linux-kernel - Re: [PATCHv4 8/8] videobuf2: handle non-contiguous DMA allocations

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Message-ID: <YSN4ms1VoJO2CWNp@google.com>
Date:   Mon, 23 Aug 2021 19:29:46 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Dafna Hirschfeld <dafna.hirschfeld@...labora.com>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Christoph Hellwig <hch@....de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 8/8] videobuf2: handle non-contiguous DMA allocations

On (21/08/03 12:15), Hans Verkuil wrote:
> >  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;
> > +
> > +		return buf->vaddr;
> >  	}
> >  
> > +	if (!buf->coherent_mem)
> > +		buf->vaddr = dma_vmap_noncontiguous(buf->dev, buf->size,
> > +						    buf->dma_sgt);
> >  	return buf->vaddr;
> >  }
> 
> This function really needs a bunch of comments.
> 
> What I want to see here specifically is under which circumstances this function
> can return NULL.
> 
> - dma_buf_vmap returns an error
> - for non-coherent memory dma_vmap_noncontiguous returns an error
> - coherent memory with DMA_ATTR_NO_KERNEL_MAPPING set.

Done.