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

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Date:   Wed, 4 Jul 2018 10:41:16 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Marc Zyngier <marc.zyngier@....com>,
        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 04/07/2018 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?

I went through the different pointers provided in the description but I
did not find a clear statement that is a hardware issue or may be I
missed it.

Are we sure there isn't another subsystem responsible on this
instability ? (eg PM or something else)

>> Also, does this make sense to CC to stable?
> 
> Probably not, as the HW never worked, so it is not a regression.
> 
> Thanks,
> 
> 	M.
> 


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