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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 1 Feb 2021 11:32:22 -0800
Message-ID: <CAHk-=wg_-_FP+B6ePabvj55_ok1YbYCsGHzYsZ064FpE4RqkTQ@mail.gmail.com>
Subject: Re: [PATCH V2] rtc: mc146818: Dont test for bit 0-5 in Register D
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Serge Belyshev <belyshev@depni.sinp.msu.ru>,
        Dirk Gouders <dirk@gouders.net>,
        =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@digikod.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Miroslav Lichvar <mlichvar@redhat.com>,
        John Stultz <john.stultz@linaro.org>,
        Prarit Bhargava <prarit@redhat.com>,
        Alessandro Zummo <a.zummo@towertech.it>,
        linux-rtc@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>
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On Mon, Feb 1, 2021 at 11:24 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> While it cures the problem on the reporters machine it breaks machines
> with Intel chipsets which use bit 0-5 of the D register. So check only
> for bit 6 being 0 which is the case on these Intel machines as well.

This looks fine, but it might also be worth it simply just checking
for the only really special value: 0xff, and going "ok, that looks
like missing hardware".

That's what a few other drivers historically do in their probing
routines, so it's not unheard of (ie you can find drivers doing that
kind of

        /* If we read 0xff from the LSR, there is no UART here. */
        if (inb(.. port ..) == 0xff)

in their init routines.

Not a big deal either way, I just think it would be more in like with
what other places do in similar situations

      Linus