[PATCH 06/10] staging: media: cedrus: mpeg2: fix the references of field pictures

[Jernej Skrabec <jernej.skrabec@gmail.com>]

The second field of an I or P frame can be predicted from the first
field of the same frame, which the hardware takes from the backward
reference. The driver programs the backward reference passed by
userspace instead, which doesn't point to the frame being decoded, so
the second field is predicted from the wrong picture.

Use the current frame as the backward reference of I and P pictures,
like the vendor library does. Also fall back to the current frame
instead of address 0 when a reference buffer is missing, so the
hardware never reads from an invalid address.

Fixes: 50e761516f2b ("media: platform: Add Cedrus VPU decoder driver")
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
---
 .../staging/media/sunxi/cedrus/cedrus_mpeg2.c | 29 ++++++++++++++-----
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/drivers/staging/media/sunxi/cedrus/cedrus_mpeg2.c b/drivers/staging/media/sunxi/cedrus/cedrus_mpeg2.c
index 4c8accee5389..0a2f8e77fb29 100644
--- a/drivers/staging/media/sunxi/cedrus/cedrus_mpeg2.c
+++ b/drivers/staging/media/sunxi/cedrus/cedrus_mpeg2.c
@@ -56,6 +56,7 @@ static int cedrus_mpeg2_setup(struct cedrus_ctx *ctx, struct cedrus_run *run)
 	dma_addr_t src_buf_addr, dst_luma_addr, dst_chroma_addr;
 	struct cedrus_dev *dev = ctx->dev;
 	struct vb2_queue *vq;
+	struct vb2_buffer *fwd, *bwd;
 	const u8 *matrix;
 	unsigned int i;
 	bool field, tff;
@@ -147,15 +148,29 @@ static int cedrus_mpeg2_setup(struct cedrus_ctx *ctx, struct cedrus_run *run)
 
 	cedrus_write(dev, VE_DEC_MPEG_PICBOUNDSIZE, reg);
 
-	/* Forward and backward prediction reference buffers. */
+	/*
+	 * Forward and backward prediction reference buffers. As the vendor
+	 * library does, I and P pictures use the current frame as backward
+	 * reference: the second field of a frame predicts from the first one.
+	 * Missing references also fall back to the current frame.
+	 */
 	vq = v4l2_m2m_get_vq(ctx->fh.m2m_ctx, V4L2_BUF_TYPE_VIDEO_CAPTURE);
 
-	cedrus_write_ref_buf_addr(ctx, vq, pic->forward_ref_ts,
-				  VE_DEC_MPEG_FWD_REF_LUMA_ADDR,
-				  VE_DEC_MPEG_FWD_REF_CHROMA_ADDR);
-	cedrus_write_ref_buf_addr(ctx, vq, pic->backward_ref_ts,
-				  VE_DEC_MPEG_BWD_REF_LUMA_ADDR,
-				  VE_DEC_MPEG_BWD_REF_CHROMA_ADDR);
+	fwd = vb2_find_buffer(vq, pic->forward_ref_ts) ?: &run->dst->vb2_buf;
+	if (pic->picture_coding_type == V4L2_MPEG2_PIC_CODING_TYPE_B)
+		bwd = vb2_find_buffer(vq, pic->backward_ref_ts) ?:
+		      &run->dst->vb2_buf;
+	else
+		bwd = &run->dst->vb2_buf;
+
+	cedrus_write(dev, VE_DEC_MPEG_FWD_REF_LUMA_ADDR,
+		     cedrus_dst_buf_addr(ctx, fwd, 0));
+	cedrus_write(dev, VE_DEC_MPEG_FWD_REF_CHROMA_ADDR,
+		     cedrus_dst_buf_addr(ctx, fwd, 1));
+	cedrus_write(dev, VE_DEC_MPEG_BWD_REF_LUMA_ADDR,
+		     cedrus_dst_buf_addr(ctx, bwd, 0));
+	cedrus_write(dev, VE_DEC_MPEG_BWD_REF_CHROMA_ADDR,
+		     cedrus_dst_buf_addr(ctx, bwd, 1));
 
 	/* Destination luma and chroma buffers. */
 
-- 
2.43.0