92 lines
3.2 KiB
ReStructuredText
92 lines
3.2 KiB
ReStructuredText
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The io_mapping functions
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========================
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API
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===
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The io_mapping functions in linux/io-mapping.h provide an abstraction for
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efficiently mapping small regions of an I/O device to the CPU. The initial
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usage is to support the large graphics aperture on 32-bit processors where
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ioremap_wc cannot be used to statically map the entire aperture to the CPU
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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,
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unsigned long size)
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'base' is the bus address of the region to be made
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mappable, while 'size' indicates how large a mapping region to
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enable. Both are in bytes.
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This _wc variant provides a mapping which may only be used with
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io_mapping_map_atomic_wc(), io_mapping_map_local_wc() or
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io_mapping_map_wc().
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With this mapping object, individual pages can be mapped either temporarily
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or long term, depending on the requirements. Of course, temporary maps are
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more efficient. They come in two flavours::
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void *io_mapping_map_local_wc(struct io_mapping *mapping,
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unsigned long offset)
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void *io_mapping_map_atomic_wc(struct io_mapping *mapping,
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unsigned long offset)
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'offset' is the offset within the defined mapping region. Accessing
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addresses beyond the region specified in the creation function yields
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undefined results. Using an offset which is not page aligned yields an
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undefined result. The return value points to a single page in CPU address
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space.
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This _wc variant returns a write-combining map to the page and may only be
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used with mappings created by io_mapping_create_wc()
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Temporary mappings are only valid in the context of the caller. The mapping
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is not guaranteed to be globally visible.
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io_mapping_map_local_wc() has a side effect on X86 32bit as it disables
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migration to make the mapping code work. No caller can rely on this side
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effect.
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io_mapping_map_atomic_wc() has the side effect of disabling preemption and
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pagefaults. Don't use in new code. Use io_mapping_map_local_wc() instead.
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Nested mappings need to be undone in reverse order because the mapping
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code uses a stack for keeping track of them::
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addr1 = io_mapping_map_local_wc(map1, offset1);
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addr2 = io_mapping_map_local_wc(map2, offset2);
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...
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io_mapping_unmap_local(addr2);
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io_mapping_unmap_local(addr1);
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The mappings are released with::
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void io_mapping_unmap_local(void *vaddr)
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void io_mapping_unmap_atomic(void *vaddr)
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'vaddr' must be the value returned by the last io_mapping_map_local_wc() or
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io_mapping_map_atomic_wc() call. This unmaps the specified mapping and
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undoes the side effects of the mapping functions.
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If you need to sleep while holding a mapping, you can use the regular
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variant, although this may be significantly slower::
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void *io_mapping_map_wc(struct io_mapping *mapping,
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unsigned long offset)
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This works like io_mapping_map_atomic/local_wc() except it has no side
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effects and the pointer is globally visible.
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The mappings are released with::
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void io_mapping_unmap(void *vaddr)
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Use 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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