[LinuxBIOS] Opteron caching of device memory

[Marc Jones]

Myles Watson wrote:
>> On 6/13/07, Myles Watson <myles at pel.cs.byu.edu> wrote:
>>     
>>> I'm using LinuxBIOS on my Tyan s2892.  I have a device that maps a lot
>>> of the memory space, but I'm struggling trying to get the Opteron to
>>> read and write to my device in larger blocks.  I have set the variable 
>>> MTRRs in the device driver to writeback (witnessed by /proc/mtrr), but I
>>>       
>
>   
>>> still get 64-bit accesses instead of 64-byte (cache line).
>>>       
>> wirte-back or write-combining?
>>
>> YH
>>     
>
> I did write-back, because I would like it to be treated as much like DRAM as
> possible.
>
> Myles
>
>
>
>   
Hi Myles,
I am not an Opteron expert but here are a few items you might want to check.

I assume that this is a PCI/E/X device. Is the device memory BAR and 
bridge memory set as prefetchable?
Check that you don't have any overlapping mtrrs. I think that they 
resolve to the most restrictive setting
Check the PAT setting for that memory. The memory page will resolve to 
the most restrictive setting. (Note I am not sure if Linux uses PAT)
See section 7.6 of the System Programmers Guide Volume 2. You can get 
here: 
http://www.amd.com/gb-uk/Processors/TechnicalResources/0,,30_182_739_7044,00.html

You might get some performance by caching the device memory space but I 
don't  think that it is the best use of the cache. If you are moving a 
lot memory between your device and system memory it is best that the 
driver DMA it in instead of reading it in with the CPU.

If you figure out the problem please let us know.

Thanks,
Marc


-- 
Marc Jones
Senior Software Engineer
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mailto:Marc.Jones at amd.com
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