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Date:   Thu, 17 Jun 2021 16:56:17 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Tomasz Figa <tfiga@chromium.org>,
        Ricardo Ribalda <ribalda@chromium.org>,
        Christoph Hellwig <hch@lst.de>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations
Message-ID: <YMsAIVs7G2hUDR2F@google.com>
References: <20210427131344.139443-1-senozhatsky@chromium.org>
 <20210427131344.139443-9-senozhatsky@chromium.org>
 <10a0903a-e295-5cba-683a-1eb89a0804ed@xs4all.nl>
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On (21/06/03 13:59), 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;
> >  	}
> >  
> > +	/* Non-coherent memory */
> > +	buf->vaddr = dma_vmap_noncontiguous(buf->dev, buf->size, buf->dma_sgt);
> > +
> 
> This function can use some comments. What is happening AFAICS is that
> buf->vaddr is either set in vb2_dc_alloc_coherent (unless
> DMA_ATTR_NO_KERNEL_MAPPING was set), it is obtained through dma_buf_vmap()
> if the buffer was attached to a dma_buf, or it is allocated via
> dma_vmap_noncontiguous() for non-coherent memory.

Yeah, it's complicated. Maybe we can make things more symmetrical.

> But this leaves coherent memory with DMA_ATTR_NO_KERNEL_MAPPING set, what
> is vaddr in that case? I think it will call dma_vmap_noncontiguous()
> incorrectly in that case, shouldn't there be a check for !buf->coherent_mem
> before the call to dma_vmap_noncontiguous()?

Thanks a lot for looking into it.

So vb2_dc_vaddr() can look like this:

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;
}