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.
Thanks,
Vivek
> sg_free_table(sg);
> kfree(sg);
> }
> --
> 2.53.0
