linux-kernel - Re: [PATCH 8/8] videobuf2: handle non-contiguous DMA allocations

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Date:   Wed, 24 Mar 2021 21:07:26 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        Christoph Hellwig <hch@....de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] videobuf2: handle non-contiguous DMA allocations

On Tue, Mar 02, 2021 at 09:46:24AM +0900, Sergey Senozhatsky wrote:
> This adds support for new noncontiguous DMA API, which
> requires allocators to have two execution branches: one
> for the current API, and one for the new one.
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> [hch: untested conversion to the ne API]
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  .../common/videobuf2/videobuf2-dma-contig.c   | 141 +++++++++++++++---
>  1 file changed, 117 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> index 1e218bc440c6..d6a9f7b682f3 100644
> --- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> +++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> @@ -17,6 +17,7 @@
>  #include <linux/sched.h>
>  #include <linux/slab.h>
>  #include <linux/dma-mapping.h>
> +#include <linux/highmem.h>
>  
>  #include <media/videobuf2-v4l2.h>
>  #include <media/videobuf2-dma-contig.h>
> @@ -42,8 +43,14 @@ struct vb2_dc_buf {
>  	struct dma_buf_attachment	*db_attach;
>  
>  	struct vb2_buffer		*vb;
> +	unsigned int			non_coherent_mem:1;
>  };
>  
> +static bool vb2_dc_is_coherent(struct vb2_dc_buf *buf)
> +{
> +	return !buf->non_coherent_mem;
> +}

nit: Given that this is just a simple negated return, do we need a dedicated
helper for it?

> +
>  /*********************************************/
>  /*        scatterlist table functions        */
>  /*********************************************/
> @@ -78,12 +85,21 @@ static void *vb2_dc_cookie(struct vb2_buffer *vb, void *buf_priv)
>  static void *vb2_dc_vaddr(struct vb2_buffer *vb, void *buf_priv)
>  {
>  	struct vb2_dc_buf *buf = buf_priv;
> -	struct dma_buf_map map;
> -	int ret;
>  
> -	if (!buf->vaddr && buf->db_attach) {
> -		ret = dma_buf_vmap(buf->db_attach->dmabuf, &map);
> -		buf->vaddr = ret ? NULL : map.vaddr;
> +	if (buf->vaddr)
> +		return buf->vaddr;
> +
> +	if (buf->db_attach) {
> +		struct dma_buf_map map;
> +
> +		if (!dma_buf_vmap(buf->db_attach->dmabuf, &map))
> +			buf->vaddr = map.vaddr;
> +	}
> +
> +	if (!vb2_dc_is_coherent(buf)) {

I believe it's not possible for both buf->db_attach and
!vb2_dc_is_coherent() to be true, but nevertheless the code can be
misleading for the reader. Would it make sense to just return early in the
if that handles db_attach?

> +		buf->vaddr = dma_vmap_noncontiguous(buf->dev,
> +						    buf->size,
> +						    buf->dma_sgt);
>  	}
>  
>  	return buf->vaddr;
> @@ -101,13 +117,26 @@ static void vb2_dc_prepare(void *buf_priv)
>  	struct vb2_dc_buf *buf = buf_priv;
>  	struct sg_table *sgt = buf->dma_sgt;
>  
> +	/* This takes care of DMABUF and user-enforced cache sync hint */
>  	if (buf->vb->skip_cache_sync_on_prepare)
>  		return;
>  
> +	/*
> +	 * Coherent MMAP buffers do not need to be synced, unlike coherent
> +	 * USERPTR and non-coherent MMAP buffers.

USERPTR buffers are always considered non-coherent.

> +	 */
> +	if (buf->vb->memory == V4L2_MEMORY_MMAP && vb2_dc_is_coherent(buf))
> +		return;
> +
>  	if (!sgt)
>  		return;
>  
> +	/* For both USERPTR and non-coherent MMAP */
>  	dma_sync_sgtable_for_device(buf->dev, sgt, buf->dma_dir);
> +
> +	/* Non-coherrent MMAP only */
> +	if (!vb2_dc_is_coherent(buf) && buf->vaddr)
> +		flush_kernel_vmap_range(buf->vaddr, buf->size);
>  }
>  
>  static void vb2_dc_finish(void *buf_priv)
> @@ -115,19 +144,46 @@ static void vb2_dc_finish(void *buf_priv)
>  	struct vb2_dc_buf *buf = buf_priv;
>  	struct sg_table *sgt = buf->dma_sgt;
>  
> +	/* This takes care of DMABUF and user-enforced cache sync hint */
>  	if (buf->vb->skip_cache_sync_on_finish)
>  		return;
>  
> +	/*
> +	 * Coherent MMAP buffers do not need to be synced, unlike coherent
> +	 * USERPTR and non-coherent MMAP buffers.

Ditto.

> +	 */
> +	if (buf->vb->memory == V4L2_MEMORY_MMAP && vb2_dc_is_coherent(buf))
> +		return;
> +
>  	if (!sgt)
>  		return;
>  
> +	/* For both USERPTR and non-coherent MMAP */
>  	dma_sync_sgtable_for_cpu(buf->dev, sgt, buf->dma_dir);
> +
> +	/* Non-coherrent MMAP only */
> +	if (!vb2_dc_is_coherent(buf) && buf->vaddr)
> +		invalidate_kernel_vmap_range(buf->vaddr, buf->size);
>  }
>  
>  /*********************************************/
>  /*        callbacks for MMAP buffers         */
>  /*********************************************/
>  
> +static void __vb2_dc_put(struct vb2_dc_buf *buf)
> +{
> +	if (vb2_dc_is_coherent(buf)) {
> +		dma_free_attrs(buf->dev, buf->size, buf->cookie,
> +			       buf->dma_addr, buf->attrs);
> +		return;
> +	}
> +
> +	if (buf->vaddr)
> +		dma_vunmap_noncontiguous(buf->dev, buf->vaddr);
> +	dma_free_noncontiguous(buf->dev, buf->size,
> +			       buf->dma_sgt, buf->dma_addr);
> +}
> +
>  static void vb2_dc_put(void *buf_priv)

nit: Unrelated to this patch or series, but could be a follow-up clean-up:
Could we rename this and the newly added function to include mmap in the
name, since it's only for MMAP buffers?

Best regards,
Tomasz