From: "Maciej W. Rozycki" <macro@orcam.me.uk>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
"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: Fri, 12 Feb 2021 00:09:20 +0100 (CET) [thread overview]
Message-ID: <alpine.DEB.2.21.2102120001450.35623@angie.orcam.me.uk> (raw)
In-Reply-To: <87wnvrbmqx.fsf@nanos.tec.linutronix.de>
On Mon, 1 Feb 2021, Thomas Gleixner 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
>
> Yeah, we can do that as well. Either way is fine.
Given that evidently vendors appear to start playing with 146818 clones
it may be worth it to peek at the D and the C register and checking they
are not 0xff both at a time for robustness before concluding no RTC is
present. The C register is supposed to hold zeros in bits 3:0. A read of
the C register will drop interrupt bits, but I guess it does not matter at
the probe time.
FWIW,
Maciej
next prev parent reply other threads:[~2021-02-11 23:17 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-06 21:46 [patch 0/8] ntp/rtc: Fixes and cleanups Thomas Gleixner
2020-12-06 21:46 ` [patch 1/8] rtc: mc146818: Prevent reading garbage Thomas Gleixner
2021-01-25 18:40 ` [patch 1/8] rtc: mc146818: Prevent reading garbage - bug Mickaël Salaün
2021-01-26 13:26 ` Thomas Gleixner
2021-01-26 14:17 ` Mickaël Salaün
2021-01-26 15:35 ` Thomas Gleixner
2021-01-26 17:02 ` [PATCH V2] rtc: mc146818: Detect and handle broken RTCs Thomas Gleixner
2021-01-26 18:00 ` Alexandre Belloni
2021-01-31 10:59 ` Dirk Gouders
2021-02-01 13:53 ` Serge Belyshev
2021-02-01 19:09 ` [PATCH] rtc: mc146818: Dont test for bit 0-5 in Register D Thomas Gleixner
2021-02-01 19:24 ` [PATCH V2] " Thomas Gleixner
2021-02-01 19:32 ` Linus Torvalds
2021-02-01 19:40 ` Thomas Gleixner
2021-02-11 23:09 ` Maciej W. Rozycki [this message]
2021-02-01 19:38 ` Alexandre Belloni
2021-02-01 20:09 ` Borislav Petkov
2021-02-01 20:15 ` Dirk Gouders
2021-02-02 4:22 ` Len Brown
2021-02-03 13:00 ` Mickaël Salaün
2020-12-06 21:46 ` [patch 2/8] rtc: mc146818: Reduce spinlock section in mc146818_set_time() Thomas Gleixner
2020-12-06 21:46 ` [patch 3/8] rtc: cmos: Make rtc_cmos sync offset correct Thomas Gleixner
2020-12-07 20:50 ` Jason Gunthorpe
2020-12-06 21:46 ` [patch 4/8] rtc: core: Make the sync offset default more realistic Thomas Gleixner
2020-12-07 20:55 ` Jason Gunthorpe
2020-12-10 23:59 ` Alexandre Belloni
2020-12-11 0:23 ` Thomas Gleixner
2020-12-11 0:28 ` Alexandre Belloni
2020-12-06 21:46 ` [patch 5/8] ntp: Make the RTC synchronization more reliable Thomas Gleixner
2020-12-07 12:47 ` Miroslav Lichvar
2020-12-07 20:59 ` Jason Gunthorpe
2020-12-29 19:41 ` Geert Uytterhoeven
2021-01-11 10:12 ` Thomas Gleixner
2021-01-11 10:40 ` Geert Uytterhoeven
2020-12-06 21:46 ` [patch 6/8] ntp, rtc: Move rtc_set_ntp_time() to ntp code Thomas Gleixner
2020-12-07 20:59 ` Jason Gunthorpe
2020-12-09 3:51 ` Alexandre Belloni
2020-12-06 21:46 ` [patch 7/8] ntp: Make the RTC sync offset less obscure Thomas Gleixner
2020-12-06 21:46 ` [patch 8/8] ntp: Consolidate the RTC update implementation Thomas Gleixner
2020-12-07 21:05 ` Jason Gunthorpe
2020-12-11 9:23 ` Thomas Gleixner
2020-12-09 0:33 ` [patch 0/8] ntp/rtc: Fixes and cleanups Thomas Gleixner
2020-12-09 4:01 ` Alexandre Belloni
2020-12-09 12:39 ` Thomas Gleixner
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