[U-Boot] [PATCH] arm: pxa: PXA270 D-Cache as ram

[Sergey Yanovich]

Dear Marek Vasut,

On Tue, 2013-05-21 at 21:02 +0200, Marek Vasut wrote:
> > The whole 256 kB of SRAM could be used for persistent storage with the
> > patch. Without it, part of SRAM should be dedicated for U-Boot stack or
> > be overwritten on boot.
> 
> This won't hold on any PXA that uses SPL, like the vpac270 with OneNAND SPL and 
> PXA3xx (which is out of tree, none of your concern ;-) )

I am no way trying to enforce D-Cache as RAM. The patch just provides an
option for those who needs it.

> > You are right. After SDRAM is configured, it is enough to turn on data
> > caching to receive its speed benefits.
> 
> You must make sure anything that uses DMA won't crash.

I wasn't sure why data cache is disabled in U-Boot for every board I saw
configuration of. Thanks for pointing out.

> But I don't understand 
> how locking cachelines as RAM and enabling dcache relate to each other in this 
> context.

I meant D-Cache is several times faster than SDRAM, so it may be
possible to get a bit faster, if stack remains on D-Cache even after
SDRAM is configured. Repeating my hedge, I don't see the full picture,
yet. It may well be impossible (if U-Boot needs more than 32 kB of
stack) or not worth the effort (if the gain is too small).