On Mon, 5 Jan 2026 03:23:01 -0500 "Michael S. Tsirkin" <[email protected]> wrote:
> When multiple small DMA_FROM_DEVICE or DMA_BIDIRECTIONAL buffers share a > cacheline, and DMA_API_DEBUG is enabled, we get this warning: > cacheline tracking EEXIST, overlapping mappings aren't supported. > > This is because when one of the mappings is removed, while another one > is active, CPU might write into the buffer. > > Add an attribute for the driver to promise not to do this, making the > overlapping safe, and suppressing the warning. > > Signed-off-by: Michael S. Tsirkin <[email protected]> LGTM. I'm not formally a reviewer, but FWIW: Reviewed-by: Petr Tesarik <[email protected]> > --- > include/linux/dma-mapping.h | 7 +++++++ > kernel/dma/debug.c | 3 ++- > 2 files changed, 9 insertions(+), 1 deletion(-) > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h > index 29ad2ce700f0..29973baa0581 100644 > --- a/include/linux/dma-mapping.h > +++ b/include/linux/dma-mapping.h > @@ -79,6 +79,13 @@ > */ > #define DMA_ATTR_MMIO (1UL << 10) > > +/* > + * DMA_ATTR_CPU_CACHE_CLEAN: Indicates the CPU will not dirty any cacheline > + * overlapping this buffer while it is mapped for DMA. All mappings sharing > + * a cacheline must have this attribute for this to be considered safe. > + */ > +#define DMA_ATTR_CPU_CACHE_CLEAN (1UL << 11) > + > /* > * A dma_addr_t can hold any valid DMA or bus address for the platform. It > can > * be given to a device to use as a DMA source or target. It is specific to > a > diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c > index 138ede653de4..7e66d863d573 100644 > --- a/kernel/dma/debug.c > +++ b/kernel/dma/debug.c > @@ -595,7 +595,8 @@ static void add_dma_entry(struct dma_debug_entry *entry, > unsigned long attrs) > if (rc == -ENOMEM) { > pr_err_once("cacheline tracking ENOMEM, dma-debug disabled\n"); > global_disable = true; > - } else if (rc == -EEXIST && !(attrs & DMA_ATTR_SKIP_CPU_SYNC) && > + } else if (rc == -EEXIST && > + !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | > DMA_ATTR_CPU_CACHE_CLEAN)) && > !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && > is_swiotlb_active(entry->dev))) { > err_printk(entry->dev, entry,
