Commit 9bf6f7ba authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

Documentation/io-mapping: Remove outdated blurb 



The implementation details in the documentation are outdated and not really
helpful. Remove them.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095858.734064977@linutronix.de
parent 351191ad

Documentation/driver-api/io-mapping.rst

+0 −22
Original line number Diff line number Diff line
@@ -73,25 +73,3 @@ 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.