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

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Message-ID: <bdfb1a31-2c4a-d258-7bf6-aaab8cee4663@sholland.org>
Date:   Tue, 3 Jul 2018 13:42:33 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Marc Zyngier <marc.zyngier@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        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 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?

Also, does this make sense to CC to stable?

Thanks,
Samuel