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

[Marek Vasut]

Dear 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.

I'd love to have a uniform way to do this cache thing, really ...

> > > 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.

Not really, enabling dcache altogether and marking DRAM region as cached would 
be much better.

> 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)

No way.

> or not worth the effort (if the gain is too small).

The larger gain would be from fixing the U-Boot drivers for PXA to work well 
with DCache ;-) Then the speedup would really be plenty significant, this can be 
well confirmed on many other ARM chips.

Best regards,
Marek Vasut