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dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning

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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: 284562e1f3 ("udmabuf: implement begin_cpu_access/end_cpu_access hooks")
Cc: stable@vger.kernel.org
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Link: https://patch.msgid.link/20260331061657.79983-1-mikhail.v.gavrilov@gmail.com
This commit is contained in:
Mikhail Gavrilov
2026-03-31 11:16:57 +05:00
committed by Vivek Kasireddy
parent f263bcdcc0
commit 504e2b4ab9
1 changed files with 2 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

4
drivers/dma-buf/udmabuf.c
View File

@@ -145,7 +145,7 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
if (ret < 0)
goto err_alloc;
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;
@@ -160,7 +160,7 @@ 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);
}