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

Marek Vasut
2013-05-21T11:38:51Z

Dear Sergey Yanovich,

> Dear Marek Vasut,
>
> On Tue, 2013-05-21 at 12:54 +0200, Marek Vasut wrote:
> > SRAM is just the in-CPU bit of fast RAM. What do you mean by
> > "battery-backup" ?
>
> Yes, you are right. It is 'for high speed code or data storage preserved
> during low-power states' using a quote from PXA270 EMTS (top of page 1).
> Battery-backup is optional. I mixed PXA270 and LP-8x4x specs.

Yes, it's just an in-CPU RAM.

> > > It can be used as a ultra-fast persistent
> > > storage in OS. Wasting a quater of it just to boot the system isn't the
> > > best choice.
> >
> > The SRAM is used for stack in U-Boot until you leave board_init_f, then
> > the stack is relocated to DRAM. The OS can use SRAM as needed, U-Boot is
> > no longer operational once you load subsequent OS. What's the problem?
>
> Anyway, SRAM preserves its state when power is off. Poweroff time could
> be in years with a backup battery. In addition, D-Cache is an order of
> magnitude faster than SRAM (approx. 9 times) for both reads and writes.

Is there any measurable difference between using DCache and SRAM? Do you have
any evidence that a speedup happens?

Still, the SRAM/DCache is only used until you leave board_init_f(), then it's
all DRAM.

Best regards,
Marek Vasut