The Mesa Redox winsys CS submit path at
src/gallium/winsys/redox/drm/redox_drm_cs.c:156 faked the seqno with
'result.seqno = rws->cs->last_seqno + 1 : 1;' instead of reading the
kernel-assigned seqno from the ioctl response. This is a correctness
bug under multi-process GPU use (the normal case for any compositor
+ GPU client setup). The kernel's seqno is global per device, but
each Mesa process had its own local counter. When process A submits
batch #1 (kernel seqno 100) and process B submits batch #2 (kernel
seqno 101), process A's local counter diverges from the kernel's
actual seqno. Fence waits keyed on the local counter would never
complete when waiting for seqnos in the kernel's namespace.
Fix (three coordinated changes):
### 1. redox-drm kernel side (local/recipes/gpu/redox-drm/)
Following the standard DRM bidirectional-ioctl pattern that
DrmAmdgpuCsWire already uses:
a) driver.rs:
- Added Default derive to RedoxPrivateCsSubmit and
RedoxPrivateCsWait structs (needed for ..Default::default() at
construction sites).
- Added response field 'seqno: u64' to RedoxPrivateCsSubmit
(bidirectional: input fields src..byte_count, output seqno).
- Added response fields to RedoxPrivateCsWait: completed(u8),
_pad([u8;7]), completed_seqno(u64).
- Updated size tests: Submit 32->40 bytes, Wait 16->32 bytes.
- Added doc comments noting the bidirectional pattern and the
kernel-Writes-Response contract.
b) scheme.rs:
- CS_SUBMIT handler writes resp.seqno back into req.seqno and
serializes req instead of serializing the separate resp
(bytes_of(&resp) -> bytes_of(&req)). This is the kernel
returning the response in the same struct.
- CS_WAIT handler similarly copies result fields into req.
- req made mutable for in-place mutation before serialization.
- All other places that construct these structs use
..Default::default() for the new response fields.
c) drivers/amd/mod.rs, intel/mod.rs, virtio/mod.rs:
- Each cs_submit and cs_wait construction site now uses
..Default::default() for the new response fields. No logic
changes (the drivers return RedoxPrivateCsSubmitResult /
RedoxPrivateCsWaitResult from the trait method; scheme.rs
copies the response into the bidirectional struct).
### 2. Mesa winsys source (local/recipes/libs/mesa/source/)
Merge the separate input/result wire structs into bidirectional
structs so the kernel's response is read back from the same struct
the caller passed to drmIoctl:
a) redox_drm_cs.c:
- Merged RedoxCsSubmitWire and RedoxCsSubmitResultWire into one
struct (RedoxCsSubmitWire now has the seqno output field).
- Merged RedoxCsWaitWire and RedoxCsWaitResultWire into one
struct (RedoxCsWaitWire now has completed + completed_seqno
fields).
- Removed 'result.seqno = rws->cs->last_seqno + 1 : 1;' fake.
Instead, reads 'submit.seqno' and 'wait.completed_seqno' from
the same struct after drmIoctl returns.
- Updated file-header comment to document the bidirectional
pattern, kernel ABI, and the multi-process correctness
consequence.
b) Patches the durability:
- Added 'mesa/26-cs-submit-bidirectional-seqno.patch' to the
patches list in local/recipes/libs/mesa/recipe.toml.
- The patch persists the C-side merge across clean re-extracts
of the upstream Mesa 26.1.4 tarball.
Note (operator runtime gate):
- The kernel ABI change requires that the ioctl bytes ARE read
back into the same user buffer on Redox schemes. This is the
standard pattern for all other DRM ioctls in redox-drm's
scheme.rs (DrmGemCreateWire, DrmAmdgpuCsWire, DrmCreateDumbWire
etc.). Verification of the runtime fix requires multi-process
GPU testing on real hardware — operator-side gate.
- Per AGENTS.md NO-FALLBACK policy: this fixes a real correctness
bug. The pre-fix 'fake seqno' code was admitted in the original
file via a '// TODO' comment with the requirement to integrate
with the actual scheme:drm protocol - now done.
Files changed:
- local/recipes/gpu/redox-drm/source/src/driver.rs
- local/recipes/gpu/redox-drm/source/src/scheme.rs
- local/recipes/gpu/redox-drm/source/src/drivers/amd/mod.rs
- local/recipes/gpu/redox-drm/source/src/drivers/intel/mod.rs
- local/recipes/gpu/redox-drm/source/src/drivers/virtio/mod.rs
- local/recipes/libs/mesa/source/src/gallium/winsys/redox/drm/redox_drm_cs.c
- local/recipes/libs/mesa/recipe.toml
- local/patches/mesa/26-cs-submit-bidirectional-seqno.patch
kellito
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