resources: add io-mapping functions to dynamically map large device apertures

[linux-2.6-block.git] / Documentation / io-mapping.txt

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+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, io_mapping_map_atomic_wc uses io_map_atomic_prot_pfn,
+which uses the fixmaps to get us a mapping to a page using an atomic fashion.
+For io_mapping_map_wc, ioremap_wc() is used to get a mapping of the region.