linux-kernel - Re: [PATCH v4] clocksource: arch_timer: Fix code to use physical timers when requested

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Date:	Thu, 20 Nov 2014 08:24:09 -0800
From:	Doug Anderson <dianders@...omium.org>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Sonny Rao <sonnyrao@...omium.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Olof Johansson <olof@...om.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	Russell King <linux@....linux.org.uk>,
	Mark Rutland <Mark.Rutland@....com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v4] clocksource: arch_timer: Fix code to use physical
 timers when requested

Catalin,

On Thu, Nov 20, 2014 at 8:10 AM, Catalin Marinas
<catalin.marinas@....com> wrote:
> On Wed, Oct 08, 2014 at 08:38:57AM +0100, Sonny Rao wrote:
>> This is a bug fix for using physical arch timers when
>> the arch_timer_use_virtual boolean is false.  It restores the
>> arch_counter_get_cntpct() function after removal in
>>
>> 0d651e4e "clocksource: arch_timer: use virtual counters"
>>
>> We need this on certain ARMv7 systems which are architected like this:
>>
>> * The firmware doesn't know and doesn't care about hypervisor mode and
>>   we don't want to add the complexity of hypervisor there.
>>
>> * The firmware isn't involved in SMP bringup or resume.
>>
>> * The ARCH timer come up with an uninitialized offset between the
>>   virtual and physical counters.  Each core gets a different random
>>   offset.
>>
>> * The device boots in "Secure SVC" mode.
>>
>> * Nothing has touched the reset value of CNTHCTL.PL1PCEN or
>>   CNTHCTL.PL1PCTEN (both default to 1 at reset)
>>
>> One example of such as system is RK3288 where it is much simpler to
>> use the physical counter since there's nobody managing the offset and
>> each time a core goes down and comes back up it will get reinitialized
>> to some other random value.
>>
>> Fixes: 0d651e4e65e9 ("clocksource: arch_timer: use virtual counters")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Sonny Rao <sonnyrao@...omium.org>
>> Acked-by: Olof Johansson <olof@...om.net>
> [...]
>> --- a/arch/arm64/include/asm/arch_timer.h
>> +++ b/arch/arm64/include/asm/arch_timer.h
>> @@ -135,6 +135,16 @@ static inline void arch_timer_evtstrm_enable(int divider)
>>  #endif
>>  }
>>
>> +static inline u64 arch_counter_get_cntpct(void)
>> +{
>> +     u64 cval;
>> +
>> +     isb();
>> +     asm volatile("mrs %0, cntpct_el0" : "=r" (cval));
>> +
>> +     return cval;
>> +}
>
> Sorry but I have to NAK the arm64 changes here. If the firmware is
> broken and does not initialise CNTVOFF properly, please fix it (at least
> on ARMv8 hardware). Also, on arm64 the vdso gettimeofday()
> implementation relies on using the virtual counter, so correct
> initialisation of CNTVOFF is essential.

Sonny's patch here just makes it so that we honor the global variable.
My patch at <https://patchwork.kernel.org/patch/5051881/> is the one
that allows the global variable to be set.  You can see in that patch
that it's impossible for the variable to be set on ARM64.

In previous discussions it was agreed that on ARM64 psci (or something
similar) was a requirement anyway and that gave us a way to get the
firmware involved again if we ever need to bring down a processor and
bring it back up in the kernel.  PSCI is not a requirement for ARM32.
There are systems that don't get the firmware involved when a
processor loses state (like if it is powered off and powered on again,
maybe for suspend/resume) and there was pushback against the kernel
itself transitioning into monitor mode to init CNTVOFF in these cases.
People agreed a month ago that these two patches were a reasonable
approach for ARM32.

There is still a question about the ARM32 vdso gettimeofday().  That
will need to get resolved eventually to, but that should be a separate
patch I believe.

NOTE: a firmware which actually implements a monitor and somehow
solves the S2R problems could modify the device tree passed to the
kernel to remove the "arm,cpu-registers-not-fw-configured" which would
switch us back into virtual timers.

-Doug
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