[patch] VRAM detection in controlfb
Timothy A. Seufert
tas at mindspring.com
Tue Jun 6 15:15:55 EST 2000
At 4:15 PM +0200 6/5/00, Geert Uytterhoeven wrote:
>On Mon, 5 Jun 2000, Michel Lanners wrote:
>> So I guess the optimum would be to mark the VRAM cacheable, but in
>>a way that
>> writes don't go into the cache. Would that be write-through?
>That's indeed write-through. Note that writes will still be cached in such a
>way that a consecutive read from the same location will return the cached
>value. But writes will immediately be sent to the host bridge.
Unfortunately a side effect of write-through mode is that it prevents
burst writes. Memory must be cacheable in copy-back mode for
existing PowerPC CPUs to perform burst writes.
Many Intel CPUs implement write combining for precisely this region.
In a memory region marked as OK for write combining, the CPU is free
to wait and collect data from several writes to perform a burst write
rather than performing a write immediately.
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