linux-kernel - Re: [PATCH 12/16] clocksource: Add clock driver for RDA8810PL SoC

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Message-ID: <e732ad15-b778-1c84-ad47-5770ee66688b@arm.com>
Date:   Tue, 20 Nov 2018 11:05:41 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Amit Kucheria <amit.kucheria@...aro.org>, zhao_steven@....net,
        Andreas Färber <afaerber@...e.de>
Subject: Re: [PATCH 12/16] clocksource: Add clock driver for RDA8810PL SoC

On 20/11/2018 08:56, Linus Walleij wrote:
> On Tue, Nov 20, 2018 at 9:17 AM Marc Zyngier <marc.zyngier@....com> wrote:
> 
>> How does this change anything with the fact that the above code is
>> broken? 56 or 64 bit, you cannot read this counter with a single
>> access, or two. The canonical way of reading such a counter is
>> something like this:
>>
>>         do {
>>                 lo = readl_relaxed(LO);
>>                 hi = readl_relaxed(HI);
>>         } while (hi != read_relaxed(HI));
> 
> To be fair, I have seen hardware that employ a logic latch
> such that when a read access is done to the LO register,
> the value of the whole counter is latched, also for the HI
> register, so when you read the HI register in the second
> step, it is never subject to wrapping. (Conversely reading
> the HI before the LO will always give you insane values
> :D)

I've seen such HW indeed, and I've also seen it being broken... ;-)

It this timer is built around such a (non-broken) logic, I'd really like
to see it spelled out. It will otherwise be a real pain to debug...

> However the above code should be fine unless you know
> for sure the hardware was constructed with a clever latch.

Let's find out!

Thanks,

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