Re: [PATCH V2] rtc: mc146818: Dont test for bit 0-5 in Register D

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Serge Belyshev" <belyshev@depni.sinp.msu.ru>,
	"Dirk Gouders" <dirk@gouders.net>,
	"Mickaël Salaün" <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>
Subject: Re: [PATCH V2] rtc: mc146818: Dont test for bit 0-5 in Register D
Date: Mon, 1 Feb 2021 11:32:22 -0800	[thread overview]
Message-ID: <CAHk-=wg_-_FP+B6ePabvj55_ok1YbYCsGHzYsZ064FpE4RqkTQ@mail.gmail.com> (raw)
In-Reply-To: <87zh0nbnha.fsf@nanos.tec.linutronix.de>

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