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iter8 Phase 7 IMP-1 experiment: LIBVA_V4L2_ZERO_CAPTURE pre-zero gate

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Env-gated CAPTURE pre-zero in BeginPicture after cap_pool_acquire. With
LIBVA_V4L2_ZERO_CAPTURE=1, the slot mmap region is memset 0 before the
kernel decode runs. Discriminates "kernel writes partial then aborts"
from "kernel writes nothing, buffer carries stale residue from prior
allocation."

Per Phase 5 IMP-1: the 16x32 patch in libva H.264 frame 1 may be either
real partial kernel write OR stale residue. This gate makes the next
sweep run deterministically zero the buffer; if the patch still appears
after, the kernel really writes it; if not, it was stale.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
claude-noether
2026-05-13 12:21:54 +00:00
parent 7eae6eab46
commit 66ecbef5c6
1 changed files with 23 additions and 0 deletions
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src/picture.c
+23
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@@ -321,6 +321,29 @@ VAStatus RequestBeginPicture(VADriverContextP context, VAContextID context_id,
if (cap_slot == NULL)
return VA_STATUS_ERROR_ALLOCATION_FAILED;
surface_bind_slot(surface_object, cap_slot);
/*
* iter8 Phase 7 IMP-1 experiment: env-gated CAPTURE buffer
* pre-zero. LIBVA_V4L2_ZERO_CAPTURE=1 wipes the slot's mmap'd
* region before kernel decode. Discriminates "kernel writes
* partial then aborts" from "kernel writes nothing and we
* see stale residue."
*/
{
static const char *zero_env = NULL;
static bool zero_env_checked = false;
if (!zero_env_checked) {
zero_env = getenv("LIBVA_V4L2_ZERO_CAPTURE");
zero_env_checked = true;
}
if (zero_env != NULL && zero_env[0] == '1') {
unsigned int b;
for (b = 0; b < cap_slot->buffers_count; b++)
if (cap_slot->map[b] != NULL)
memset(cap_slot->map[b], 0,
cap_slot->map_lengths[b]);
}
}
}
/*