Re: [PATCH v4 5/8] [Media] vcodec: mediatek: Add Mediatek V4L2 Video Encoder Driver

[Hans Verkuil]

Hi Tiffany,

>>>>> +int mtk_vcodec_enc_queue_init(void *priv, struct vb2_queue *src_vq,
>>>>> +			   struct vb2_queue *dst_vq)
>>>>> +{
>>>>> +	struct mtk_vcodec_ctx *ctx = priv;
>>>>> +	int ret;
>>>>> +
>>>>> +	src_vq->type		= V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE;
>>>>> +	src_vq->io_modes	= VB2_DMABUF | VB2_MMAP | VB2_USERPTR;
>>>>
>>>> I recomment dropping VB2_USERPTR. That only makes sense for scatter-gather dma,
>>>> and you use physically contiguous DMA.
>>>>
>>> Now our userspace app use VB2_USERPTR. I need to check if we could drop
>>> VB2_USERPTR.
>>> We use src_vq->mem_ops = &vb2_dma_contig_memops;
>>> And there are
>>> 	.get_userptr	= vb2_dc_get_userptr,
>>> 	.put_userptr	= vb2_dc_put_userptr,
>>> I was confused why it only make sense for scatter-gather.
>>> Could you kindly explain more?
>>
>> VB2_USERPTR indicates that the application can use malloc to allocate buffers
>> and pass those to the driver. Since malloc uses virtual memory the physical
>> memory is scattered all over. And the first page typically does not start at
>> the beginning of the page but at a random offset.
>>
>> To support that the DMA generally has to be able to do scatter-gather.
>>
>> Now, where things get ugly is that a hack was added to the USERPTR support where
>> apps could pass a pointer to physically contiguous memory as a user pointer. This
>> was a hack for embedded systems that preallocated a pool of buffers and needed to
>> pass those pointers around somehow. So the dma-contig USERPTR support is for that
>> 'feature'. If you try to pass a malloc()ed buffer to a dma-contig driver it will
>> reject it. One big problem is that this specific hack isn't signaled anywhere, so
>> applications have no way of knowing if the USERPTR support is the proper version
>> or the hack where physically contiguous memory is expected.
>>
>> This hack has been replaced with DMABUF which is the proper way of passing buffers
>> around.
>>
>> New dma-contig drivers should not use that old hack anymore. Use dmabuf to pass
>> external buffers around.
>>
>> How do you use it in your app? With malloc()ed buffers? Or with 'special' pointers
>> to physically contiguous buffers?
>>
> Understood now. Thanks for your explanation.
> Now our app use malloc()ed buffers and we hook vb2_dma_contig_memops. 
> I don't know why that dma-contig driver do not reject it.
> I will try to figure it out.

Is there an iommu involved that turns the scatter-gather list into what looks like
contiguous memory for the DMA?

At the end of vb2_dc_get_userptr() in videobuf2-dma-contig.c there is a check
'if (contig_size < size)' that verifies that the sg DMA is contiguous. This would
work if there is an iommu involved (if I understand it correctly).

If that's the case, then it's OK to keep VB2_USERPTR because you have real sg
support (although not via the DMA engine, but via iommu mappings).

Regards,

	Hans