Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size

[Tomasz Figa]

On Wed, Aug 30, 2023 at 5:50 PM Hui Fang <hui.fang@xxxxxxx> wrote:
>
> On Mon, Aug 28, 2023 at 8:57 AM Anle Pan <anle.pan@xxxxxxx> wrote:
> >
> > When allocating from pages, the size of the sg segment is unlimited and the
> > default is UINT_MAX. This will cause the DMA stream mapping failed later
> > with a "swiotlb buffer full" error. The default maximum mapping size is 128
> > slots x 2K = 256K, determined by "IO_TLB_SEGSIZE".
> > To fix the issue, limit the sg segment size according to
> > "dma_max_mapping_size" to match the mapping limit.
>
> I wonder if only NXP met the "swiotlb buffer full" issue. In theory,
> when format is YUYV, those resolutions no greater than 320x240 (153600 bytes,
> which less than the max mapping size 256K ) can't meet the issue.
> But resolutions no less than 640x480 (307200 bytes), may have chances to
> trigger the issue.

I think a combination of a device that can support scatter-gather, but
requires swiotlb is kind of rare. Most drivers would require a single
contiguous DMA address (and thus use videobuf2-dma-contig) and
physical discontinuity would be handled by an IOMMU (transparently to
vb2).

I guess one thing to ask you about your specific setup is whether it's
expected that the swiotlb ends up being used at all?

Best regards,
Tomasz