On Tue, Aug 29, 2023 at 8:14 PM Robin Murphy <robin.murphy@xxxxxxx> 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@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). > The extra complexity needed here convinces me even more that we need a helper... > > 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. That code probably predates drm_prime_pages_to_sg(), but that's probably also the reason that it has its own issues... I'm tempted to send a patch adding such a helper, but Christoph didn't sound very keen on that idea. Hmm. Best regards, Tomasz
