io_remap_pfn_range()

io_remap_page_range() has always been a strange function. Its stated purpose is to portably map I/O memory into a process's address space. Its prototype has always differed from one system to the next, however, making portable use difficult. On most architectures it looks like this:

    int io_remap_page_range(struct vm_area_struct *vma, unsigned long virt_addr,
                            unsigned long phys_addr, unsigned long size, 
                            pgprot_t prot);

The sparc64 architecture, however, defines it this way:

    int io_remap_page_range(struct vm_area_struct *vma, unsigned long virt_addr,
                            unsigned long phys_addr, unsigned long size, 
                            pgprot_t prot, int space);

The extra argument (space) was necessary to deal with the inconvenient fact that I/O addresses on the sparc64 architecture would not fit into an unsigned long variable.

The change from remap_page_range() to remap_pfn_range() was done, in part, to address (so to speak) this issue. Since remapping must be done on a page-aligned basis anyway, there is no real point in using a regular physical address, which contains the offset within the page. Said offset, after all, must be zero. By using a page frame number instead, the range of the phys_addr argument is extended far enough to reach into I/O memory on all architectures. The remap_pfn_range() work stopped short of actually fixing the io_remap_page_range() problem, however.

Randy Dunlap has now finished the task with a set of patches adding io_remap_pfn_range():

    int io_remap_pfn_range(struct vm_area_struct *vma, unsigned long from,
                           unsigned long pfn, unsigned long size, 
                           pgprot_t prot);

This function has the same prototype on all architectures. In-tree callers have been modified, and the feature removal schedule has been updated: io_remap_page_range() will go away in September, 2005.

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