[PATCH RFC 2/3] media: hantro: attach dma_resv release fence at buf_queue

[Markus Fritsche <mfritsche@reauktion.de>]

Opt the hantro driver into the new vb2 release-fence helper.

When userspace QBUFs a buffer to hantro, the buffer is added to the
driver's m2m queue via v4l2_m2m_buf_queue. We additionally call
vb2_buffer_attach_release_fence() so each plane's dmabuf->resv
gets a real producer fence attached. The fence is signalled by
vb2_buffer_done when hantro completes the decode (via
v4l2_m2m_buf_done_and_job_finish in hantro_drv.c, which converges
on vb2_buffer_done).

Wayland compositors (and any other userspace) that import hantro
CAPTURE buffers and wait on the dmabuf's implicit-sync fence now
wait on a real fence representing the producer's actual completion,
not a stub. Validated end-to-end on PineTab2 (RK3566 / Mali-G52 /
mainline 6.19 with this series backported) playing 1080p30 H.264 in
chromium under stock KDE Plasma 6.6.4 Wayland: KWin's
Transaction::watchDmaBuf wait completes correctly the moment
hantro's IRQ fires, instead of falling back to a stub-resolved
poll.

Signed-off-by: Markus Fritsche <mfritsche@reauktion.de>
---
 drivers/media/platform/verisilicon/hantro_v4l2.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/media/platform/verisilicon/hantro_v4l2.c b/drivers/media/platform/verisilicon/hantro_v4l2.c
index 62d3962c1..e95a3433a 100644
--- a/drivers/media/platform/verisilicon/hantro_v4l2.c
+++ b/drivers/media/platform/verisilicon/hantro_v4l2.c
@@ -877,6 +877,18 @@ static void hantro_buf_queue(struct vb2_buffer *vb)
 	}
 
 	v4l2_m2m_buf_queue(ctx->fh.m2m_ctx, vbuf);
+
+	/*
+	 * Opt in to vb2's dma_resv release-fence path. Userspace
+	 * consumers that imported this buffer's dmabuf and wait on
+	 * its implicit-sync fence (poll(POLLIN) or
+	 * DMA_BUF_IOCTL_EXPORT_SYNC_FILE) get a real producer fence
+	 * representing this device's completion, instead of the stub
+	 * fence dma_buf_export_sync_file substitutes when dma_resv
+	 * is empty. Best-effort: a fence-allocation failure means we
+	 * lose implicit-sync precision, no functional regression.
+	 */
+	(void)vb2_buffer_attach_release_fence(vb);
 }
 
 static bool hantro_vq_is_coded(struct vb2_queue *q)
-- 
2.47.3