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98 lines
3.3 KiB
Plaintext
98 lines
3.3 KiB
Plaintext
========================
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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
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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
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or not, depending on the necessary scheduling environment. Of course, atomic
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maps are more efficient::
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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.
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Accessing addresses beyond the region specified in the
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creation function yields undefined results. Using an offset
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which is not page aligned yields an undefined result. The
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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
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page and may only be used with mappings created by
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io_mapping_create_wc
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Note that the task may not sleep while holding this page
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mapped.
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::
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void io_mapping_unmap_atomic(void *vaddr)
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'vaddr' must be the value returned by the last
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io_mapping_map_atomic_wc call. This unmaps the specified
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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
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variant, although they may be significantly slower.
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::
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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_wc except it allows
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the task to sleep while holding the page mapped.
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::
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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
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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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======================
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The initial implementation of these functions uses existing mapping
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mechanisms and so provides only an abstraction layer and no new
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functionality.
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On 64-bit processors, io_mapping_create_wc calls ioremap_wc for the whole
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range, creating a permanent kernel-visible mapping to the resource. The
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map_atomic and map functions add the requested offset to the base of the
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virtual address returned by ioremap_wc.
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On 32-bit processors with HIGHMEM defined, io_mapping_map_atomic_wc uses
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kmap_atomic_pfn to map the specified page in an atomic fashion;
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kmap_atomic_pfn isn't really supposed to be used with device pages, but it
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provides an efficient mapping for this usage.
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On 32-bit processors without HIGHMEM defined, io_mapping_map_atomic_wc and
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io_mapping_map_wc both use ioremap_wc, a terribly inefficient function which
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performs an IPI to inform all processors about the new mapping. This results
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in a significant performance penalty.
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