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io-mapping.txt: standardize document format 

Each text file under Documentation follows a different
format. Some doesn't even have titles!

Change its representation to follow the adopted standard,
using ReST markups for it to be parseable by Sphinx:

- Add a title for the document and for API chapter;
- mark literal blocks;
- Adjust whitespacing.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mauro Carvalho Chehab 2017-05-14 14:32:35 -03:00 committed by Jonathan Corbet
parent 45d8514626
commit 9cf5116d5b
1 changed files with 41 additions and 26 deletions
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67
Documentation/io-mapping.txt
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@ -1,66 +1,81 @@
========================
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
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.
'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.
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:
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.
'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.
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.
'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.
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.
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:
At driver close time, the io_mapping object must be freed::
void io_mapping_free(struct io_mapping *mapping)
Current Implementation:
Current Implementation
======================
The initial implementation of these functions uses existing mapping
mechanisms and so provides only an abstraction layer and no new