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

[Tomasz Figa]

Hi Anle,

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.

Thanks for the patch. Good catch.

> 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.
>
> Signed-off-by: Anle Pan <anle.pan@xxxxxxx>
> ---
>  drivers/media/common/videobuf2/videobuf2-dma-sg.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-sg.c b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> index fa69158a65b1..b608a7c5f240 100644
> --- a/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> +++ b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> @@ -105,6 +105,7 @@ static void *vb2_dma_sg_alloc(struct vb2_buffer *vb, struct device *dev,
>         struct sg_table *sgt;
>         int ret;
>         int num_pages;
> +       size_t max_segment = 0;
>
>         if (WARN_ON(!dev) || WARN_ON(!size))
>                 return ERR_PTR(-EINVAL);
> @@ -134,8 +135,12 @@ static void *vb2_dma_sg_alloc(struct vb2_buffer *vb, struct device *dev,
>         if (ret)
>                 goto fail_pages_alloc;
>
> -       ret = sg_alloc_table_from_pages(buf->dma_sgt, buf->pages,
> -                       buf->num_pages, 0, size, GFP_KERNEL);
> +       if (dev)
> +               max_segment = dma_max_mapping_size(dev);
> +       if (max_segment == 0)
> +               max_segment = UINT_MAX;
> +       ret = sg_alloc_table_from_pages_segment(buf->dma_sgt, buf->pages,
> +               buf->num_pages, 0, size, max_segment, GFP_KERNEL);

One thing that I'm not sure about here is that we use
sg_alloc_table_from_pages_segment(), but we actually don't pass the
max segment size (as returned by dma_get_max_seg_size()) to it.
I'm also not exactly sure what's the difference between "max mapping
size" and "max seg size".
+Robin Murphy +Christoph Hellwig I think we could benefit from your
expertise here.

Generally looking at videobuf2-dma-sg, I feel like we would benefit
from some kind of dma_alloc_table_from_pages() that simply takes the
struct dev pointer and does everything necessary.

Best regards,
Tomasz