Re: [RFC][PATCH 12/15] videobuf2: add begin/end cpu_access callbacks to dma-sg

[Hans Verkuil]

On 1/28/20 5:38 AM, Tomasz Figa wrote:
> On Fri, Jan 10, 2020 at 7:13 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>>
>> On 12/17/19 4:20 AM, Sergey Senozhatsky wrote:
>>> Provide begin_cpu_access() and end_cpu_access() dma_buf_ops
>>> callbacks for cache synchronisation on exported buffers.
>>>
>>> Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
>>> ---
>>>  .../media/common/videobuf2/videobuf2-dma-sg.c | 22 +++++++++++++++++++
>>>  1 file changed, 22 insertions(+)
>>>
>>> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-sg.c b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
>>> index 6db60e9d5183..bfc99a0cb7b9 100644
>>> --- a/drivers/media/common/videobuf2/videobuf2-dma-sg.c
>>> +++ b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
>>> @@ -470,6 +470,26 @@ static void vb2_dma_sg_dmabuf_ops_release(struct dma_buf *dbuf)
>>>       vb2_dma_sg_put(dbuf->priv);
>>>  }
>>>
>>
>> There is no corresponding vb2_sg_buffer_consistent function here.
>>
>> Looking more closely I see that vb2_dma_sg_alloc doesn't pass the dma_attrs
>> argument to dma_map_sg_attrs, thus V4L2_FLAG_MEMORY_NON_CONSISTENT has no
>> effect on dma-sg buffers.
> 
> videobuf2-dma-sg allocates the memory using the page allocator
> directly, which means that there is no memory consistency guarantee.
> 
>>
>> Is there a reason why dma_attrs isn't passed on to dma_map_sg_attrs()?
>>
> 
> V4L2_FLAG_MEMORY_NON_CONSISTENT is a flag for dma_alloc_attrs(). It
> isn't supposed to do anything for dma_map_sg_attrs(), which is only
> supposed to create the device (e.g. IOMMU) mapping for already
> allocated memory.

Ah, right.

But could vb2_dma_sg_alloc_compacted() be modified so that is uses
dma_alloc_attrs() instead of alloc_pages()? Sorry, that might be a stupid
question, I'm not an expert in this area. All I know is that I hate inconsistent
APIs where something works for one thing, but not another.

Regards,

	Hans

> 
>> I suspect it was just laziness in the past, and that it should be wired
>> up, just as for dma-contig.
>>
>> Regards,
>>
>>         Hans
>>
>>> +static int vb2_dma_sg_dmabuf_ops_begin_cpu_access(struct dma_buf *dbuf,
>>> +                                     enum dma_data_direction direction)
>>> +{
>>> +     struct vb2_dma_sg_buf *buf = dbuf->priv;
>>> +     struct sg_table *sgt = buf->dma_sgt;
>>> +
>>> +     dma_sync_sg_for_cpu(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir);
>>> +     return 0;
>>> +}
>>> +
>>> +static int vb2_dma_sg_dmabuf_ops_end_cpu_access(struct dma_buf *dbuf,
>>> +                                     enum dma_data_direction direction)
>>> +{
>>> +     struct vb2_dma_sg_buf *buf = dbuf->priv;
>>> +     struct sg_table *sgt = buf->dma_sgt;
>>> +
>>> +     dma_sync_sg_for_device(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir);
>>> +     return 0;
>>> +}
>>> +
>>>  static void *vb2_dma_sg_dmabuf_ops_vmap(struct dma_buf *dbuf)
>>>  {
>>>       struct vb2_dma_sg_buf *buf = dbuf->priv;
>>> @@ -488,6 +508,8 @@ static const struct dma_buf_ops vb2_dma_sg_dmabuf_ops = {
>>>       .detach = vb2_dma_sg_dmabuf_ops_detach,
>>>       .map_dma_buf = vb2_dma_sg_dmabuf_ops_map,
>>>       .unmap_dma_buf = vb2_dma_sg_dmabuf_ops_unmap,
>>> +     .begin_cpu_access = vb2_dma_sg_dmabuf_ops_begin_cpu_access,
>>> +     .end_cpu_access = vb2_dma_sg_dmabuf_ops_end_cpu_access,
>>>       .vmap = vb2_dma_sg_dmabuf_ops_vmap,
>>>       .mmap = vb2_dma_sg_dmabuf_ops_mmap,
>>>       .release = vb2_dma_sg_dmabuf_ops_release,
>>>
>>