linux-kernel - Re: [PATCH RESEND 2/6] rtc-cmos: dont touch alarm registers during update

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Message-ID: <YW0NGiZDhZf2RrjN@kroah.com>
Date:   Mon, 18 Oct 2021 07:58:50 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mateusz Jończyk <mat.jonczyk@...pl>
Cc:     linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH RESEND 2/6] rtc-cmos: dont touch alarm registers during
 update

On Sun, Oct 17, 2021 at 09:39:23PM +0200, Mateusz Jończyk wrote:
> Some Intel chipsets disconnect the time and date RTC registers when the
> clock update is in progress: during this time reads may return bogus
> values and writes fail silently. This includes the RTC alarm registers.
> [1]
> 
> cmos_read_alarm() and cmos_set_alarm() did not take account for that,
> which caused alarm time reads to sometimes return bogus values. This can
> be shown with a test patch that I am attaching to this patch series.
> Setting the alarm clock also probably did fail sometimes.
> 
> To make this patch suitable for inclusion in stable kernels, I'm using a
> simple method for avoiding the RTC update cycle. This method is used in
> mach_set_rtc_mmss() in arch/x86/kernel/rtc.c. A more elaborate algorithm
> - as in mc146818_get_time() in drivers/rtc/rtc-mc146818-lib.c - would be
> too complcated for stable. [2]

No, just do it properly the first time, do not worry about stable
kernels, we can just take the also-correct version for backporting if
needed.

thanks,

greg k-h