linux-kernel - Re: [PATCH] rtc: ds1307: set uie_unsupported if no interrupt is available

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Date:   Tue, 30 Mar 2021 08:52:08 +0200
From:   Lukasz Stelmach <l.stelmach@...sung.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>, linux-rtc@...r.kernel.org,
        Bartłomiej Żolnierkiewicz 
        <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rtc: ds1307: set uie_unsupported if no interrupt is
 available

It was <2021-03-30 wto 02:02>, when Alexandre Belloni wrote:
> On 16/03/2021 19:04:14+0100, Lukasz Stelmach wrote:
>> OK, you are right. The problem seems to be elsewhere.
>> 
>> How about this scnario? We call rtc_update_irq_enable(). We read rtc
>> with __rtc_read_time() and calculate the alarm time. We get through
>> rtc_timer_enqueue() and down to __rtc_set_alarm(). We loose the race
>> condition (I can do it, I've got really slow connection to DS3231) and
>> we return -ETIME from __rtc_set_alarm()
>> 
>>     if (scheduled <= now)
>>         return -ETIME;
>> 
>> and 0 from rtc_timer_enqueue() and the very same zero from
>> rtc_update_irq_enable(). The caller of ioctl() thinks they can expect
>> interrupts when, in fact, they won't receive any.
>> 
>> The really weird stuff happens in rtc_timer_do_work(). For the timer to
>> be dequeued __rtc_set_alarm() needs to return EINVAL three times in a
>> row. In my setup this doesn't happen and the code keeps running loops
>> around "reporogram" and "again" labels.
>> 
>> With my patch we never risk the above race condition between
>> __rtc_read_time() in rtc_update_irq_enable() and the one in
>> __rtc_set_alarm(), because we know rtc doesn't support alarms before we
>> start the race. In fact there is another race between __rtc_read_time()
>> and actually setting the alarm in the chip.
>> 
>> IMHO the solution is to introduce RTC_HAS_ALARM flag for struct
>> rtc_device and check it at the very beginning of __rtc_set_alarm() the
>> same way it is being done in ds1337_set_alarm(). What are your thoughts?
>> 
>
> I did introduce RTC_FEATURE_ALARM for that in v5.12. I'm sending patches
> that are not well tested but should solve your issue.

Oh, I didn't see that one coming (-; I was working on a slightly larger
feature elsewhere in the tree and didn't rebase too often. I will test
the patches.

-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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