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

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Message-ID: <1d75c538-9166-5eec-a08c-b168f9770bd1@arm.com>
Date:   Wed, 4 Jul 2018 09:39:11 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Samuel Holland <samuel@...lland.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sunxi@...glegroups.com, Mark Rutland <Mark.Rutland@....com>
Subject: Re: [PATCH 0/2] Allwinner A64 timer workaround

On 04/07/18 09:23, Daniel Lezcano wrote:
> On 04/07/2018 10:16, Marc Zyngier 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.
> 
> If the patches fix a bug which already exist, it makes sense to
> propagated the fix back to the stable versions.
That's your call, but I'm not supportive of that decision, specially as
we have information from the person developing the workaround that this
doesn't fully address the issue.

	M.
-- 
Jazz is not dead. It just smells funny...