On Wed, 05 Jul 2006 20:40:36 -0700 (PDT), David Miller wrote:
>> I.e., X did a simple PROT_READ|PROT_WRITE MAP_SHARED mmap() of
>> something PCI-related, presumably the ATI card. The protection
>> bits passed into io_remap_pfn_range() are 0x80...0788, while
>> pg_iobits are 0x80...0f8a. Current kernels obey the prot bits,
>> which, if I read things correctly, means that _PAGE_W_4U and
>> _PAGE_MODIFIED_4U don't get set any more.
>>
>> I guess something else in the kernel should have set those
>> bits before they got to io_remap_pfn_range()?
>
>The problem is with X, it should not be doing a MAP_SHARED
>mmap() of the framebuffer device. It should be using
>MAP_PRIVATE instead.
>
>The kernel is trying to provide copy-on-write semantics for
>the mapping, which doesn't make any sense for device registers.
>That's why the kernel isn't setting the writable or modified
>bits in the protection bitmask.
Now I'm confused. That COW behaviour would be consistent with
MAP_PRIVATE, not MAP_SHARED which is what X did use.
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