diff options
Diffstat (limited to 'Documentation/io-mapping.txt')
| -rw-r--r-- | Documentation/io-mapping.txt | 82 |
1 files changed, 82 insertions, 0 deletions
diff --git a/Documentation/io-mapping.txt b/Documentation/io-mapping.txt new file mode 100644 index 00000000000..473e43b2d58 --- /dev/null +++ b/Documentation/io-mapping.txt @@ -0,0 +1,82 @@ +The io_mapping functions in linux/io-mapping.h provide an abstraction for +efficiently mapping small regions of an I/O device to the CPU. The initial +usage is to support the large graphics aperture on 32-bit processors where +ioremap_wc cannot be used to statically map the entire aperture to the CPU +as it would consume too much of the kernel address space. + +A mapping object is created during driver initialization using + + struct io_mapping *io_mapping_create_wc(unsigned long base, + unsigned long size) + + 'base' is the bus address of the region to be made + mappable, while 'size' indicates how large a mapping region to + enable. Both are in bytes. + + This _wc variant provides a mapping which may only be used + with the io_mapping_map_atomic_wc or io_mapping_map_wc. + +With this mapping object, individual pages can be mapped either atomically +or not, depending on the necessary scheduling environment. Of course, atomic +maps are more efficient: + + void *io_mapping_map_atomic_wc(struct io_mapping *mapping, + unsigned long offset) + + 'offset' is the offset within the defined mapping region. + Accessing addresses beyond the region specified in the + creation function yields undefined results. Using an offset + which is not page aligned yields an undefined result. The + return value points to a single page in CPU address space. + + This _wc variant returns a write-combining map to the + page and may only be used with mappings created by + io_mapping_create_wc + + Note that the task may not sleep while holding this page + mapped. + + void io_mapping_unmap_atomic(void *vaddr) + + 'vaddr' must be the the value returned by the last + io_mapping_map_atomic_wc call. This unmaps the specified + page and allows the task to sleep once again. + +If you need to sleep while holding the lock, you can use the non-atomic +variant, although they may be significantly slower. + + void *io_mapping_map_wc(struct io_mapping *mapping, + unsigned long offset) + + This works like io_mapping_map_atomic_wc except it allows + the task to sleep while holding the page mapped. + + void io_mapping_unmap(void *vaddr) + + This works like io_mapping_unmap_atomic, except it is used + for pages mapped with io_mapping_map_wc. + +At driver close time, the io_mapping object must be freed: + + void io_mapping_free(struct io_mapping *mapping) + +Current Implementation: + +The initial implementation of these functions uses existing mapping +mechanisms and so provides only an abstraction layer and no new +functionality. + +On 64-bit processors, io_mapping_create_wc calls ioremap_wc for the whole +range, creating a permanent kernel-visible mapping to the resource. The +map_atomic and map functions add the requested offset to the base of the +virtual address returned by ioremap_wc. + +On 32-bit processors with HIGHMEM defined, io_mapping_map_atomic_wc uses +kmap_atomic_pfn to map the specified page in an atomic fashion; +kmap_atomic_pfn isn't really supposed to be used with device pages, but it +provides an efficient mapping for this usage. + +On 32-bit processors without HIGHMEM defined, io_mapping_map_atomic_wc and +io_mapping_map_wc both use ioremap_wc, a terribly inefficient function which +performs an IPI to inform all processors about the new mapping. This results +in a significant performance penalty. |