Messages in this thread | | | Date | Tue, 29 Aug 2023 12:14:44 +0100 | Subject | Re: [PATCH] media: videobuf2-dma-sg: limit the sg segment size | From | Robin Murphy <> |
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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.
Thanks, Robin.
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