Re: [RFCv2 PATCH 3/9] v4l: vb2: Add dma-contig allocator as dma_buf user

[Tomasz Stanislawski <t.stanislaws@samsung.com>]

Hi Laurent,
Please refer to the comments below.

On 03/22/2012 12:04 PM, Laurent Pinchart wrote:
> Hi Tomasz,
> 
> On Tuesday 13 March 2012 11:17:01 Tomasz Stanislawski wrote:
>> From: Sumit Semwal <sumit.semwal@ti.com>
>>
>> This patch makes changes for adding dma-contig as a dma_buf user. It
>> provides function implementations for the {attach, detach, map,
>> unmap}_dmabuf() mem_ops of DMABUF memory type.
>>
>> Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
>> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
>> 	[author of the original patch]
>> Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
>> 	[integration with refactored dma-contig allocator]
>> ---
>>  drivers/media/video/videobuf2-dma-contig.c |  116
>> ++++++++++++++++++++++++++++ 1 files changed, 116 insertions(+), 0
>> deletions(-)
>>
>> diff --git a/drivers/media/video/videobuf2-dma-contig.c
>> b/drivers/media/video/videobuf2-dma-contig.c index c1dc043..746dd5f 100644
>> --- a/drivers/media/video/videobuf2-dma-contig.c
>> +++ b/drivers/media/video/videobuf2-dma-contig.c
>> @@ -35,6 +35,9 @@ struct vb2_dc_buf {
>>
>>  	/* USERPTR related */
>>  	struct vm_area_struct		*vma;
>> +
>> +	/* DMABUF related */
>> +	struct dma_buf_attachment	*db_attach;
>>  };
>>
>>  /*********************************************/
>> @@ -485,6 +488,115 @@ fail_buf:
>>  }
>>
>>  /*********************************************/
>> +/*       callbacks for DMABUF buffers        */
>> +/*********************************************/
>> +
>> +static int vb2_dc_map_dmabuf(void *mem_priv)
>> +{
>> +	struct vb2_dc_buf *buf = mem_priv;
>> +	struct sg_table *sgt;
>> +	unsigned long contig_size;
>> +	int ret = 0;
>> +
>> +	if (WARN_ON(!buf->db_attach)) {
>> +		printk(KERN_ERR "trying to pin a non attached buffer\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (WARN_ON(buf->dma_sgt)) {
>> +		printk(KERN_ERR "dmabuf buffer is already pinned\n");
>> +		return 0;
>> +	}
>> +
>> +	/* get the associated scatterlist for this buffer */
>> +	sgt = dma_buf_map_attachment(buf->db_attach, buf->dma_dir);
>> +	if (IS_ERR_OR_NULL(sgt)) {
>> +		printk(KERN_ERR "Error getting dmabuf scatterlist\n");
>> +		return -EINVAL;
>> +	}
> 
> I've checked why dma_map_sg() was missing from here, and then remembered that 
> the exporter is still responsible for mapping the buffer to the importer's 
> device address space :-) I'll raise that topic separately.
> 
>> +
>> +	/* checking if dmabuf is big enough to store contiguous chunk */
>> +	contig_size = vb2_dc_get_contiguous_size(sgt);
>> +	if (contig_size < buf->size) {
>> +		printk(KERN_ERR "contiguous chunk of dmabuf is too small\n");
>> +		ret = -EFAULT;
>> +		goto fail_map;
>> +	}
>> +
>> +	buf->dma_addr = sg_dma_address(sgt->sgl);
>> +	buf->dma_sgt = sgt;
>> +
>> +	return 0;
>> +
>> +fail_map:
>> +	dma_buf_unmap_attachment(buf->db_attach, sgt);
>> +
>> +	return ret;
>> +}
>> +
>> +static void vb2_dc_unmap_dmabuf(void *mem_priv)
>> +{
>> +	struct vb2_dc_buf *buf = mem_priv;
>> +	struct sg_table *sgt = buf->dma_sgt;
>> +
>> +	if (WARN_ON(!buf->db_attach)) {
>> +		printk(KERN_ERR "trying to unpin a not attached buffer\n");
>> +		return;
>> +	}
>> +
>> +	if (WARN_ON(!sgt)) {
>> +		printk(KERN_ERR "dmabuf buffer is already unpinned\n");
>> +		return;
>> +	}
>> +
>> +	dma_buf_unmap_attachment(buf->db_attach, sgt);
>> +
>> +	buf->dma_addr = 0;
>> +	buf->dma_sgt = NULL;
>> +}
>> +
>> +static void vb2_dc_detach_dmabuf(void *mem_priv)
>> +{
>> +	struct vb2_dc_buf *buf = mem_priv;
>> +
>> +	if (buf->dma_addr)
>> +		vb2_dc_unmap_dmabuf(buf);
> 
> Can detach() be called with the buffer still mapped() ? Wouldn't that be a bug 
> in the caller ?
> 

No, it is not. The functions from vb2_dc_*_dmabuf are called by vb2-core.
It is not a part of DMABUF API itself. Therefore usage can be a bit different.
Please refer to the function __qbuf_dmabuf function vb2-core. If new DMABUF
is passed to in the QBUF then old DMABUF is released by calling detach_dmabuf
without unmap. However, this part of code is probably not reachable if the
buffer was not dequeued.

Detach without unmap may happen in a case of closing a video fd
without prior dequeuing of all buffers.

Do you think it would be a good idea to move a call map_dmabuf to
__enqueue_in_driver and add calling unmap_dmabuf to vb2_buffer_done?

>> +
>> +	/* detach this attachment */
>> +	dma_buf_detach(buf->db_attach->dmabuf, buf->db_attach);
>> +	kfree(buf);
>> +}
> 
> There's nothing allocator-specific in the attach/detach operations. Shouldn't 
> they be moved to the vb2 core ?
> 

Calling dma_buf_attach could be moved to vb2-core. But it gives little gain.
First, the pointer of dma_buf_attachment would have to be added to struct vb2_plane.
Second, the allocator would have to keep in the copy of this pointer in its
context structure for use of vb2_dc_(un)map_dmabuf functions.
Third, dma_buf_attach requires a pointer to 'struct device' which is not
available in the vb2-core layer.

Because of the mentioned reasons I decided to keep attach_dmabuf in
allocator-specific code.

>> +
>> +static void *vb2_dc_attach_dmabuf(void *alloc_ctx, struct dma_buf *dbuf,
>> +	unsigned long size, int write)
>> +{
>> +	struct vb2_dc_buf *buf;
>> +	struct dma_buf_attachment *dba;
>> +
>> +	if (dbuf->size < size)
>> +		return ERR_PTR(-EFAULT);
>> +
>> +	buf = kzalloc(sizeof *buf, GFP_KERNEL);
>> +	if (!buf)
>> +		return ERR_PTR(-ENOMEM);
>> +
>> +	buf->dev = alloc_ctx;
>> +	/* create attachment for the dmabuf with the user device */
>> +	dba = dma_buf_attach(dbuf, buf->dev);
>> +	if (IS_ERR(dba)) {
>> +		printk(KERN_ERR "failed to attach dmabuf\n");
>> +		kfree(buf);
>> +		return dba;
>> +	}
>> +
>> +	buf->dma_dir = write ? DMA_FROM_DEVICE : DMA_TO_DEVICE;
>> +	buf->size = size;
>> +	buf->db_attach = dba;
>> +
>> +	return buf;
>> +}
>> +
>> +/*********************************************/
>>  /*       DMA CONTIG exported functions       */
>>  /*********************************************/
>>
>> @@ -498,6 +610,10 @@ const struct vb2_mem_ops vb2_dma_contig_memops = {
>>  	.put_userptr	= vb2_dc_put_userptr,
>>  	.prepare	= vb2_dc_prepare,
>>  	.finish		= vb2_dc_finish,
>> +	.map_dmabuf	= vb2_dc_map_dmabuf,
>> +	.unmap_dmabuf	= vb2_dc_unmap_dmabuf,
>> +	.attach_dmabuf	= vb2_dc_attach_dmabuf,
>> +	.detach_dmabuf	= vb2_dc_detach_dmabuf,
>>  	.num_users	= vb2_dc_num_users,
>>  };
>>  EXPORT_SYMBOL_GPL(vb2_dma_contig_memops);
> 

Best regards,
Tomasz Stanislawski