On 2026-07-10 23:31, Alex Hung wrote:
> [WHY]
> The out fence was signalled on the first vblank after arming, before the
> DMA finished copying, and the old code worked around this with an
> mdelay() in the IRQ handler.
>
> [HOW]
> Hold a vblank reference while writeback is pending and signal the out
> fence on the second vblank instead of using mdelay(). Add
> amdgpu_dm_crtc_complete_writeback() to finish and clean up writeback
> from both the IRQ and teardown paths.
>
> This can be verified by running IGT's kms_writeback 20 times without
> timeout errors.
>
> Assisted-by: Copilot:Claude-Opus-4.8
> Signed-off-by: Alex Hung <[email protected]>
Reviewed-by: Harry Wentland <[email protected]>
Harry
> ---
> drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h | 1 +
> .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 2 ++
> .../drm/amd/display/amdgpu_dm/amdgpu_dm_irq.c | 33 ++++++++++---------
> 3 files changed, 21 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h
> b/drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h
> index 8069fc41cc7f..7c784277396a 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_mode.h
> @@ -509,6 +509,7 @@ struct amdgpu_crtc {
> struct drm_pending_vblank_event *event;
>
> bool wb_pending;
> + bool wb_frame_done;
> bool wb_enabled;
> struct drm_writeback_connector *wb_conn;
> };
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> index d67dcaa3fa8f..0f5453649200 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -4521,6 +4521,7 @@ bool amdgpu_dm_crtc_complete_writeback(struct
> amdgpu_crtc *acrtc)
> spin_lock_irqsave(&acrtc->wb_conn->job_lock, flags);
> pending = acrtc->wb_pending;
> acrtc->wb_pending = false;
> + acrtc->wb_frame_done = false;
> spin_unlock_irqrestore(&acrtc->wb_conn->job_lock, flags);
>
> if (!pending)
> @@ -4988,6 +4989,7 @@ static void dm_set_writeback(struct
> amdgpu_display_manager *dm,
> * cannot run its matching vblank_put before this get.
> */
> WARN_ON(drm_crtc_vblank_get(&acrtc->base));
> + acrtc->wb_frame_done = false;
> acrtc->wb_pending = true;
> }
>
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_irq.c
> b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_irq.c
> index c5467f34c51f..4de7fb264cb2 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_irq.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_irq.c
> @@ -1974,23 +1974,26 @@ static void dm_crtc_high_irq(void *interrupt_params)
> return;
>
> if (acrtc->wb_conn && acrtc->wb_pending) {
> - struct dc_stream_state *stream = acrtc->dm_irq_params.stream;
> - unsigned int v_total, refresh_hz;
> -
> - v_total = stream->adjust.v_total_max ?
> - stream->adjust.v_total_max : stream->timing.v_total;
> - refresh_hz = div_u64((uint64_t) stream->timing.pix_clk_100hz *
> - 100LL, (v_total * stream->timing.h_total));
> - mdelay(1000 / refresh_hz);
> -
> - /*
> - * Completion (signalling the out fence and releasing the vblank
> - * reference taken in dm_set_writeback()) is handled by the
> shared
> - * helper, which is also used by the teardown path.
> - */
> - if (amdgpu_dm_crtc_complete_writeback(acrtc))
> + if (acrtc->wb_frame_done) {
> + /*
> + * Second vblank: the DMA for the captured frame has
> + * had a full frame period to flush to memory. Signal
> + * the out fence now.
> + */
> + amdgpu_dm_crtc_complete_writeback(acrtc);
> + } else {
> + /*
> + * First vblank after arming: the frame has been
> + * scanned out and the DMA is finishing. Disable
> + * writeback immediately to prevent the hardware from
> + * starting a new capture that would overwrite the
> + * buffer. Signal completion on the next vblank to
> + * ensure the DMA is fully flushed to memory.
> + */
> dc_stream_fc_disable_writeback(adev->dm.dc,
>
> acrtc->dm_irq_params.stream, 0);
> + acrtc->wb_frame_done = true;
> + }
> }
>
> vrr_active = amdgpu_dm_crtc_vrr_active_irq(acrtc);
> --
> 2.43.0
>
