linux-kernel - Re: [RESEND PATCH v3] rtc: cros-ec: return -ETIME when refused to set alarms in the past

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Date:   Mon, 26 Feb 2018 19:55:56 -0800
From:   Brian Norris <briannorris@...gle.com>
To:     Jeffy Chen <jeffy.chen@...k-chips.com>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>,
        Chris Zhong <zyw@...k-chips.com>,
        Doug Anderson <dianders@...gle.com>,
        Julius Werner <jwerner@...omium.org>,
        linux-rtc@...r.kernel.org,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Alessandro Zummo <a.zummo@...ertech.it>
Subject: Re: [RESEND PATCH v3] rtc: cros-ec: return -ETIME when refused to set
 alarms in the past

Hi Jeffy,

On Mon, Feb 26, 2018 at 6:50 PM, Jeffy Chen <jeffy.chen@...k-chips.com> wrote:
> Since accessing a Chrome OS EC based rtc is a slow operation, there is a
> race window where if the alarm is set for the next second and the second
> ticks over right before calculating the alarm offset.
>
> In this case the current driver is setting a 0-second alarm, which would
> be considered as disabling alarms by the EC(EC_RTC_ALARM_CLEAR).
>
> This breaks, e.g., hwclock which relies on RTC_UIE_ON ->
> rtc_update_irq_enable(), which sets a 1-second alarm and expects it to
> fire an interrupt.
>
> So return -ETIME when the alarm is in the past, follow __rtc_set_alarm().
>
> Signed-off-by: Jeffy Chen <jeffy.chen@...k-chips.com>

Commit message is better. Thanks! And this is exactly what I was
already testing. I still need to get through a bit more testing to
ensure it has resolved all the races I saw, but it's good on several
of them:

Reviewed-by: Brian Norris <briannorris@...omium.org>
Tested-by: Brian Norris <briannorris@...omium.org>

> ---
>
> Changes in v3:
> Fix alarm time comparing.
>
> Changes in v2:
> Rewrite commit message as Brian suggested.
> Check alarm time only when that alarm is enabled.
>
>  drivers/rtc/rtc-cros-ec.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/rtc/rtc-cros-ec.c b/drivers/rtc/rtc-cros-ec.c
> index f0ea6899c731..bf7ced095c94 100644
> --- a/drivers/rtc/rtc-cros-ec.c
> +++ b/drivers/rtc/rtc-cros-ec.c
> @@ -197,10 +197,10 @@ static int cros_ec_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
>                 cros_ec_rtc->saved_alarm = (u32)alarm_time;
>         } else {
>                 /* Don't set an alarm in the past. */
> -               if ((u32)alarm_time < current_time)
> -                       alarm_offset = EC_RTC_ALARM_CLEAR;
> -               else
> -                       alarm_offset = (u32)alarm_time - current_time;
> +               if ((u32)alarm_time <= current_time)
> +                       return -ETIME;
> +
> +               alarm_offset = (u32)alarm_time - current_time;
>         }
>
>         ret = cros_ec_rtc_set(cros_ec, EC_CMD_RTC_SET_ALARM, alarm_offset);
> --
> 2.11.0
>
>