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

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Message-ID: <54AFE775.3090209@gmail.com>
Date:	Fri, 09 Jan 2015 17:36:37 +0300
From:	Dmitry Osipenko <digetx@...il.com>
To:	Peter De Schrijver <pdeschrijver@...dia.com>,
	Thierry Reding <thierry.reding@...il.com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	linux-arm-kernel@...ts.infradead.org,
	Alexandre Courbot <gnurou@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Sekhar Nori <nsekhar@...com>, linux-kernel@...r.kernel.org,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	stable@...r.kernel.org, Joseph Lo <josephl@...dia.com>,
	linux-tegra@...r.kernel.org, Shawn Guo <shawn.guo@...aro.org>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH] ARM: tegra: Use PMC scratch register 40 for tegra_resume()
 location store

09.01.2015 13:29, Peter De Schrijver пишет:
> On Fri, Jan 09, 2015 at 10:51:35AM +0100, Thierry Reding wrote:
>> * PGP Signed by an unknown key
>>
>> 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:
>>>>> Old 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 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.
>>>>
>>>
>>> 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?
> 
> No. I can't think of a reason this flag could not be in IRAM.
> 
> Cheers,
> 
> Peter.
> 

IRAM is a good candidate. Just tested draft patch version in QEMU, will give it
more testing and polish during weekend and send next week.

-- 
Dmitry
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