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

[Robin Murphy]

On 2023-08-29 11:03, Tomasz Figa wrote:
> 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)

dev can't be NULL, see the context above.

> > +               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.

dma_get_max_seg_size() represents a capability of the device itself, 
namely the largest contiguous range it can be programmed to access in a 
single DMA descriptor/register/whatever. Conversely, 
dma_max_mapping_size() is a capablity of the DMA API implementation, and 
represents the largest contiguous mapping it is guaranteed to be able to 
handle (each segment in the case of dma_map_sg(), or the whole thing for 
dma_map_page()). Most likely the thing you want here is 
min_not_zero(max_seg_size, max_mapping_size).

> 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.

Possibly; this code already looks lifted from drm_prime_pages_to_sg(), 
and if it's needed here then presumably vb2_dma_sg_get_userptr() also 
needs it, at the very least.

Thanks,
Robin.