[PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer

Doug Anderson dianders at chromium.org
Thu Sep 11 17:01:11 PDT 2014


On Thu, Sep 11, 2014 at 4:56 PM, Stephen Boyd <sboyd at codeaurora.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?

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.


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