RE: [PATCH] dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning

[Kasireddy, Vivek]

Hi Mikhail,

> Subject: Re: [PATCH] dma-buf/udmabuf: skip redundant cpu sync to fix
> cacheline EEXIST warning
> 
> On Tue, Mar 17, 2026 at 10:37 AM Mikhail Gavrilov
> <mikhail.v.gavrilov@xxxxxxxxx> wrote:
> >
> > 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: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
> > ---
> >  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
> >
> 
> Gentle ping on this patch.
> 
> To summarize the review so far:
>   Vivek: "Looks OK to me"
>   Christian: Acked-by
> 
> Note: while my separate dma-debug patch [1] would also suppress
> this specific warning on x86, the udmabuf fix here is still
> valuable as it removes the redundant CPU sync at map/unmap time
> and aligns udmabuf with other dma-buf exporters that already
> pass DMA_ATTR_SKIP_CPU_SYNC.
> 
> [1] https://lore.kernel.org/all/20260327124156.24820-1-
> mikhail.v.gavrilov@xxxxxxxxx/
> 
> Gerd, could you take a look when you have a chance?
I am convinced that this patch is useful in some cases, so I'll add Christian's
Ack and push it (into drm-misc-next) in a day or two.

Thanks,
Vivek

> 
> --
> Best Regards,
> Mike Gavrilov.