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-rw-r--r--Documentation/DMA-API-HOWTO.txt85
1 files changed, 49 insertions, 36 deletions
diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index 2e435ad..98ce517 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
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