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.
