[RFC v1 1/4] dma-mapping: introduce the DMA_ATTR_MAY_SLEEP attribute

[Petr Tesarik]

From: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>

Introduce a DMA attribute to tell the DMA-mapping subsystem that
the operation is allowed to sleep.

This patch merely adds the flag, which is not used for anything at
the moment. It should be used by users who can sleep (e.g. dma-buf
ioctls) to allow page reclaim and/or allocations from CMA.

Signed-off-by: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx>
---
 Documentation/core-api/dma-attributes.rst | 10 ++++++++++
 include/linux/dma-mapping.h               |  6 ++++++
 2 files changed, 16 insertions(+)

diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
index 1887d92e8e92..6481ce2acf5d 100644
--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -130,3 +130,13 @@ accesses to DMA buffers in both privileged "supervisor" and unprivileged
 subsystem that the buffer is fully accessible at the elevated privilege
 level (and ideally inaccessible or at least read-only at the
 lesser-privileged levels).
+
+DMA_ATTR_MAY_SLEEP
+------------------
+
+This tells the DMA-mapping subsystem that it is allowed to sleep. For example,
+if mapping needs a bounce buffer, software IO TLB may use CMA for the
+allocation if this flag is given.
+
+This attribute is not used for dma_alloc_* functions. Instead, the provided
+GFP flags are used to determine whether the allocation may sleep.
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 0ee20b764000..7a75c503ac38 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -61,6 +61,12 @@
  */
 #define DMA_ATTR_PRIVILEGED		(1UL << 9)
 
+/*
+ * DMA_ATTR_MAY_SLEEP: This tells the DMA-mapping subsystem that it is allowed
+ * to sleep.
+ */
+#define DMA_ATTR_MAY_SLEEP		(1UL << 10)
+
 /*
  * 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
-- 
2.25.1