drmtap_plane_rotation(): the rotation the primary plane scans out with - #64
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…ut with A captured frame is upright only when the compositor rotated the output in hardware: then the framebuffer holds the logical desktop and the plane turns it on scanout. When the plane cannot rotate, the compositor draws the framebuffer already turned and the capture comes out upside down at 180 and sideways at 90/270. wl_output cannot tell the two apart; the plane rotation property can. Measured before writing it: on i915 + mutter (GNOME 50) a 180 output sets the primary plane to rotate-180 and a scanout dump is upright, identical to the unrotated one; on virtio-gpu (no rotation property) the same 180 gives an upside-down dump and 90/270 give the logical desktop turned inside the native mode framebuffer. So a consumer turns the frame by (output transform - plane rotation), and -ENOTSUP (no property) counts as rotate-0. The property id is looked up once per plane and reused, so a call costs one GETPLANE plus one OBJ_GETPROPERTIES. Exported in the map, bound in the -sys crate and as Context::plane_rotation() -> Option<u32> in the wrapper. The integration capture test reads it after a grab: exactly one ROTATE bit, or -ENOTSUP. Verified: i915 (Sigma-26) 0x1 at 0 and 0x4 with HDMI-A-1 at 180, the other output untouched; amdgpu RX560 0x1; appletbdrm (Touch Bar) -ENOTSUP.
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📝 SummarySummary by CodeRabbit
WalkthroughThe change adds a C API that reads the primary plane’s DRM rotation bitmask, caches plane and property lookup state, and exposes the query through a Rust wrapper. It also updates version metadata and adds capture-test assertions. ChangesPrimary Plane Rotation Query
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant Caller
participant drmtap_plane_rotation
participant current_primary_plane
participant DRM
Caller->>drmtap_plane_rotation: Request rotation bitmask
drmtap_plane_rotation->>current_primary_plane: Find bound primary plane
current_primary_plane->>DRM: Check cached plane or find primary plane
drmtap_plane_rotation->>DRM: Read plane properties and rotation value
DRM-->>drmtap_plane_rotation: Return property data
drmtap_plane_rotation-->>Caller: Return bitmask or error code
Merge Risk: 🔵 Low · up to On a DRM configuration with an active overlay but no recognized active primary plane, the query can report the overlay's rotation and lead consumers to orient captured frames incorrectly. This is a narrow, configuration-dependent risk; reject the non-primary fallback before relying on the result. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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🧪 Generate unit tests (beta)
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Actionable comments posted: 5
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@bindings/rust/libdrmtap/src/lib.rs`:
- Line 373: Move the existing “Get the cursor state” documentation comment so it
directly precedes get_cursor(), ensuring it documents that method rather than
plane_rotation().
In `@CHANGELOG.md`:
- Line 22: Update the release note to name the method as
DrmTap::plane_rotation() instead of Context::plane_rotation(), keeping the
documented return type unchanged.
In `@src/drm_grab.c`:
- Line 2550: Move the static helper current_primary_plane() into the
private-function section before the public functions. Apply the same
organization change at src/drm_grab.c lines 2550-2550 and
bindings/rust/libdrmtap-sys/csrc/drm_grab.c lines 2550-2550, keeping both copies
consistent.
- Around line 2557-2558: Update drmtap_plane_rotation() to return rotation only
for a verified primary plane: check the cached plane’s type before reusing it,
and reject the non-primary fallback from find_primary_plane() on the initial
lookup. Apply the same correction to the bundled implementation.
- Line 2608: Update the rotation-property lookup that assigns
ctx->rot_prop_state so a NULL result from drmModeGetProperty leaves the lookup
pending or propagates the read error, rather than caching the property as
missing. Apply this behavior in both implementations while retaining -1 for a
successfully completed lookup that confirms the property is absent.
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CHANGELOG.mdbindings/rust/libdrmtap-sys/Cargo.tomlbindings/rust/libdrmtap-sys/csrc/drm_grab.cbindings/rust/libdrmtap-sys/csrc/drmtap.hbindings/rust/libdrmtap-sys/csrc/drmtap_internal.hbindings/rust/libdrmtap-sys/src/lib.rsbindings/rust/libdrmtap/Cargo.tomlbindings/rust/libdrmtap/src/lib.rsinclude/drmtap.hmeson.buildsrc/drm_grab.csrc/drmtap_internal.htests/test_capture.c
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🪛 Clang (14.0.6)
bindings/rust/libdrmtap-sys/csrc/drm_grab.c
[warning] 2553-2553: variable 'still' is not initialized
(cppcoreguidelines-init-variables)
src/drm_grab.c
[warning] 2553-2553: variable 'still' is not initialized
(cppcoreguidelines-init-variables)
| if (still) { | ||
| return ctx->rot_plane_id; |
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🎯 Functional Correctness | 🟠 Major | ⚡ Quick win
🔎 Supported by static analysis
🏁 Script executed:
grep -n 'find_primary_plane' -r src | head
sed -n '2539,2630p' src/drm_grab.cRepository: fxd0h/libdrmtap
Length of output: 3780
🏁 Script executed:
sed -n '200,350p' src/drm_grab.c
printf '%s\n' '--- bundled helper and current function ---'
sed -n '200,350p' bindings/rust/libdrmtap-sys/csrc/drm_grab.c
sed -n '2535,2570p' bindings/rust/libdrmtap-sys/csrc/drm_grab.c
printf '%s\n' '--- context and plane-type references ---'
rg -n -C 3 'rot_plane_id|DRM_PLANE_TYPE|type.*primary|PRIMARY|find_primary_plane' src/drm_grab.c bindings/rust/libdrmtap-sys/csrc/drm_grab.c src include bindings/rust/libdrmtap-sys/csrc --glob '*.{c,h}' | head -240Repository: fxd0h/libdrmtap
Length of output: 30489
🏁 Script executed:
sed -n '225,325p' src/drm_grab.c
sed -n '225,325p' bindings/rust/libdrmtap-sys/csrc/drm_grab.cRepository: fxd0h/libdrmtap
Length of output: 7397
Use only a verified primary plane for rotation.
find_primary_plane() returns the first active plane when no primary plane is found. current_primary_plane() then reuses that cached plane when it remains bound with a framebuffer, without checking its type. Therefore, drmtap_plane_rotation() can report an active overlay’s rotation, including after a primary plane becomes active.
Check the cached plane’s type before returning it. Also reject the non-primary fallback from find_primary_plane() for this API; checking only the cache does not prevent an overlay from being returned on the first call. Apply the same correction to the bundled implementation.
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In `@src/drm_grab.c` around lines 2557 - 2558, Update drmtap_plane_rotation() to
return rotation only for a verified primary plane: check the cached plane’s type
before reusing it, and reject the non-primary fallback from find_primary_plane()
on the initial lookup. Apply the same correction to the bundled implementation.
After applying the fix, consider running `coderabbit review --agent` for local
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… failed read find_primary_plane() falls back to the first active plane bound to the CRTC when no primary-typed plane carries a framebuffer, and the accessor cached whatever it found as long as GETPLANE said it was still bound. A later grab could then read a different plane than the one the rotation was answered for. Now do_grab and the fast path record the plane they read from, and the accessor answers for that plane, so the rotation always belongs to the frame it is called next to; before the first grab it uses the plane a grab would use. A drmModeGetProperty failure during the name lookup was stored as "no such property" and every later call returned -ENOTSUP. It now leaves the lookup pending and returns the errno, so the next call looks again. Also: the changelog names the wrapper type (DrmTap, not Context), the cursor doc comment stays with get_cursor(), and the static helper is gone (the accessor sits in the plane discovery section, after find_primary_plane). Verified again on Sigma i915: 0x1 at 0, 0x4 with HDMI-A-1 at 180, 0x1 restored; unit 10/10, integration 2/2 as root.
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Inline comments:
In `@src/drm_grab.c`:
- Line 915: Update the grab flow so `ctx->grab_plane_id` is committed only after
framebuffer acquisition and the grab succeeds, preserving its previous value on
every error path. Apply the same change at src/drm_grab.c:915-915 and
bindings/rust/libdrmtap-sys/csrc/drm_grab.c:915-915.
- Line 1975: In the fast-grab implementation, remove the initialization-time
assignment to `ctx->grab_plane_id` and update it to `ctx->fast_plane_id`
immediately before every successful `return 0`, including cache-hit paths. Apply
the same changes to both C implementations.
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do_grab recorded the plane it chose before acquiring the framebuffer, and the fast path recorded it at initialisation, so a grab that then failed could leave drmtap_plane_rotation() describing a plane no frame came from. Both paths now write grab_plane_id right before each successful return, and keep the previous value on every error path.
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Inline comments:
In `@src/drm_grab.c`:
- Line 1226: Update the helper V2 path in src/drm_grab.c at line 1226 and
bindings/rust/libdrmtap-sys/csrc/drm_grab.c at line 1226 so grab_plane_id
reflects only the plane from the last successful public grab. Defer the update
until drmtap_grab_desc accepts the frame, or roll it back when that call rejects
the frame.
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drmtap_grab_desc fails closed with -ENOTSUP when do_grab produced pixels instead of a transferable dma-buf (the helper V2 fallback), but do_grab had already named that plane as the one drmtap_plane_rotation() answers for. The caller never received that frame, so the previous name is restored.
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a captured frame is upright only when the compositor rotated the output in hardware: then the framebuffer holds the logical desktop and the plane turns it on scanout. when the plane cannot rotate, the compositor draws the framebuffer already turned and the capture comes out upside down at 180 and sideways at 90/270. wl_output cannot tell the two apart; the plane rotation property can.
measured before writing it:
so a consumer turns the frame by (output transform - plane rotation), and -ENOTSUP (no property) counts as rotate-0.
drmtap_plane_rotation(ctx, &mask)reads the property now, for the plane the last grab read its framebuffer from, so it is called right after the grab it describes. the property id is looked up once per plane and reused: a call costs one GETPLANE plus one OBJ_GETPROPERTIES. exported in the map, bound in the -sys crate and asDrmTap::plane_rotation() -> Option<u32>in the wrapper. the integration capture test reads it after a grab and expects exactly one ROTATE bit, or -ENOTSUP.verified: i915 0x1 at 0 and 0x4 with one output at 180 (the other output stays 0x1); amdgpu rx560 0x1; appletbdrm (touch bar) -ENOTSUP. unit 10/10, integration 2/2 as root.
no version bump here; 0.5.8 when it ships.