linux-kernel - Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer

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Date:	Fri, 12 Sep 2014 07:43:04 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Marc Zyngier <marc.zyngier@....com>
CC:	Doug Anderson <dianders@...omium.org>,
	Will Deacon <Will.Deacon@....com>,
	"olof@...om.net" <olof@...om.net>,
	Sonny Rao <sonnyrao@...omium.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to
 specify the physical timer

Hi Marc,

On 09/11/2014 01:43 PM, Marc Zyngier wrote:
> On 11/09/14 18:29, Doug Anderson wrote:
>> Marc,
>>
>> On Thu, Sep 11, 2014 at 10:22 AM, Marc Zyngier <marc.zyngier@....com> wrote:
>>>> We would need to run this code potentially at processor bringup and
>>>> after suspend/resume, but that seems possible too.
>>>
>>> Note that this would be an ARMv7 only thing (you can't do that on ARMv8,
>>> at all).
>>
>> Yes, of course.
>>
>>
>>>> Is the transition to monitor mode and back simple?  Where would you
>>>> suggest putting this code?  It would definitely need to be pretty
>>>> early.  We'd also need to be able to detect that we're in Secure SVC
>>>> and not mess up anyone else who happened to boot in Non Secure SVC.
>>>
>>> This would have to live in some very early platform-specific code. The
>>> ugly part is that you cannot find out what world you're in (accessing
>>> SCR is going to send you to UNDEF-land if accessed from NS).
>>
>> Yup, so the question is: would such code be accepted upstream, or are
>> we going to embark on a big job for someone to figure out how to do
>> this only to get NAKed?
>>
>> If there was some indication that folks would take this, I think we
>> might be able to get it coded up.  If someone else wanted to volunteer
>> to code it that would make me even happier, but maybe that's pushing
>> my luck.  ;)
> 
> Writing the code is a 5 minute job. Getting it accepted is another
> story, and I'm not sure everyone would agree on that.
> 
>>> If I was suicidal, I'd suggest you could pass a parameter to the command
>>> line, interpreted by the timer code... But I since I'm not, let's
>>> pretend I haven't said anything... ;-)
>>
>> I did this in the past (again, see Sonny's thread), but didn't
>> consider myself knowledgeable to know if that was truly a good test:
>>
>>        asm volatile("mrc p15, 0, %0, c1, c1, 0" : "=r" (val));
>>        pr_info("DOUG: val is %#010x", val);
>>        val |= (1 << 2);
>>        asm volatile("mcr p15, 0, %0, c1, c1, 0" : : "r" (val));
>>        val = 0xffffffff;
>>        asm volatile("mrc p15, 0, %0, c1, c1, 0" : "=r" (val));
>>        pr_info("DOUG: val is %#010x", val);
>>
>> The idea being that if you can make modifications to the SCR register
>> (and see your changes take effect) then you must be in secure mode.
>> In my case the first printout was 0x0 and the second was 0x4.
> 
> The main issue is when you're *not* in secure mode. It is likely that
> this will explode badly. This is why I suggested something that is set
> by the bootloader (after all. it knows which mode it is booted in), and
> that the timer driver can use when the CPU comes up.

What exactly does "exploding badly" look like? Causing and undefined
instruction exception? That's just a branch with a mode switch. Any reason the
code couldn't deal with that or even use that to its advantage?

Thanks,
Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
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