linux-kernel - Re: [PATCH] alarmtimer: check RTC features instead of ops

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Date:   Fri, 30 Apr 2021 10:59:53 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] alarmtimer: check RTC features instead of ops

On Fri, Apr 30 2021 at 10:10, Alexandre Belloni wrote:
> On 30/04/2021 09:16:40+0200, Thomas Gleixner wrote:
>> On Thu, Apr 29 2021 at 23:49, Alexandre Belloni wrote:
>> > Test RTC_FEATURE_ALARM instead of relying on ops->set_alarm to know whether
>> > alarms are available.
>> >
>> > Fixes: 7ae41220ef58 ("rtc: introduce features bitfield")
>> > Signed-off-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
>> > ---
>> > Hello,
>> >
>> > This doesn't seem much but this solve an issue where following a change in the
>> > RTC driver, this part of the code will think the RTC is alarm capable while it
>> > is not, then breaking the alarmtimer functionnality.
>> 
>> So a driver has the set_alarm() callback but does not advertise
>> RTC_FEATURE_ALARM for whatever reason and why ever this makes sense.
>> 
>
> No, it would be the other way around. The issue happens when you have
> two RTCs, rtc0 is not alarm capable and rtc1 has alarms.
>
> The driver for rtc0 used to not have .set_alarm() to signal it didn't
> support alarms, it then switched to RTC_FEATURE_ALARM, making the
> alarmtimer code select that RTC instead of rtc1, breaking suspend/resume
> on the platform.

I'm even more confused. So RTC0 does not have .set_alarm() but why does
it turn on RTC_FEATURE_ALARM? I'm obviously misinterpreting the above...

Thanks,

        tglx