98 lines
3.3 KiB
Plaintext
98 lines
3.3 KiB
Plaintext
========================
|
|
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.
|