1 files changed, 49 insertions, 36 deletions

diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index 2e435adfbd6b..98ce51796f71 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -639,6 +639,36 @@ is planned to completely remove virt_to_bus() and bus_to_virt() as
 they are entirely deprecated.  Some ports already do not provide these
 as it is impossible to correctly support them.
 
+                        Handling Errors
+
+DMA address space is limited on some architectures and an allocation
+failure can be determined by:
+
+- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
+
+- checking the returned dma_addr_t of dma_map_single and dma_map_page
+  by using dma_mapping_error():
+
+        dma_addr_t dma_handle;
+
+        dma_handle = dma_map_single(dev, addr, size, direction);
+        if (dma_mapping_error(dev, dma_handle)) {
+                /*
+                 * reduce current DMA mapping usage,
+                 * delay and try again later or
+                 * reset driver.
+                 */
+        }
+
+Networking drivers must call dev_kfree_skb to free the socket buffer
+and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook
+(ndo_start_xmit). This means that the socket buffer is just dropped in
+the failure case.
+
+SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping
+fails in the queuecommand hook. This means that the SCSI subsystem
+passes the command to the driver again later.
+
                 Optimizing Unmap State Space Consumption
 
 On many platforms, dma_unmap_{single,page}() is simply a nop.
@@ -703,42 +733,25 @@ to "Closing".
 
 1) Struct scatterlist requirements.
 
-   Struct scatterlist must contain, at a minimum, the following
-   members:
-
-        struct page *page;
-        unsigned int offset;
-        unsigned int length;
-
-   The base address is specified by a "page+offset" pair.
-
-   Previous versions of struct scatterlist contained a "void *address"
-   field that was sometimes used instead of page+offset.  As of Linux
-   2.5., page+offset is always used, and the "address" field has been
-   deleted.
-
-2) More to come...
-
-                        Handling Errors
-
-DMA address space is limited on some architectures and an allocation
-failure can be determined by:
-
-- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
-
-- checking the returned dma_addr_t of dma_map_single and dma_map_page
-  by using dma_mapping_error():
-
-        dma_addr_t dma_handle;
-
-        dma_handle = dma_map_single(dev, addr, size, direction);
-        if (dma_mapping_error(dev, dma_handle)) {
-                /*
-                 * reduce current DMA mapping usage,
-                 * delay and try again later or
-                 * reset driver.
-                 */
-        }
+   Don't invent the architecture specific struct scatterlist; just use
+   <asm-generic/scatterlist.h>. You need to enable
+   CONFIG_NEED_SG_DMA_LENGTH if the architecture supports IOMMUs
+   (including software IOMMU).
+
+2) ARCH_KMALLOC_MINALIGN
+
+   Architectures must ensure that kmalloc'ed buffer is
+   DMA-safe. Drivers and subsystems depend on it. If an architecture
+   isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in
+   the CPU cache is identical to data in main memory),
+   ARCH_KMALLOC_MINALIGN must be set so that the memory allocator
+   makes sure that kmalloc'ed buffer doesn't share a cache line with
+   the others. See arch/arm/include/asm/cache.h as an example.
+
+   Note that ARCH_KMALLOC_MINALIGN is about DMA memory alignment
+   constraints. You don't need to worry about the architecture data
+   alignment constraints (e.g. the alignment constraints about 64-bit
+   objects).
 
                            Closing