linux-kernel - Re: [PATCH 0/2] Allwinner A64 timer workaround

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Message-ID: <CAGb2v67NvccReOk1OMBpUrr+A8i_f7swc23_Ko8rxXr_tJ6HUg@mail.gmail.com>
Date:   Wed, 4 Jul 2018 16:19:39 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Samuel Holland <samuel@...lland.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        Mark Rutland <Mark.Rutland@....com>
Subject: Re: [PATCH 0/2] Allwinner A64 timer workaround

On Wed, Jul 4, 2018 at 4:16 PM, Marc Zyngier <marc.zyngier@....com> wrote:
> On 03/07/18 19:42, Samuel Holland wrote:
>> On 07/03/18 10:09, Marc Zyngier wrote:
>>> On 11/05/18 03:27, Samuel Holland wrote:
>>>> Hello,
>>>>
>>>> Several people (including me) have experienced extremely large system
>>>> clock jumps on their A64-based devices, apparently due to the architectural
>>>> timer going backward, which is interpreted by Linux as the timer wrapping
>>>> around after 2^56 cycles.
>>>>
>>>> Investigation led to discovery of some obvious problems with this SoC's
>>>> architectural timer, and this patch series introduces what I believe is
>>>> the simplest workaround. More details are in the commit message for patch
>>>> 1. Patch 2 simply enables the workaround in the device tree.
>>>
>>> What's the deal with this series? There was a couple of nits to address, and
>>> I was more or less expecting a v2.
>>
>> I got reports that people were still occasionally having clock jumps after
>> applying this series, so I wanted to attempt a more complete fix, but I haven't
>> had time to do any deeper investigation. I think this series is still beneficial
>> even if it's not a complete solution, so I'll come back with another patch on
>> top of this if/once I get it fully fixed.
>>
>> I'll prepare a v2 with a bounded loop. Presumably, 3 * (max CPU Hz) / (24MHz
>> timer) ≈ 150 should be a conservative iteration limit?
>
> Should be OK.
>
> Maxime: How do you want to deal with the documentation aspect? We need
> an erratum number, but AFAIU the concept hasn't made it into the silicom
> vendor's brain yet. Any chance you could come up with something that
> uniquely identifies this?
>
>> Also, does this make sense to CC to stable?
>
> Probably not, as the HW never worked, so it is not a regression.

A64 support has been in for a few releases now. So while this is not
fixing a regression, people will still benefit from it being in stable.

ChenYu