Re: [RFC] MMIO accessors & barriers documentation

Eric W. Biederman
2006-09-12T05:50:11Z

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

>> > "Except where the underlying device is marked as cachable or
>> > prefetchable"
>>
>> You aren't supposed to use MMIO accessors on cacheable memory, are you ?
>
> Why not. Providing it is in MMIO space, consider ROMs for example or
> write path consider frame buffers.

Frame buffers are rarely cachable as such, on x86 they are usually
write-combining. Which means that the writes can be merged and
possibly reordered while they are being written but they can't be
cached. Most arches I believe have something that roughly corresponds
to write combining.

Ensuring we can still use this optimization to mmio space is
moderately important.

Eric