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

Russell King - ARM Linux
2011-01-25T13:04:22Z

On Tue, Jan 25, 2011 at 12:56:56PM +0000, Russell King - ARM Linux wrote:
> 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.

Actually, we can do this safely - byte stores are permitted to SCU
registers probably for this very reason.