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

Tomasz Figa
2023-09-06T09:27:11Z

On Wed, Sep 6, 2023 at 5:52 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> Hi all,
>
> On 29/08/2023 13:14, Robin Murphy wrote:
> > On 2023-08-29 11:03, Tomasz Figa wrote:
> >> Hi Anle,
> >>
> >> On Mon, Aug 28, 2023 at 8:57 AM Anle Pan <anle.pan@nxp.com> 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@nxp.com>
> >>> ---
> >>> 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.
>
> I also see sg_alloc_table_from_pages being called in vb2_dc_get_userptr in videobuf2-dma-contig.c,
> presumably that needs the same fix?
>
> From what I gather from this thread there are no improved helpers on the immediate
> horizon, so this issue has to be fixed in videobuf2 for now.

Agreed. (Although I suspect the real issue that NXP is having isn't
really this and this is just a side effect.)

>
> So this requires a v2 that fixes this also in vb2_dma_sg_get_userptr and vb2_dc_get_userptr,
> correct? If so, then it would be nice if Anle can post a v2 with those changes.

If we need to fix 3 different callers, could we at least add an
internal vb2 helper for this (e.g. vb2_asg_alloc_table_from_pages())?

>
> Note that when I grepped for sg_alloc_table_from_pages_segment users, I noticed that
> in most cases dma_max_mapping_size is used, but in one case it uses dma_get_max_seg_size
> (drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c). I have no idea what the difference is
> between the two.

FWIW, Robin explained that earlier in the thread. To be fully correct,
we need to consider both...

>
> One small change that would make sg_alloc_table_from_pages_segment() a bit easier
> to work with is if it would replace a max_segment value of 0 with UINT_MAX. Then
> you can just stick in dma_max_mapping_size(dev) as the argument.
>
> Alternatively, if we can be certain that dma_max_mapping_size(dev) never returns 0,
> then that 'if (max_segment == 0)' part can just be dropped.

That's also the reason I wanted a helper. Not sure if it's so easy to
change the calling convention now.

>
> I also wonder if any of the other (non-media) users of sg_alloc_table_from_pages
> would need to use sg_alloc_table_from_pages_segment as well. Is this really
> media specific? Or is media just the most likely subsystem to hit this issue due
> to the large amounts of memory it uses?

I don't think it's media-specific, but also as I said earlier, it
should be really rare to hit it - swiotlb isn't normally expected for
reasonably capable systems and buffer allocation being done correctly.

Best regards,
Tomasz