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Diffstat (limited to 'Documentation/driver-api/io-mapping.rst')
| -rw-r--r-- | Documentation/driver-api/io-mapping.rst | 96 |
1 files changed, 45 insertions, 51 deletions
diff --git a/Documentation/driver-api/io-mapping.rst b/Documentation/driver-api/io-mapping.rst index a966239f04e4..a0cfb15988df 100644 --- a/Documentation/driver-api/io-mapping.rst +++ b/Documentation/driver-api/io-mapping.rst @@ -20,78 +20,72 @@ A mapping object is created during driver initialization using:: 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. +This _wc variant provides a mapping which may only be used with +io_mapping_map_atomic_wc(), io_mapping_map_local_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:: +With this mapping object, individual pages can be mapped either temporarily +or long term, depending on the requirements. Of course, temporary maps are +more efficient. They come in two flavours:: + + void *io_mapping_map_local_wc(struct io_mapping *mapping, + unsigned long offset) 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. +'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 +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. +Temporary mappings are only valid in the context of the caller. The mapping +is not guaranteed to be globaly visible. -:: +io_mapping_map_local_wc() has a side effect on X86 32bit as it disables +migration to make the mapping code work. No caller can rely on this side +effect. - void io_mapping_unmap_atomic(void *vaddr) +io_mapping_map_atomic_wc() has the side effect of disabling preemption and +pagefaults. Don't use in new code. Use io_mapping_map_local_wc() instead. + +Nested mappings need to be undone in reverse order because the mapping +code uses a stack for keeping track of them:: -'vaddr' must be 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. + addr1 = io_mapping_map_local_wc(map1, offset1); + addr2 = io_mapping_map_local_wc(map2, offset2); + ... + io_mapping_unmap_local(addr2); + io_mapping_unmap_local(addr1); + +The mappings are released with:: + + void io_mapping_unmap_local(void *vaddr) + void io_mapping_unmap_atomic(void *vaddr) -If you need to sleep while holding the lock, you can use the non-atomic -variant, although they may be significantly slower. +'vaddr' must be the value returned by the last io_mapping_map_local_wc() or +io_mapping_map_atomic_wc() call. This unmaps the specified mapping and +undoes the side effects of the mapping functions. -:: +If you need to sleep while holding a mapping, you can use the regular +variant, although this 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. +This works like io_mapping_map_atomic/local_wc() except it has no side +effects and the pointer is globaly visible. - -:: +The mappings are released with:: 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. +Use 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. |