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DEBUG: cache-fix CAPTURE dump + VIDIOC_G_EXT_CTRLS readback

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Tier 3E + 3F observability hardening from the libva-multiplanar
campaign Phase 6 follow-up. Improves diagnostic reliability for
future probes; no functional decode path change.

Tier 3E (cache-fix): patch-0010's CAPTURE Y-plane dump now calls
msync(p, 32, MS_SYNC|MS_INVALIDATE) before the read so userspace
sees what the kernel actually DMA-wrote, not a stale CPU cache
line. Without this, the previous version of the dump consistently
showed the patch-0011 sentinel (0xab) even when the kernel had
overwritten it — caused half a day of mistaken "kernel never wrote
the buffer" diagnosis. Also computes a luma min/max/variance
signal so a uniform fill (variance=0) is visually obvious vs
real pixel data (variance > 0).

Tier 3F (VIDIOC_G_EXT_CTRLS readback): after v4l2_set_controls
in h264_set_controls, reads back DECODE_PARAMS + PPS via
v4l2_get_controls (added by patch 0003) on the request fd.
Logs key fields:

  dec.idr_pic_id, poc_lsb, refmark_bits, poc_bits  — confirms
    slice-header parser outputs landed in the V4L2 control batch.
  dec.top_foc / bot_foc                             — confirms
    patch-0015 POC sentinel strip actually applied (should NOT
    show 65536 unless the strip mis-fired).
  dec.frame_num                                     — cross-checks
    against VAAPI's pre-parsed frame_num (also already logged by
    patch 0014).
  pps.flags + (SMP=...)                             — confirms
    SCALING_MATRIX_PRESENT bit set this build.
  pps.refidx_l0/l1                                  — confirms
    Tier 1B num_ref_idx writes landed.

Discriminates "we wrote X but kernel saw Y" from "we wrote zero
all along" — the failure mode the original patch series didn't
catch when slice-header bit_size fields were left zero.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Markus Fritsche
2026-05-04 12:58:52 +00:00
parent 9de1be34ef
commit a047926dbc
2 changed files with 81 additions and 9 deletions
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src/surface.c
+28 -9
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@@ -367,13 +367,24 @@ VAStatus RequestSyncSurface(VADriverContextP context, VASurfaceID surface_id)
/*
* DEBUG INSTRUMENTATION (0010): hex-dump first 32 bytes of the
* decoded CAPTURE Y-plane after DQBUF. If the kernel actually
* decoded the frame, these should reflect a real Y-luma pattern
* (varied bytes). All-zero or all-identical means no decode
* landed pixels in the buffer. Removed once Step 1 is verified.
* decoded CAPTURE Y-plane after DQBUF, plus a 32-byte luma
* variance signal (max - min) so we can tell at a glance whether
* the bytes are real decoded pixels or a uniform fill.
*
* Cache hygiene fix (2026-05-04): without msync(MS_SYNC|
* MS_INVALIDATE) the previous version of this dump consistently
* showed the patch-0011 sentinel (0xab) even when the kernel had
* DMA-overwritten the buffer with real pixels — userspace's
* cached mmap line for the first 32 bytes contained the sentinel
* write and was never invalidated by DQBUF alone on this hantro
* CMA-backed config. Caused half a day of mistaken "kernel never
* wrote the buffer" diagnosis (see libva-multiplanar campaign
* phase0_evidence/2026-05-04-kernel-trace/findings.md). The
* msync() forces the kernel to invalidate userspace cache for
* this region so the read goes to physical memory.
*/
{
const unsigned char *p =
unsigned char *p =
(unsigned char *)surface_object->destination_map[0];
char hex[32 * 3 + 1] = { 0 };
unsigned int i;
@@ -381,11 +392,19 @@ VAStatus RequestSyncSurface(VADriverContextP context, VASurfaceID surface_id)
request_log("CAPTURE[idx=%u, plane0]: (NULL)\n",
surface_object->destination_index);
} else {
for (i = 0; i < 32; i++)
snprintf(hex + i * 3, 4, " %02x", p[i]);
request_log("CAPTURE[idx=%u, plane0]:%s\n",
unsigned char ymin = 0xff, ymax = 0;
(void)msync(p, 32, MS_SYNC | MS_INVALIDATE);
for (i = 0; i < 32; i++) {
unsigned char y = p[i];
snprintf(hex + i * 3, 4, " %02x", y);
if (y < ymin) ymin = y;
if (y > ymax) ymax = y;
}
request_log("CAPTURE[idx=%u, plane0]:%s "
"(ymin=0x%02x ymax=0x%02x var=%u)\n",
surface_object->destination_index,
hex);
hex, ymin, ymax,
(unsigned)(ymax - ymin));
}
}