Re: [RFC Patch] use MTRR for write combining if PAT is not available

[Jan Beulich]

>>> Thomas Schlichter <thomas.schlichter@xxxxxx> 13.10.09 23:29 >>>
>Jan Beulich wrote:
>> >>> Thomas Schlichter <thomas.schlichter@xxxxxx> 12.10.09 20:32 >>>
>> >+	if (!pat_enabled) {
>> >+		void __iomem *ret = ioremap_nocache(phys_addr, size);
>> >+		if (ret)
>> >+			mtrr_add(phys_addr, size, MTRR_TYPE_WRCOMB, false);
>> 
>> This won't work if phys_addr or size aren't page aligned, or if size isn't
>> a power of two.
>
>No, at least the comments in mtrr_add and mtrr_check state that it is just 
>required that phys_addr and size are multiple of PAGE_SIZE. And I'm not sure 
>if it is always safe to round these up/down to the next PAGE boundary. If it 
>is not, maybe it is better to fail...

That function isn't the limiting factor, generic_validate_add_page() is
what you need to look at (plus the spec on how the MTRR ranges are
calculated by the CPU from the base/mask register pairs).

Jan

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