[PATCH RFC 03/13] dma-mapping: add DMA_ATTR_CPU_CACHE_CLEAN

["Michael S. Tsirkin" <mst@redhat.com>]

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 <mst@redhat.com>
---
 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 47b7de3786a1..8216a86cd0c2 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -78,6 +78,13 @@
  */
 #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,
-- 
MST