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

[Christopher Covington <cov@codeaurora.org>]

Hi Sonny,

On 09/15/2014 06:04 PM, Sonny Rao wrote:
> On Mon, Sep 15, 2014 at 2:52 PM, Sonny Rao <sonnyrao@chromium.org> wrote:
>> On Mon, Sep 15, 2014 at 2:49 PM, Stephen Boyd <sboyd@codeaurora.org> wrote:
>>> On 09/15/14 14:47, Sonny Rao wrote:
>>>> On Mon, Sep 15, 2014 at 1:33 PM, Stephen Boyd <sboyd@codeaurora.org> wrote:
>>>>> On 09/15/14 04:10, Catalin Marinas wrote:
>>>>>> On Fri, Sep 12, 2014 at 07:59:29PM +0100, Stephen Boyd wrote:
>>>>>>> On 09/12/14 05:14, Marc Zyngier wrote:
>>>>>>>> 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
>>>>>> I think the context was slightly different. As I re-read the thread, it
>>>>>> seems that the discussion was around whether to use some SMC interface
>>>>>> or not based on whether the kernel is running secure or non-secure. The
>>>>>> argument made by Will was to actually specify the type of the firmware
>>>>>> SMC interface in the DT and use it in the kernel (and probably assume
>>>>>> the kernel is running in secure mode if no smc interface is specified in
>>>>>> the DT; you could have both though, running in secure mode and also
>>>>>> having firmware).
>>>>>>
>>>>>> In this arch timer case, we need to work around a firmware bug (or
>>>>>> feature as 32-bit ARM kernels never required CNTVOFF initialisation by
>>>>>> firmware, no matter how small such firmware is). We don't expect a
>>>>>> specific SMC call to initialise CNTVOFF, so we can't describe it in the
>>>>>> DT.
>>>>> Agreed, we can't described SMC calls that don't exist. From my
>>>>> perspective it's just another part of the cpu boot sequence that needs
>>>>> to be handled in the kernel, so describing the requirement via the
>>>>> cpu-boot method seems appropriate. It seems like we're making it harder
>>>>> than it should be by handling the undef when we could have slightly
>>>>> different SMP boot code (and suspend/resume code) depending on the boot
>>>>> method property.
>>>>
>>>> +heiko
>>>>
>>>> So, for the case of rk3288, based on this discussion what I'm going to
>>>> propose is to add code to rockchip.c which looks for a particular SMP
>>>> enable method -- say something like "rockchip,rk3288-smp-secure-svc"
>>>> which will then assume we have been booted in secure SVC mode and do
>>>> the CNTVOFF fixup.  I believe, it will need to do this on the boot CPU
>>>> as well, so I think it will need to scan the DT fairly early on the
>>>> boot CPU and also perform the function there.
>>>>
>>>> I'll look into implementing this and post code.  Comments and
>>>> suggestions appreciated, thanks.
>>>
>>> What goes wrong if we read the cntvoff from the boot CPU during
>>> smp_prepare_cpus() phase and use that to set the cntvoff on the other
>>> CPUs? That avoids needing to do anything very early by making the value
>>> the same. It does mean that cntvoff is some random out of reset value
>>> for CPU0, but at least it's consistent.
>>
>> I think we cannot read the value if we're not in hyp mode.
> 
> Well, thinking about it a little more, I think you still have a good point.
> 
> We don't need to do this early on, as long as we haven't started using
> the arch timers yet.  If we are still able to do this at the point
> where we're executing the code in arch/arm/mach-rockchip/platsmp.c
> that finds the enable method then we can just handle it there.

I've been playing around with the probe-based approach and while I need to do
a lot more testing, it seems to be working for the first tens of instructions.
I hope to be able to share a draft of that soon. Basically, I just read the
current NSACR value and write it back (although maybe in the long term we
would want to make sure a few of those bits are set or cleared). If that
succeeds, we know we're in secure SVC and can proceed to set up MON and HYP.

Christopher

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