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Russell King
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Russell King
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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. | ||
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A mapping object is created during driver initialization using | ||
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struct io_mapping *io_mapping_create_wc(unsigned long base, | ||
unsigned long size) | ||
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'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. | ||
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This _wc variant provides a mapping which may only be used | ||
with the io_mapping_map_atomic_wc or io_mapping_map_wc. | ||
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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: | ||
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void *io_mapping_map_atomic_wc(struct io_mapping *mapping, | ||
unsigned long offset) | ||
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'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. | ||
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This _wc variant returns a write-combining map to the | ||
page and may only be used with mappings created by | ||
io_mapping_create_wc | ||
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Note that the task may not sleep while holding this page | ||
mapped. | ||
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void io_mapping_unmap_atomic(void *vaddr) | ||
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'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. | ||
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If you need to sleep while holding the lock, you can use the non-atomic | ||
variant, although they may be significantly slower. | ||
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void *io_mapping_map_wc(struct io_mapping *mapping, | ||
unsigned long offset) | ||
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This works like io_mapping_map_atomic_wc except it allows | ||
the task to sleep while holding the page mapped. | ||
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void io_mapping_unmap(void *vaddr) | ||
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This works like io_mapping_unmap_atomic, except it is used | ||
for pages mapped with io_mapping_map_wc. | ||
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At driver close time, the io_mapping object must be freed: | ||
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void io_mapping_free(struct io_mapping *mapping) | ||
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Current Implementation: | ||
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The initial implementation of these functions uses existing mapping | ||
mechanisms and so provides only an abstraction layer and no new | ||
functionality. | ||
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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. | ||
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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. | ||
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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. |
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