linux-kernel - Re: [PATCH] ARM: tegra: Use PMC scratch register 40 for tegra

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Date:	Thu, 8 Jan 2015 11:57:43 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Dmitry Osipenko <digetx@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Joseph Lo <josephl@...dia.com>, stable@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Sekhar Nori <nsekhar@...com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: tegra: Use PMC scratch register 40 for
 tegra_resume() location store

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 пишет:
> >>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.

Changing the storage location for tegra_resume() isn't such a good idea
since that's a documented use of PMC_SCRATCH41.

Thierry

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