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

[Sonny Rao]

On Mon, Sep 15, 2014 at 2:49 PM, Stephen Boyd <sboyd@xxxxxxxxxxxxxx> wrote:
> On 09/15/14 14:47, Sonny Rao wrote:
>> On Mon, Sep 15, 2014 at 1:33 PM, Stephen Boyd <sboyd@xxxxxxxxxxxxxx> 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.


>
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