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

[Stephen Boyd <sboyd-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 09/12/14 05:14, Marc Zyngier wrote:
> Hi Christopher,
>
> On 12/09/14 12:43, Christopher Covington wrote:
>> Hi Marc,
>>
>> On 09/11/2014 01:43 PM, Marc Zyngier wrote:
>>> On 11/09/14 18:29, Doug Anderson wrote:
>>>
>>>> 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?
> We surely can handle the UNDEF and do something there. We just can't do
> it the way Doug described it above.
>
>

I suggested doing that for something else a while ago and Will and Dave
we're not thrilled[1]. The suggestion back then was to use DT to
indicate what mode the kernel is running in.

[1]
http://lists.infradead.org/pipermail/linux-arm-kernel/2012-June/105321.html

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

--
To unsubscribe from this list: send the line "unsubscribe devicetree" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html