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feat: drmtap_crtc_refresh(), the exact refresh of the captured CRTC as a fraction (0.5.9) - #66
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…s a fraction (0.5.9) drmtap_display.refresh_hz is the kernel's vrefresh, rounded to whole hertz: 59.94 reads 60, 23.976 reads 24, 1280x1024 at 60.02 reads 60. A consumer that paces capture to the panel needs the real rate. drmtap_crtc_refresh(ctx, &num, &den) returns it as the reduced fraction num/den hertz, computed from the CRTC's current mode the way the kernel computes vrefresh (pixel clock over the totals, with interlace, doublescan and vscan) but not rounded. No connector probe: once the CRTC is known it is one GETCRTC, so it can be called again to follow a mode change. On a crtc_id 0 context the first call picks the CRTC a grab would pick; the grab and this call now share one helper for that choice. -ENODATA when the CRTC has no mode (disabled). The mode-to-fraction step is pure (drmtap_mode_refresh) and unit-tested on CEA-861 timings, interlace, doublescan, vscan and the widest field values; test_enumerate checks on hardware that each active display's fraction rounds to its vrefresh. Measured on i915: 60/1, 6750000/112463 (1280x1024, 60.0197 Hz) and 131375/4382 (3840x2160, 29.9806 Hz); -ENODATA on a disabled pipe. Rust: libdrmtap-sys drmtap_crtc_refresh, wrapper DrmTap::crtc_refresh() -> Option<(u64, u64)>. Version 0.5.9.
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🧠 Learnings (1)📓 Common learnings🪛 LanguageToolCHANGELOG.md[style] ~16-~16: Consider using “who” when you are referring to a person instead of an object. (THAT_WHO) 📝 SummarySummary by CodeRabbit
WalkthroughThe change adds exact refresh-rate calculation from DRM mode timings and exposes the current CRTC rate through C and Rust APIs. It also adds tests and updates release documentation for version 0.5.9. ChangesExact CRTC refresh reporting
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant App
participant DrmTap
participant CRTC
participant ModeCalculator
App->>DrmTap: request current CRTC refresh
DrmTap->>CRTC: select or read CRTC mode
CRTC-->>DrmTap: mode timings
DrmTap->>ModeCalculator: calculate reduced refresh fraction
ModeCalculator-->>DrmTap: numerator and denominator
DrmTap-->>App: fraction or error
Merge Risk: ⚪ Minimal · up to The exact-refresh API is consistent across the C implementation, Rust wrapper, and shared-library ABI, with no actionable merge-blocking risk remaining. Security Architecture ReviewSecurity architecture risk: ⚪ Minimal · up to The new query reads metadata from the already-open DRM device without granting additional capture privileges or invoking privileged helper or frame-export paths. The reviewed selection and failure behavior preserves existing capture boundaries; no material security regression was identified. Retained concerns Security review detailsSecurity Blast Radius
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Inline comments:
Review comments at @bindings/rust/libdrmtap/src/lib.rs:
- Line 408: Update the errno comparison in the function containing the rc check
to use libc’s target-specific ENODATA constant instead of the hard-coded value,
and add libc as a dependency in the crate manifest so disabled CRTCs continue to
return Ok(None) across architectures.
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CHANGELOG.mdREADME.mdbindings/rust/libdrmtap-sys/Cargo.tomlbindings/rust/libdrmtap-sys/csrc/drm_enumerate.cbindings/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/README.mdbindings/rust/libdrmtap/src/lib.rsdocs/research/05_api_and_architecture.mdinclude/drmtap.hmeson.buildsrc/drm_enumerate.csrc/drm_grab.csrc/drmtap_internal.htests/test_enumerate.ctests/test_refresh.c
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Source excerpt: | Rule | Convention | |---|---| | Indent | 4 spaces, never tabs | | Naming: functions | `snake_case`, prefixed `drmtap_` for public API | | Naming: variables | `snake_case` | | Naming: macros/constants | `UPPER_SNAKE_CASE`,...
📄 CodeRabbit inference engine (AGENTS.md)
Files:
src/drm_enumerate.cbindings/rust/libdrmtap-sys/csrc/drm_enumerate.cbindings/rust/libdrmtap-sys/csrc/drmtap_internal.hsrc/drm_grab.cbindings/rust/libdrmtap-sys/csrc/drm_grab.ctests/test_enumerate.cinclude/drmtap.hsrc/drmtap_internal.hbindings/rust/libdrmtap-sys/csrc/drmtap.htests/test_refresh.c
Source excerpt: Architecture changes → update `docs/research/05_api_and_architecture.md`
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docs/research/05_api_and_architecture.md
Source excerpt: Source excerpt: Public API changes → update `include/drmtap.h` comments
📄 CodeRabbit inference engine (AGENTS.md)
Files:
include/drmtap.h
🪛 Clang (14.0.6)
src/drm_enumerate.c
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🪛 LanguageTool
CHANGELOG.md
[style] ~16-~16: Consider using “who” when you are referring to a person instead of an object.
Context: ...29.98) read as if they were. A consumer that paces capture to the panel needs the re...
(THAT_WHO)
🔇 Additional comments (1)
src/drm_grab.c (1)
341-341: 🗄️ Data Integrity & IntegrationThe export concern does not apply.
libdrmtap.maplistsdrmtap_crtc_refreshin itsglobalblock, so the shared library exports the new API.
plane_rotation() matched -95 and crtc_refresh() -61: ENOTSUP and ENODATA on x86 and ARM, but not on SPARC, MIPS or PA-RISC, where a plane without the rotation property or a CRTC with no mode came back as an Err instead of Ok(None).
- the header said the wrapper crate carries its own version; all four share one since 0.5.1 - refresh_hz: the whole-hertz vrefresh "on Linux 5.9 and later" - wrapper: crtc_refresh() on a crtc_id 0 context with no CRTC to pick is an Err (ENOENT), not None; version() returns a tuple, not a packed integer; the crate now ships its LICENSE - README: 27 public functions, not ~20; the hotspot measurement link is rustdesk#16122 (#15897 closed unmerged); the integration-test command pointed DRM_DEVICE at a card meson overrides and root ignores - AGENTS.md: test_cursor.c and test_refresh.c in the test tree, eight more unit suites; the integration commands point at CONTRIBUTING.md - CONTRIBUTING.md: when test_capture passes without grabbing a frame - CHANGELOG 0.5.9: the measurements under Added, not under the errno fix; link references in order; no apostrophes - tools/README.md: publish the wrapper on every release, after -sys
1 participant
drmtap_display.refresh_hzis the kernel's vrefresh, rounded to whole hertz: 59.94 reads 60, 23.976 reads 24, and 1280x1024 at 60.02 reads 60. a consumer that paces capture to the panel needs the real rate (rustdesk/rustdesk#16175: the drm producer will pace to the encoder rate, with the panel refresh as its floor).what changes:
1-
int drmtap_crtc_refresh(drmtap_ctx *ctx, uint64_t *num, uint64_t *den): the refresh of the captured CRTC in hertz as the reduced fraction num/den, from its current mode, computed the way the kernel computes vrefresh (pixel clock over the totals, interlace, doublescan, vscan) but not rounded. no connector probe: once the CRTC is known it is one GETCRTC.-ENODATAfor a CRTC with no mode,-ENOENTwhen there is none to pick,-ENOTSUPon a render-only context2- the CRTC auto-selection of a
crtc_id0 context moved into one helper that the grab and this call share; the grab behaves as before3- exported in
libdrmtap.map;libdrmtap-sysgets the extern and the wrapperDrmTap::crtc_refresh() -> Result<Option<(u64, u64)>>4- version 0.5.9, changelog, README rows
it is the rate the mode is programmed to: the kernel keeps the pixel clock in kHz, so a 1080p "59.94" mode at 148352 kHz reads 148352/2475 (59.94020 Hz), a few parts per million from the nominal 60000/1001.
tests:
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test_refresh(unit, no hardware): CEA-861 timings at 60, 59.94, 50, 29.97, 23.976 and 119.88, 1080i, doublescan, vscan, two real odd timings, empty modes, null arguments and the widest field values. removing interlace, doublescan, vscan, the reduction or the zero-timing refusal each turns it red2-
test_enumerate(integration): on every active display the fraction rounds to the enumerated vrefresh, and acrtc_id0 context picks a CRTC and keeps itmeasured on i915 (Sigma-26): 60/1 at 1920x1080@60, 6750000/112463 (60.0197 Hz) at 1280x1024, 131375/4382 (29.9806 Hz) at 3840x2160@30;
-ENODATAon a disabled pipe; the auto-selected CRTC on acrtc_id0 context. unit 11/11, integration 2/2, the rust crates build and test.