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
> 
> > >
> > > 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.
I ran into a merge conflict while applying this patch on top of other
udmabuf patches. So, please resend this patch after rebasing on top
of drm-tip:
https://gitlab.freedesktop.org/drm/tip/-/commits/drm-tip?ref_type=HEADS

Thanks,
Vivek

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