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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/DMA-mapping.txt | 103 |
1 files changed, 0 insertions, 103 deletions
diff --git a/Documentation/DMA-mapping.txt b/Documentation/DMA-mapping.txt index 028614cdd06..e07f2530326 100644 --- a/Documentation/DMA-mapping.txt +++ b/Documentation/DMA-mapping.txt @@ -664,109 +664,6 @@ It is that simple. Well, not for some odd devices. See the next section for information about that. - DAC Addressing for Address Space Hungry Devices - -There exists a class of devices which do not mesh well with the PCI -DMA mapping API. By definition these "mappings" are a finite -resource. The number of total available mappings per bus is platform -specific, but there will always be a reasonable amount. - -What is "reasonable"? Reasonable means that networking and block I/O -devices need not worry about using too many mappings. - -As an example of a problematic device, consider compute cluster cards. -They can potentially need to access gigabytes of memory at once via -DMA. Dynamic mappings are unsuitable for this kind of access pattern. - -To this end we've provided a small API by which a device driver -may use DAC cycles to directly address all of physical memory. -Not all platforms support this, but most do. It is easy to determine -whether the platform will work properly at probe time. - -First, understand that there may be a SEVERE performance penalty for -using these interfaces on some platforms. Therefore, you MUST only -use these interfaces if it is absolutely required. %99 of devices can -use the normal APIs without any problems. - -Note that for streaming type mappings you must either use these -interfaces, or the dynamic mapping interfaces above. You may not mix -usage of both for the same device. Such an act is illegal and is -guaranteed to put a banana in your tailpipe. - -However, consistent mappings may in fact be used in conjunction with -these interfaces. Remember that, as defined, consistent mappings are -always going to be SAC addressable. - -The first thing your driver needs to do is query the PCI platform -layer if it is capable of handling your devices DAC addressing -capabilities: - - int pci_dac_dma_supported(struct pci_dev *hwdev, u64 mask); - -You may not use the following interfaces if this routine fails. - -Next, DMA addresses using this API are kept track of using the -dma64_addr_t type. It is guaranteed to be big enough to hold any -DAC address the platform layer will give to you from the following -routines. If you have consistent mappings as well, you still -use plain dma_addr_t to keep track of those. - -All mappings obtained here will be direct. The mappings are not -translated, and this is the purpose of this dialect of the DMA API. - -All routines work with page/offset pairs. This is the _ONLY_ way to -portably refer to any piece of memory. If you have a cpu pointer -(which may be validly DMA'd too) you may easily obtain the page -and offset using something like this: - - struct page *page = virt_to_page(ptr); - unsigned long offset = offset_in_page(ptr); - -Here are the interfaces: - - dma64_addr_t pci_dac_page_to_dma(struct pci_dev *pdev, - struct page *page, - unsigned long offset, - int direction); - -The DAC address for the tuple PAGE/OFFSET are returned. The direction -argument is the same as for pci_{map,unmap}_single(). The same rules -for cpu/device access apply here as for the streaming mapping -interfaces. To reiterate: - - The cpu may touch the buffer before pci_dac_page_to_dma. - The device may touch the buffer after pci_dac_page_to_dma - is made, but the cpu may NOT. - -When the DMA transfer is complete, invoke: - - void pci_dac_dma_sync_single_for_cpu(struct pci_dev *pdev, - dma64_addr_t dma_addr, - size_t len, int direction); - -This must be done before the CPU looks at the buffer again. -This interface behaves identically to pci_dma_sync_{single,sg}_for_cpu(). - -And likewise, if you wish to let the device get back at the buffer after -the cpu has read/written it, invoke: - - void pci_dac_dma_sync_single_for_device(struct pci_dev *pdev, - dma64_addr_t dma_addr, - size_t len, int direction); - -before letting the device access the DMA area again. - -If you need to get back to the PAGE/OFFSET tuple from a dma64_addr_t -the following interfaces are provided: - - struct page *pci_dac_dma_to_page(struct pci_dev *pdev, - dma64_addr_t dma_addr); - unsigned long pci_dac_dma_to_offset(struct pci_dev *pdev, - dma64_addr_t dma_addr); - -This is possible with the DAC interfaces purely because they are -not translated in any way. - Optimizing Unmap State Space Consumption On many platforms, pci_unmap_{single,page}() is simply a nop. |