Documentation: Add io-mapping.rst to driver-api manual

Add io-mapping.rst under Documentation/driver-api and reference it from
Sphinx TOC Tree present in Documentation/driver-api/index.rst

Signed-off-by: Pragat Pandya <pragat.pandya@gmail.com>
Link: https://lore.kernel.org/r/20200303050301.5412-2-pragat.pandya@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 97 deletions

diff --git a/Documentation/io-mapping.txt b/Documentation/io-mapping.txt
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-========================
-The io_mapping functions
-========================
-
-API
-===
-
-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 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.