-sigma/protocol.c
index 94fd8de044356a9a999b9a32cea11cd8c998f3ae..93d2623951e565f099ab68cc02e4af8ac66ccf32 100644 (file)
--- a/src/hardware/asix-sigma/protocol.c
+++ b/src/hardware/asix-sigma/protocol.c
@@ -168,11 +168,17 @@ static int sigma_read_pos(uint32_t *stoppos, uint32_t *triggerpos,
        *triggerpos = result[0] | (result[1] << 8) | (result[2] << 16);
        *stoppos = result[3] | (result[4] << 8) | (result[5] << 16);
 
-       /* Not really sure why this must be done, but according to spec. */
+       /*
+        * These "position" values point to after the event (end of
+        * capture data, trigger condition matched). This is why they
+        * get decremented here. Sample memory consists of 512-byte
+        * chunks with meta data in the upper 64 bytes. Thus when the
+        * decrements takes us into this upper part of the chunk, then
+        * further move backwards to the end of the chunk's data part.
+        */
        if ((--*stoppos & 0x1ff) == 0x1ff)
                *stoppos -= 64;
-
-       if ((*--triggerpos & 0x1ff) == 0x1ff)
+       if ((--*triggerpos & 0x1ff) == 0x1ff)
                *triggerpos -= 64;
 
        return 1;
@@ -1009,8 +1015,6 @@ static int download_capture(struct sr_dev_inst *sdi)
 
        devc = sdi->priv;
        dl_events_in_line = 64 * 7;
-       trg_line = ~0;
-       trg_event = ~0;
 
        dram_line = g_try_malloc0(chunks_per_read * sizeof(*dram_line));
        if (!dram_line)
@@ -1038,6 +1042,8 @@ static int download_capture(struct sr_dev_inst *sdi)
 
        /* Check if trigger has fired. */
        modestatus = sigma_get_register(READ_MODE, devc);
+       trg_line = ~0;
+       trg_event = ~0;
        if (modestatus & RMR_TRIGGERED) {
                trg_line = triggerpos >> 9;
                trg_event = triggerpos & 0x1ff;