Re: [PATCHv8 20/26] v4l: vb2-dma-contig: add support for DMABUF exporting

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Tomasz,

Thanks for the patch.

Just a couple of small comments below.

On Tuesday 14 August 2012 17:34:50 Tomasz Stanislawski wrote:
> This patch adds support for exporting a dma-contig buffer using
> DMABUF interface.
> 
> Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> ---
>  drivers/media/video/videobuf2-dma-contig.c |  204 +++++++++++++++++++++++++
>  1 file changed, 204 insertions(+)
> 
> diff --git a/drivers/media/video/videobuf2-dma-contig.c
> b/drivers/media/video/videobuf2-dma-contig.c index 7fc71a0..bb2b4ac8 100644
> --- a/drivers/media/video/videobuf2-dma-contig.c
> +++ b/drivers/media/video/videobuf2-dma-contig.c

[snip]

> +static struct sg_table *vb2_dc_dmabuf_ops_map(
> +	struct dma_buf_attachment *db_attach, enum dma_data_direction dir)
> +{
> +	struct vb2_dc_attachment *attach = db_attach->priv;
> +	/* stealing dmabuf mutex to serialize map/unmap operations */

Why isn't this operation serialized by the dma-buf core itself ?

> +	struct mutex *lock = &db_attach->dmabuf->lock;
> +	struct sg_table *sgt;
> +	int ret;
> +
> +	mutex_lock(lock);
> +
> +	sgt = &attach->sgt;
> +	/* return previously mapped sg table */
> +	if (attach->dir == dir) {
> +		mutex_unlock(lock);
> +		return sgt;
> +	}
> +
> +	/* release any previous cache */
> +	if (attach->dir != DMA_NONE) {
> +		dma_unmap_sg(db_attach->dev, sgt->sgl, sgt->orig_nents,
> +			attach->dir);
> +		attach->dir = DMA_NONE;
> +	}
> +
> +	/* mapping to the client with new direction */
> +	ret = dma_map_sg(db_attach->dev, sgt->sgl, sgt->orig_nents, dir);
> +	if (ret <= 0) {
> +		pr_err("failed to map scatterlist\n");
> +		mutex_unlock(lock);
> +		return ERR_PTR(-EIO);
> +	}
> +
> +	attach->dir = dir;
> +
> +	mutex_unlock(lock);
> +
> +	return sgt;
> +}

[snip]

> +static int vb2_dc_dmabuf_ops_mmap(struct dma_buf *dbuf,
> +	struct vm_area_struct *vma)
> +{
> +	/* Dummy support for mmap */
> +	return -ENOTTY;

What about calling the dma-contig mmap handler here ? Is there a specific 
reason why you haven't implemented mmap support for dmabuf ?

> +}
> +
> +static struct dma_buf_ops vb2_dc_dmabuf_ops = {
> +	.attach = vb2_dc_dmabuf_ops_attach,
> +	.detach = vb2_dc_dmabuf_ops_detach,
> +	.map_dma_buf = vb2_dc_dmabuf_ops_map,
> +	.unmap_dma_buf = vb2_dc_dmabuf_ops_unmap,
> +	.kmap = vb2_dc_dmabuf_ops_kmap,
> +	.kmap_atomic = vb2_dc_dmabuf_ops_kmap,
> +	.vmap = vb2_dc_dmabuf_ops_vmap,
> +	.mmap = vb2_dc_dmabuf_ops_mmap,
> +	.release = vb2_dc_dmabuf_ops_release,
> +};
> +
> +static struct sg_table *vb2_dc_get_base_sgt(struct vb2_dc_buf *buf)
> +{
> +	int ret;
> +	struct sg_table *sgt;
> +
> +	sgt = kmalloc(sizeof(*sgt), GFP_KERNEL);
> +	if (!sgt) {
> +		dev_err(buf->dev, "failed to alloc sg table\n");
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
> +	ret = dma_get_sgtable(buf->dev, sgt, buf->vaddr, buf->dma_addr,
> +		buf->size);
> +	if (ret < 0) {
> +		dev_err(buf->dev, "failed to get scatterlist from DMA API\n");
> +		kfree(sgt);
> +		return ERR_PTR(ret);
> +	}

As this function is only used below, where the exact value of the error code 
is ignored, what about just returning NULL on failure ? Another option is to 
return the error code in vb2_dc_get_dmabuf (not sure if that would be useful 
though).

> +
> +	return sgt;
> +}
> +
> +static struct dma_buf *vb2_dc_get_dmabuf(void *buf_priv)
> +{
> +	struct vb2_dc_buf *buf = buf_priv;
> +	struct dma_buf *dbuf;
> +	struct sg_table *sgt = buf->sgt_base;
> +
> +	if (!sgt)
> +		sgt = vb2_dc_get_base_sgt(buf);
> +	if (WARN_ON(IS_ERR(sgt)))
> +		return NULL;
> +
> +	/* cache base sgt for future use */
> +	buf->sgt_base = sgt;

You can move this assignment inside the first if, there's no need to execute 
it every time. The WARN_ON can also be moved inside the first if, as buf-
>sgt_base will either be NULL or valid. You can then get rid of the sgt 
variable initialization by testing if (!buf->sgt_base).

> +	dbuf = dma_buf_export(buf, &vb2_dc_dmabuf_ops, buf->size, 0);
> +	if (IS_ERR(dbuf))
> +		return NULL;
> +
> +	/* dmabuf keeps reference to vb2 buffer */
> +	atomic_inc(&buf->refcount);
> +
> +	return dbuf;
> +}

-- 
Regards,

Laurent Pinchart