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    SubjectRe: [PATCH] Add io-mapping functions to dynamically map large device apertures
    On Thu, 23 Oct 2008 00:14:46 -0700 Keith Packard wrote:

    > Documentation/io-mapping.txt | 84 ++++++++++++++++++++++++++++
    > include/linux/io-mapping.h | 125 ++++++++++++++++++++++++++++++++++++++++++
    > 2 files changed, 209 insertions(+), 0 deletions(-)
    > create mode 100644 Documentation/io-mapping.txt
    > create mode 100644 include/linux/io-mapping.h
    >
    > diff --git a/Documentation/io-mapping.txt b/Documentation/io-mapping.txt
    > new file mode 100644
    > index 0000000..ebf6dc5
    > --- /dev/null
    > +++ b/Documentation/io-mapping.txt
    > @@ -0,0 +1,84 @@
    > +The io_mapping functions in linux/io.h provide an abstraction for

    io-mapping.h ?

    > +efficiently mapping small regions of an io device to the CPU. The initial

    IO or I/O, please

    > +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

    with <TBD>...

    > +
    > + Note that the task may not sleep while holding this page
    > + mapped.
    > +
    > + void
    > + io_mapping_unmap_atomic(void *vaddr)
    > +
    > + '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.

    s/,//

    > +
    > +If you need to sleep while holding the lock, you can use the non-atomic
    > +variant, although they may be significantly slower;

    s/;/./

    > +
    > + 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 use existing mapping

    uses

    > +mechanisms and so provide only an abstraction layer and no new

    provides

    > +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.


    And I wish you could lose that horrible (non-Linux kernel) style of function
    return type on a separate line.

    ---
    ~Randy


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