Re: [PATCH 4/5] ARM: scu: Move register defines to header file

Russell King - ARM Linux
2011-01-25T12:57:30Z

On Tue, Jan 25, 2011 at 06:06:27PM +0530, Santosh Shilimkar wrote:
> Ok. I missed some information my last email.
> The SCU power status programming is used to take CPU in/out
> of coherency as an alternative to SMP bit. We don't
> have an access to SMP bit on OMAP4. ARM has already
> confirmed SCU programming is same as SMP bit enable/disable.
>
> I don't know how safe is to use spin lock when one CPU is
> goes out of coherency after programming the power state. The
> spin lock release may not even be visible to other CPU.

Erm, I do hope that's not the case, as that means it is unsafe for CPUs in
a SMP system to write to this register without them potentially trampling
over each other.

If it is the case, then the solutions are either:
1. Fix the hardware so that coherency requests only yet turned off
when entering the WFI state.
2. Fix the hardware such that each CPU has a separate register.

I can't see a software solution to this as we can't use ldrex/strex anything
but memory regions, and memory regions without coherency won't work.
Maybe ARM Support can help by suggesting how a 4-CPU system is supposed
to safely read/modify/write the SCU power control register...