[PATCH v4 1/2] media: videobuf2: Fix dmabuf cache sync/flush in dma-contig

[Nicolas Dufresne]

Hi Mikhail,

Le lundi 03 mars 2025 à 14:40 +0300, Mikhail Rudenko a écrit :
> When support for V4L2_FLAG_MEMORY_NON_CONSISTENT was removed in
> commit 129134e5415d ("media: media/v4l2: remove
> V4L2_FLAG_MEMORY_NON_CONSISTENT flag"),
> vb2_dc_dmabuf_ops_{begin,end}_cpu_access() functions were made
> no-ops. Later, when support for V4L2_MEMORY_FLAG_NON_COHERENT was
> introduced in commit c0acf9cfeee0 ("media: videobuf2: handle
> V4L2_MEMORY_FLAG_NON_COHERENT flag"), the above functions remained
> no-ops, making cache maintenance for non-coherent dmabufs allocated
> by
> dma-contig impossible.
> 
> Fix this by reintroducing dma_sync_sgtable_for_{cpu,device} and
> {flush,invalidate}_kernel_vmap_range calls to
> vb2_dc_dmabuf_ops_{begin,end}_cpu_access() functions for non-coherent
> buffers.
> 
> Fixes: c0acf9cfeee0 ("media: videobuf2: handle
> V4L2_MEMORY_FLAG_NON_COHERENT flag")
> Cc: stable at vger.kernel.org
> Signed-off-by: Mikhail Rudenko <mike.rudenko at gmail.com>
> ---
>  .../media/common/videobuf2/videobuf2-dma-contig.c  | 22
> ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> index
> a13ec569c82f6da2d977222b94af32e74c6c6c82..d41095fe5bd21faf815d6b035d7
> bc888a84a95d5 100644
> --- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> +++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> @@ -427,6 +427,17 @@ static int
>  vb2_dc_dmabuf_ops_begin_cpu_access(struct dma_buf *dbuf,
>  				   enum dma_data_direction
> direction)
>  {
> +	struct vb2_dc_buf *buf = dbuf->priv;
> +	struct sg_table *sgt = buf->dma_sgt;
> +
> +	if (!buf->non_coherent_mem)
> +		return 0;
> +
> +	if (buf->vaddr)
> +		invalidate_kernel_vmap_range(buf->vaddr, buf->size);

What would make me a lot more confortable with this change is if you
enable kernel mappings for one test. This will ensure you cover the
call to "invalidate" in your testing. I'd like to know about the
performance impact. With this implementation it should be identical to
the VB2 one.

What I was trying to say in previous comments, is that my impression is
that we can skip this for CPU read access, since we don't guaranty
concurrent access anyway. Both address space can keep their cache in
that case. Though, I see RKISP does not use kernel mapping plus I'm not
reporting a bug, but checking if we should leave a comment for possible
users of kernel mapping in the future ?

> +
> +	dma_sync_sgtable_for_cpu(buf->dev, sgt, direction);
> +
>  	return 0;
>  }
>  
> @@ -434,6 +445,17 @@ static int
>  vb2_dc_dmabuf_ops_end_cpu_access(struct dma_buf *dbuf,
>  				 enum dma_data_direction direction)
>  {
> +	struct vb2_dc_buf *buf = dbuf->priv;
> +	struct sg_table *sgt = buf->dma_sgt;
> +
> +	if (!buf->non_coherent_mem)
> +		return 0;
> +
> +	if (buf->vaddr)
> +		flush_kernel_vmap_range(buf->vaddr, buf->size);
> +
> +	dma_sync_sgtable_for_device(buf->dev, sgt, direction);
> +
>  	return 0;
>  }
>  
>