Write combining for PowerPC processors?

[Kendall Bennett]

Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:

> > We are working on some PowerPC machines and noticed that the boxes don't 
> > appear to support the equivalent of Write Combining that we get on x86 
> > boxes. Copies to Video Memory on our Motorola Sandpoint box run about 
> > 10Mb/s, which is terribly, terribly slow!
> 
> Your north bridge should implement it (provided you don't eieio
> between every writes). If not, then the only way is to mark the
> space cacheable, but that means you have to deal with flushing it
> appropriately. 

Which would work if we flushed the CPU cache after we have compeleted all 
software rendering, right? We have to support that anyway as a lot of 
modern cards have write buffers that need to be flushed after software 
rendering is done, so we could put that the same place in the code.

How do we enable caching? Is that something that needs to be done in the 
northbridge also? 

I assume either caching or write combining would be something for the 
platform specific code in the Linux kernel, right?

Thanks!

---
Kendall Bennett
Chief Executive Officer
SciTech Software, Inc.
Phone: (530) 894 8400
http://www.scitechsoft.com

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