Re: [PATCH] ARM: tegra: Use PMC scratch register 40 for tegra_resume() location store

[Stephen Warren]

On 12/22/2014 10:27 AM, Dmitry Osipenko wrote:
> 22.12.2014 19:17, Stephen Warren ÐÐÑÐÑ:
> > On 12/21/2014 03:52 PM, Dmitry Osipenko wrote:
> > > Commit 7232398abc6a ("ARM: tegra: Convert PMC to a driver") changed
> > > tegra_resume()
> > > location storing from late to early and as result broke suspend on tegra20.
> > > PMC scratch register 41 was used by tegra lp1 suspend core code for storing
> > > physical memory address of common resume function and in the same time used by
> > > tegra20 cpuidle driver for storing cpu1 "resettable" status, so it implied
> > > strict order of scratch register use. Fix it by using scratch 40 instead of 41
> > > for tegra_resume() location store.
> >
> > You likely can't simply change the PMC scratch register usage arbitrarily;
> > specific registers are designated for specific purposes, and code outside the
> > Linux kernel (bootloaders, LP0 resume code, secure monitors, etc.) may depend on
> > those specific values being in those registers. Without significant research,
> > I'd suggest not changing the PMC scratch register usage.
>
> Sure, that's why I asked to verify if scratch register 40 is in use in the
> comment after commit message.

Sorry, I didn't notice that.

> I've checked that u-boot doesn't use it (since
> upstream kernel doesn't care about any other bootloader), but no idea about
> secure monitor. It's definitely safer to avoid changing scratch regs usage, I
> thought that proposed solution would be best from the pure code point of view.
> So, I'm considering your answer as a rejection of the patch (please, let me know
> if I'm wrong) and will prepare another one. Btw, it would be nice to have
> scratch registers usage publicly documented somewhere (on "Tegra Public
> Application Notes" webpage for example), if it's possible, of course.

At this stage in Tegra20 development, I think it'd be best to avoid 
changing any scratch register usage if at all possible.

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