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

[Thierry Reding]

On Thu, Jan 08, 2015 at 02:37:09PM +0200, Peter De Schrijver wrote:
> On Thu, Jan 08, 2015 at 11:57:43AM +0100, Thierry Reding wrote:
> > * PGP Signed by an unknown key
> > 
> > On Mon, Dec 22, 2014 at 11:00:16AM -0700, Stephen Warren wrote:
> > > 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.
> > 
> > Sorry, I had completely missed this discussion. When looking at the code
> > it doesn't look like this particular "resettable" status needs to be
> > stored in a PMC scratch register. It can't be stored in RAM because that
> > goes into self-refresh as part of LP1, but how about just putting it
> > into IRAM? That stays on in both LP1 and LP2, so should be suitable for
> > this use-case. It would make the code slightly more complex but using a
> > single scratch register for multiple purposes sounds brittle and easy to
> > break (as evidenced by the offending commit).
> > 
> > Otherwise it would seem that PMC_SCRATCH40 is only used to store EMC
> > configuration data across LP0 suspend/resume, so I wouldn't think it'd
> > cause problems if we used that instead of PMC_SCRATCH41 to store the
> > "resettable" state.
> > 
> 
> No. Usually the scratch registers for EMC config data are setup once by the
> bootloader and never touched by the kernel after that. So I would not
> recommend reusing those registers for different purposes.

Right, I misread the code in the downstream kernel. Though it's not the
bootloader that does it (at least on Tegra20), but some early code in
the kernel.

IRAM sounds like a good candidate still. Or do you know of anything that
would exclude IRAM as storage location for this data?

Thierry

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