Write combining for PowerPC processors?

[Benjamin Herrenschmidt]

On Fri, 2004-12-10 at 13:21 -0800, Kendall Bennett wrote:
> Hi Guys,
> 
> 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.

> Does anyone know if it is possible to do something similar to Write 
> Combining for the PowerPC architecture, to speed up CPU access to the 
> linear framebuffer? Part of the problem is that for video overlay support 
> (not motion compensation) you have to dump the entire YUV frame into 
> video memory for the hardware overlay, and even on a 1GHz PPC box playing 
> an MPEG2 stream is not possible as X takes up over 80% of the CPU just to 
> copy the YUV data to video memory!

Use DMA :)

> Obviously bus mastering will help solve this problem, but it would be 
> better if there was a way to enabling faster CPU access to the 
> framebuffer as well.

Either use cacheable space, or look into your north bridge specs for
write combining capabilities.

Ben.