On 3/18/26 06:40, Kasireddy, Vivek wrote:
> Hi Mikhail,
>
>> Subject: [PATCH] dma-buf/udmabuf: skip redundant cpu sync to fix
>> cacheline EEXIST warning
>>
>> 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);
> Looks OK to me but it would be nice if Christian or someone else can
> provide an Ack for this patch.
The details of the udmabuf handling is absolutely not my field of expertise.
Feel free to add my Acked-by since it obviously seems to fix a bug, but it
would be nice if somebody could do an in deep review as well.
Regards,
Christian.
>
> Thanks,
> Vivek
>
>> sg_free_table(sg);
>> kfree(sg);
>> }
>> --
>> 2.53.0
>
