Re: [PATCH V2] rtc: mc146818: Detect and handle broken RTCs

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 26/01/2021 18:02:11+0100, Thomas Gleixner wrote:
> The recent fix for handling the UIP bit unearthed another issue in the RTC
> code. If the RTC is advertised but the readout is straight 0xFF because
> it's not available, the old code just proceeded with crappy values, but the
> new code hangs because it waits for the UIP bit to become low.
> 
> Add a sanity check in the RTC CMOS probe function which reads the RTC_VALID
> register (Register D) which should have bit 0-6 cleared. If that's not the
> case then fail to register the CMOS.
> 
> Add the same check to mc146818_get_time(), warn once when the condition
> is true and invalidate the rtc_time data.
> 
> Reported-by: Mickaël Salaün <mic@digikod.net>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Tested-by: Mickaël Salaün <mic@linux.microsoft.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

> ---
> V2: Fixed the sizeof() as spotted by Mickaël
> ---
>  drivers/rtc/rtc-cmos.c         |    8 ++++++++
>  drivers/rtc/rtc-mc146818-lib.c |    7 +++++++
>  2 files changed, 15 insertions(+)
> 
> --- a/drivers/rtc/rtc-cmos.c
> +++ b/drivers/rtc/rtc-cmos.c
> @@ -805,6 +805,14 @@ cmos_do_probe(struct device *dev, struct
>  
>  	spin_lock_irq(&rtc_lock);
>  
> +	/* Ensure that the RTC is accessible. Bit 0-6 must be 0! */
> +	if ((CMOS_READ(RTC_VALID) & 0x7f) != 0) {
> +		spin_unlock_irq(&rtc_lock);
> +		dev_warn(dev, "not accessible\n");
> +		retval = -ENXIO;
> +		goto cleanup1;
> +	}
> +
>  	if (!(flags & CMOS_RTC_FLAGS_NOFREQ)) {
>  		/* force periodic irq to CMOS reset default of 1024Hz;
>  		 *
> --- a/drivers/rtc/rtc-mc146818-lib.c
> +++ b/drivers/rtc/rtc-mc146818-lib.c
> @@ -21,6 +21,13 @@ unsigned int mc146818_get_time(struct rt
>  
>  again:
>  	spin_lock_irqsave(&rtc_lock, flags);
> +	/* Ensure that the RTC is accessible. Bit 0-6 must be 0! */
> +	if (WARN_ON_ONCE((CMOS_READ(RTC_VALID) & 0x7f) != 0)) {
> +		spin_unlock_irqrestore(&rtc_lock, flags);
> +		memset(time, 0xff, sizeof(*time));
> +		return 0;
> +	}
> +
>  	/*
>  	 * Check whether there is an update in progress during which the
>  	 * readout is unspecified. The maximum update time is ~2ms. Poll

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com