Kernel configuration options
wmb at laptop.org
Wed Jan 2 13:01:36 EST 2008
John Richard Moser wrote:
> Bernardo Innocenti wrote:
>> Tom Sylla wrote:
>>> msr: 0000.1810 fdfff000.fd000111. \ Video (write through), fbsize
>>> which is setting the framebuffer as write-combining. (the "write
>>> through" comment is incorrect)
>> This takes care of the physical mapping, but how would userspace
>> be able to mmap the framebuffer into virtual memory without
>> additional MMU programming?
>> I was under the impression that we also need to cover the whole
>> region with small 4KB MMU pages. This degrades performance
>> somewhat due to TLB misses when the CPU accesses the framebuffer.
> I missed whether or not the Geode actually has 4MiB huge pages, I
> thought someone said it does. This being the case, why can't you access
> the 16 (or was it 24?) MiB of memory via a handful (about 1/1024th) of
> huge mappings? Does x86 MMU not allow for huge MMIO?
> The Geode GX has 64 TLB entries right? I don't know how many the Geode
> LX has, or if there's an L2 TLB. Obviously, though, this would be a
> major performance boon, what with there being (assuming 24MiB of vram) 5
> probably often used mappings instead of 5120 in an often-used set with a
> probably uneven distribution.
The magnitude of the performance benefit is not at all obvious. The
Geode's graphics accelerator uses physical addressing.
>> But I must confess I have limited understanding of the Geode
>> architecture, so I may be overlooking something.
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