From: Will Davis <wdavis@xxxxxxxxxx>
Add references to both the general API documentation as well as the HOWTO.
Signed-off-by: Will Davis <wdavis@xxxxxxxxxx>
---
Documentation/DMA-API-HOWTO.txt | 39 +++++++++++++++++++++++++++++++++++++--
Documentation/DMA-API.txt | 36 +++++++++++++++++++++++++++++++-----
2 files changed, 68 insertions(+), 7 deletions(-)
diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index 0f7afb2..89bd730 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -138,6 +138,10 @@ What about block I/O and networking buffers? The block I/O and
networking subsystems make sure that the buffers they use are valid
for you to DMA from/to.
+In some systems, it may also be possible to DMA to and/or from a peer
+device's MMIO region, as described by a 'struct resource'. This is
+referred to as a peer-to-peer mapping.
+
DMA addressing limitations
Does your device have any DMA addressing limitations? For example, is
@@ -648,6 +652,35 @@ Every dma_map_{single,sg}() call should have its dma_unmap_{single,sg}()
counterpart, because the bus address space is a shared resource and
you could render the machine unusable by consuming all bus addresses.
+Peer-to-peer DMA mappings can be obtained using dma_map_resource() to map
+another device's MMIO region for the given device:
+
+ struct resource *peer_mmio_res = &other_dev->resource[0];
+ dma_addr_t dma_handle = dma_map_resource(dev, peer_mmio_res,
+ offset, size, direction);
+ if (dma_handle == 0 || dma_mapping_error(dev, dma_handle))
+ {
+ /*
+ * If dma_handle == 0, dma_map_resource() is not
+ * implemented, and peer-to-peer transactions will not
+ * work.
+ */
+ goto map_error_handling;
+ }
+
+ ...
+
+ dma_unmap_resource(dev, dma_handle, size, direction);
+
+Here, "offset" means byte offset within the given resource.
+
+You should both check for a 0 return value and call dma_mapping_error(),
+as dma_map_resource() can either be not implemented or fail and return
+error as outlined under the dma_map_single() discussion.
+
+You should call dma_unmap_resource() when DMA activity is finished, e.g.,
+from the interrupt which told you that the DMA transfer is done.
+
If you need to use the same streaming DMA region multiple times and touch
the data in between the DMA transfers, the buffer needs to be synced
properly in order for the CPU and device to see the most up-to-date and
@@ -765,8 +798,8 @@ failure can be determined by:
- checking if dma_alloc_coherent() returns NULL or dma_map_sg returns 0
-- checking the dma_addr_t returned from dma_map_single() and dma_map_page()
- by using dma_mapping_error():
+- checking the dma_addr_t returned from dma_map_single(), dma_map_resource(),
+ and dma_map_page() by using dma_mapping_error():
dma_addr_t dma_handle;
@@ -780,6 +813,8 @@ failure can be determined by:
goto map_error_handling;
}
+- checking if dma_map_resource() returns 0
+
- unmap pages that are already mapped, when mapping error occurs in the middle
of a multiple page mapping attempt. These example are applicable to
dma_map_page() as well.
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 5208840..c25c549 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -283,14 +283,40 @@ and <size> parameters are provided to do partial page mapping, it is
recommended that you never use these unless you really know what the
cache width is.
+dma_addr_t
+dma_map_resource(struct device *dev, struct resource *res,
+ unsigned long offset, size_t size,
+ enum dma_data_direction_direction)
+
+API for mapping resources. This API allows a driver to map a peer
+device's resource for DMA. All the notes and warnings for the other
+APIs apply here. Also, the success of this API does not validate or
+guarantee that peer-to-peer transactions between the device and its
+peer will be functional. They only grant access so that if such
+transactions are possible, an IOMMU will not prevent them from
+succeeding.
