linux-kernel - Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations

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Date:   Fri, 25 Jun 2021 12:10:30 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>
Cc:     Tomasz Figa <tfiga@...omium.org>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Christoph Hellwig <hch@....de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations

Hi Hans,

On (21/06/17 16:56), Sergey Senozhatsky wrote:
[..]
> static void *vb2_dc_vaddr(struct vb2_buffer *vb, void *buf_priv)
> {
>         struct vb2_dc_buf *buf = buf_priv;
> 
>         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;
> 
>                 return buf->vaddr;
>         }
> 
>         if (!buf->coherent_mem)
>                 buf->vaddr = dma_vmap_noncontiguous(buf->dev, buf->size,
>                                                     buf->dma_sgt);
>         return buf->vaddr;
> }
> 
> And in vb2_dc_alloc functions set vaddr for !DMA_ATTR_NO_KERNEL_MAPPING
> in both coherent and non-coherent. So that we probably can have less
> branches when ->vaddr is NULL for one type of allocations, and is not
> NULL for another.
> 
> static int vb2_dc_alloc_coherent(struct vb2_dc_buf *buf)
> {
>         struct vb2_queue *q = buf->vb->vb2_queue;
> 
>         buf->cookie = dma_alloc_attrs(buf->dev,
>                                       buf->size,
>                                       &buf->dma_addr,
>                                       GFP_KERNEL | q->gfp_flags,
>                                       buf->attrs);
>         if (!buf->cookie)
>                 return -ENOMEM;
> 
>         if (q->dma_attrs & DMA_ATTR_NO_KERNEL_MAPPING)
>                 return 0;
> 
>         buf->vaddr = buf->cookie;
>         return 0;
> }
> 
> static int vb2_dc_alloc_non_coherent(struct vb2_dc_buf *buf)
> {
>         struct vb2_queue *q = buf->vb->vb2_queue;
> 
>         buf->dma_sgt = dma_alloc_noncontiguous(buf->dev,
>                                                buf->size,
>                                                buf->dma_dir,
>                                                GFP_KERNEL | q->gfp_flags,
>                                                buf->attrs);
>         if (!buf->dma_sgt)
>                 return -ENOMEM;
> 
>         if (q->dma_attrs & DMA_ATTR_NO_KERNEL_MAPPING)
>                 return 0;
> 
>         buf->vaddr = dma_vmap_noncontiguous(buf->dev, buf->size, buf->dma_sgt);
>         if (!buf->vaddr) {
>                 dma_free_noncontiguous(buf->dev, buf->size,
>                                        buf->dma_sgt, buf->dma_addr);
>                 return -ENOMEM;
>         }
>         return 0;
> }

I guess this should address the case when

"after allocating the buffer, the buffer is exported as a dma_buf and
another device calls dma_buf_ops vb2_dc_dmabuf_ops_vmap, which in turn
calls dma_buf_map_set_vaddr(map, buf->vaddr); with a NULL buf->vaddr"

Because ->vaddr will not be NULL now after allocation for both coherent
and non-coherent buffers (modulo DMA_ATTR_NO_KERNEL_MAPPING requests).

What do you think?