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

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Message-ID: <5412C909.50004@arm.com>
Date:	Fri, 12 Sep 2014 11:20:57 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Doug Anderson <dianders@...omium.org>
CC:	Stephen Boyd <sboyd@...eaurora.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>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Christopher Covington <cov@...eaurora.org>,
	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

On 12/09/14 01:01, Doug Anderson wrote:
> Stephen,
> 
> On Thu, Sep 11, 2014 at 4:56 PM, Stephen Boyd <sboyd@...eaurora.org> wrote:
>> On 09/11/14 10:43, Marc Zyngier wrote:
>>>>> 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.
>>
>> Where does this platform jump to when a CPU comes up? Is it
>> rockchip_secondary_startup()? I wonder if that path could have this
>> little bit of assembly to poke the cntvoff in monitor mode and then jump
>> to secondary_startup()? Before we boot any secondary CPUs we could also
>> read the cntvoff for CPU0 in the platform specific layer (where we know
>> we're running in secure mode) and then use that value as the "reset"
>> value for the secondaries. Or does this platform boot up in secure mode
>> some times and non-secure mode other times?
> 
> I guess it would depend a whole lot on the bootloader, wouldn't it?

Yes, hence my suggestion of hooking such a thing from the timer code,
where we could have a clue what to do (and what not to).

> With our current "get out of the way" bootloader, Linux always sees
> "Secure SVC".  ...but if someone decided to put a new bootloader on
> the system that wanted to do something different (implement security
> and boot the kernel in nonsecure HYP or implement a hypervisor and
> boot the kernel in nonsecure SVC) then everything would be different.
> 
> If someone were to write a bootloader like that (or perhaps if we're
> running in a VM?) then I'd imagine that the whole world would be
> different.  Somehow this secure bootloader and/or hypervisor would
> _have_ to be involved in processor bringup and suspend/resume.  Since
> I've never looked at code implementing either of these I'm just making
> assumptions, though.

Exactly. That's why we're so hell bent on PSCI, because it solves all
these issues (and that's why I've added some rudimentary support for it
in u-boot).

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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