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 > <[email protected]> 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: [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); > > 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- > [email protected]/ > > 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.
