When CONFIG_DMA_API_DEBUG_SG is enabled, importing a udmabuf into a DRM
driver (e.g. amdgpu for video playback in GNOME Videos / Showtime)
triggers a spurious warning:
DMA-API: amdgpu 0000:03:00.0: cacheline tracking EEXIST, \
overlapping mappings aren't supported
WARNING: kernel/dma/debug.c:619 at add_dma_entry+0x473/0x5f0
The call chain is:
amdgpu_cs_ioctl
-> amdgpu_ttm_backend_bind
-> dma_buf_map_attachment
-> [udmabuf] map_udmabuf -> get_sg_table
-> dma_map_sgtable(dev, sg, direction, 0) // attrs=0
-> debug_dma_map_sg -> add_dma_entry -> EEXIST
This happens because udmabuf builds a per-page scatter-gather list via
sg_set_folio(). When begin_cpu_udmabuf() has already created an sg
table mapped for the misc device, and an importer such as amdgpu maps
the same pages for its own device via map_udmabuf(), the DMA debug
infrastructure sees two active mappings whose physical addresses share
cacheline boundaries and warns about the overlap.
The DMA_ATTR_SKIP_CPU_SYNC flag suppresses this check in
add_dma_entry() because it signals that no CPU cache maintenance is
performed at map/unmap time, making the cacheline overlap harmless.
All other major dma-buf exporters already pass this flag:
- drm_gem_map_dma_buf() passes DMA_ATTR_SKIP_CPU_SYNC
- amdgpu_dma_buf_map() passes DMA_ATTR_SKIP_CPU_SYNC
The CPU sync at map/unmap time is also redundant for udmabuf:
begin_cpu_udmabuf() and end_cpu_udmabuf() already perform explicit
cache synchronization via dma_sync_sgtable_for_cpu/device() when CPU
access is requested through the dma-buf interface.
Pass DMA_ATTR_SKIP_CPU_SYNC to dma_map_sgtable() and
dma_unmap_sgtable() in udmabuf to suppress the spurious warning and
skip the redundant sync.
Fixes: 284562e1f348 ("udmabuf: implement begin_cpu_access/end_cpu_access hooks")
Cc: [email protected]
Signed-off-by: Mikhail Gavrilov <[email protected]>
---
drivers/dma-buf/udmabuf.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index 94b8ecb892bb..9c6f8785a28a 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -162,7 +162,7 @@ static struct sg_table *get_sg_table(struct device *dev,
struct dma_buf *buf,
sg_set_folio(sgl, ubuf->folios[i], PAGE_SIZE,
ubuf->offsets[i]);
- ret = dma_map_sgtable(dev, sg, direction, 0);
+ ret = dma_map_sgtable(dev, sg, direction, DMA_ATTR_SKIP_CPU_SYNC);
if (ret < 0)
goto err_map;
return sg;
@@ -177,7 +177,7 @@ static struct sg_table *get_sg_table(struct device *dev,
struct dma_buf *buf,
static void put_sg_table(struct device *dev, struct sg_table *sg,
enum dma_data_direction direction)
{
- dma_unmap_sgtable(dev, sg, direction, 0);
+ dma_unmap_sgtable(dev, sg, direction, DMA_ATTR_SKIP_CPU_SYNC);
sg_free_table(sg);
kfree(sg);
}
--
2.53.0
