linux-kernel - Re: [PATCH v1 1/1] arm64: Early boot time stamps

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Date:   Tue, 20 Nov 2018 14:55:23 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     catalin.marinas@....com, will.deacon@....com,
        Andrew Morton <akpm@...ux-foundation.org>,
        rppt@...ux.vnet.ibm.com, Michal Hocko <mhocko@...e.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        andrew.murray@....com, james.morse@....com, sboyd@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/1] arm64: Early boot time stamps

On 20/11/2018 14:40, Pavel Tatashin wrote:
>>> +static __init void sched_clock_early_init(void)
>>> +{
>>> +     u64 freq = arch_timer_get_cntfrq();
>>> +     u64 (*read_time)(void) = arch_counter_get_cntvct;
>>
>> We already have arch_timer_read_counter which is exposed from
>> arm_arch_timer.h.
> 
> OK
> 
>>
>>> +
>>> +     /* Early clock is available only on platforms with known freqs */
>>
>> This comment is misleading. It should read something like:
>>
>>         /*
>>          * The arm64 boot protocol mandates that CNTFRQ_EL0 reflects
>>          * the timer frequency. To avoid breakage on misconfigured
>>          * systems, do not register the early sched_clock if the
>>          * programmed value if zero. Other random values will just
>>          * result in random output.
>>          */
>>
> 
> OK
> 
>>> +     if (!freq)
>>> +             return;
>>> +
>>> +     sched_clock_register(read_time, BITS_PER_LONG, freq);
>>
>> This doesn't seem right. The counter has an architected minimum of 56
>> bits, and you can't assume that it is going to be more than that.
> 
> Yeah, I saw 56 is used in arm_arch_timer.c, but I could not find where
> this minimum is defined in aarch64 specs. I will change it to 56.

See D10.1.2 (The system counter) in the ARM ARM[1], which says:

<quote>
The Generic Timer provides a system counter with the following
specification:
Width	At least 56 bits wide.
	The value returned by any 64-bit read of the counter is
	zero-extended to 64 bits.
</quote>

This is not AArch64-specific though, and 32bit systems implementing the
generic timers have the exact same requirements.

> I will send v2 soon.

Please wait a bit and give others a chance to review this too.

Thanks,

	M.

[1] https://static.docs.arm.com/ddi0487/da/DDI0487D_a_armv8_arm.pdf
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