On 02/17/16 09:01, tiffany lin wrote: > On Tue, 2016-02-16 at 14:48 +0100, Hans Verkuil wrote: >> 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? >> > Yes, We have iommu that could make scatter-gather list looks like > contiguous memory. > >> 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). >> > I see. We saw this error before we add iommu support. > >> 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). >> > Got it. We will keep VB2_USERPTR.
Can you add a comment here mentioning that VB2_USERPTR works with dma-contig because there is an iommu? That should clarify this. Regards, Hans