diff options
Diffstat (limited to 'Documentation/DMA-API.txt')
| -rw-r--r-- | Documentation/DMA-API.txt | 30 |
1 files changed, 15 insertions, 15 deletions
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt index 52088408668a..7eba542eff7c 100644 --- a/Documentation/DMA-API.txt +++ b/Documentation/DMA-API.txt @@ -18,10 +18,10 @@ Part I - dma_ API To get the dma_ API, you must #include <linux/dma-mapping.h>. This provides dma_addr_t and the interfaces described below. -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. A CPU cannot -reference a dma_addr_t directly because there may be translation between -its physical address space and the bus address space. +A dma_addr_t can hold any valid DMA address for the platform. It can be +given to a device to use as a DMA source or target. A CPU cannot reference +a dma_addr_t directly because there may be translation between its physical +address space and the DMA address space. Part Ia - Using large DMA-coherent buffers ------------------------------------------ @@ -42,7 +42,7 @@ It returns a pointer to the allocated region (in the processor's virtual address space) or NULL if the allocation failed. It also returns a <dma_handle> which may be cast to an unsigned integer the -same width as the bus and given to the device as the bus address base of +same width as the bus and given to the device as the DMA address base of the region. Note: consistent memory can be expensive on some platforms, and the @@ -193,7 +193,7 @@ dma_map_single(struct device *dev, void *cpu_addr, size_t size, enum dma_data_direction direction) Maps a piece of processor virtual memory so it can be accessed by the -device and returns the bus address of the memory. +device and returns the DMA address of the memory. The direction for both APIs may be converted freely by casting. However the dma_ API uses a strongly typed enumerator for its @@ -212,20 +212,20 @@ contiguous piece of memory. For this reason, memory to be mapped by this API should be obtained from sources which guarantee it to be physically contiguous (like kmalloc). -Further, the bus address of the memory must be within the +Further, the DMA address of the memory must be within the dma_mask of the device (the dma_mask is a bit mask of the -addressable region for the device, i.e., if the bus address of -the memory ANDed with the dma_mask is still equal to the bus +addressable region for the device, i.e., if the DMA address of +the memory ANDed with the dma_mask is still equal to the DMA address, then the device can perform DMA to the memory). To ensure that the memory allocated by kmalloc is within the dma_mask, the driver may specify various platform-dependent flags to restrict -the bus address range of the allocation (e.g., on x86, GFP_DMA -guarantees to be within the first 16MB of available bus addresses, +the DMA address range of the allocation (e.g., on x86, GFP_DMA +guarantees to be within the first 16MB of available DMA addresses, as required by ISA devices). Note also that the above constraints on physical contiguity and dma_mask may not apply if the platform has an IOMMU (a device which -maps an I/O bus address to a physical memory address). However, to be +maps an I/O DMA address to a physical memory address). However, to be portable, device driver writers may *not* assume that such an IOMMU exists. @@ -296,7 +296,7 @@ reduce current DMA mapping usage or delay and try again later). dma_map_sg(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) -Returns: the number of bus address segments mapped (this may be shorter +Returns: the number of DMA address segments mapped (this may be shorter than <nents> passed in if some elements of the scatter/gather list are physically or virtually adjacent and an IOMMU maps them with a single entry). @@ -340,7 +340,7 @@ must be the same as those and passed in to the scatter/gather mapping API. Note: <nents> must be the number you passed in, *not* the number of -bus address entries returned. +DMA address entries returned. void dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size, @@ -507,7 +507,7 @@ it's asked for coherent memory for this device. phys_addr is the CPU physical address to which the memory is currently assigned (this will be ioremapped so the CPU can access the region). -device_addr is the bus address the device needs to be programmed +device_addr is the DMA address the device needs to be programmed with to actually address this memory (this will be handed out as the dma_addr_t in dma_alloc_coherent()). |