Kernel configuration options

Mitch Bradley wmb at laptop.org
Wed Jan 2 13:01:36 EST 2008


John Richard Moser wrote:
> Bernardo Innocenti wrote:
>   
>> Tom Sylla wrote:
>>
>>     
>>> http://openbios.org/viewvc/cpu/x86/pc/olpc/lxmsrs.fth?view=markup&revision=739&root=OpenFirmware
>>> has:
>>>   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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