Subject: Re: [RFC PATCH] watchdog: s3c2410_wdt: Add max and min timeout values
To: Javier Martinez Canillas <javier@osg.samsung.com>,
        linux-kernel@vger.kernel.org
References: <1456850717-3670-1-git-send-email-javier@osg.samsung.com>
 <56D62979.5040009@samsung.com> <56D7233B.50907@osg.samsung.com>
Cc: Guenter Roeck <linux@roeck-us.net>, Kukjin Kim <kgene@kernel.org>,
        Wim Van Sebroeck <wim@iguana.be>, linux-samsung-soc@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-watchdog@vger.kernel.org,
        Chanwoo Choi <cw00.choi@samsung.com>
From: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Message-id: <56D7838D.4060602@samsung.com>
Date: Thu, 03 Mar 2016 09:21:33 +0900
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On 03.03.2016 02:30, Javier Martinez Canillas wrote:
> Hello Krzysztof,
> 
> On 03/01/2016 08:44 PM, Krzysztof Kozlowski wrote:
>> On 02.03.2016 01:45, Javier Martinez Canillas wrote:
>>> The watchdog maximum timeout value is determined by the number of bits
>>> for the interval timer counter, its source clock frequency, the number
>>> of bits of the prescaler and maximum divider value.
>>>
>>> This can be calculated with the following equation:
>>>
>>> max_timeout = counter / (freq / (max_prescale + 1) / max_divider)
>>>
>>> Setting a maximum timeout value will allow the watchdog core to refuse
>>> user-space calls to the WDIOC_SETTIMEOUT ioctl that sets not supported
>>> timeout values.
>>>
>>> For example, systemd tries to set a timeout of 10 minutes on reboot to
>>> ensure that the machine will be rebooted even if a reboot failed. This
>>> leads to the following error message on an Exynos5422 Odroid XU4 board:
>>>
>>> [  147.986045] s3c2410-wdt 101d0000.watchdog: timeout 600 too big
>>>
>>> Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
>>> Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
>>>
>>> ---
>>> Hello,
>>>
>>> I'm sending this as an RFC because I don't have access to user manuals
>>> for all the Exynos SoCs that this driver supports. But at least in all
>>> the ones I have here, the maximum prescaler and divider values are the
>>> same and also all have a 16-bit interval timer counter.
>>>
>>> So this patch assumes that all IP blocks supported by this driver are
>>> the same on that regard, if that's not the case then we can move these
>>> values on a per IP block basis using struct of_device_id .data field.
>>
>> It looks correct at least for Exynos4210 and newer.
>>
> 
> Great, thanks a lot for the confirmation.
> 
>>>
>>> Best regards,
>>> Javier
>>>
>>>   drivers/watchdog/s3c2410_wdt.c | 16 ++++++++++++++++
>>>   1 file changed, 16 insertions(+)
>>>
>>> diff --git a/drivers/watchdog/s3c2410_wdt.c
>>> b/drivers/watchdog/s3c2410_wdt.c
>>> index 0093450441fe..f289c9fc353a 100644
>>> --- a/drivers/watchdog/s3c2410_wdt.c
>>> +++ b/drivers/watchdog/s3c2410_wdt.c
>>> @@ -47,6 +47,8 @@
>>>   #define S3C2410_WTDAT        0x04
>>>   #define S3C2410_WTCNT        0x08
>>>
>>> +#define S3C2410_WTCNT_MAXCNT    0xffff
>>> +
>>>   #define S3C2410_WTCON_RSTEN    (1 << 0)
>>>   #define S3C2410_WTCON_INTEN    (1 << 2)
>>>   #define S3C2410_WTCON_ENABLE    (1 << 5)
>>> @@ -56,8 +58,11 @@
>>>   #define S3C2410_WTCON_DIV64    (2 << 3)
>>>   #define S3C2410_WTCON_DIV128    (3 << 3)
>>>
>>> +#define S3C2410_WTCON_MAXDIV    0x80
>>> +
>>>   #define S3C2410_WTCON_PRESCALE(x)    ((x) << 8)
>>>   #define S3C2410_WTCON_PRESCALE_MASK    (0xff << 8)
>>> +#define S3C2410_WTCON_PRESCALE_MAX    0xff
>>>
>>>   #define CONFIG_S3C2410_WATCHDOG_ATBOOT        (0)
>>>   #define CONFIG_S3C2410_WATCHDOG_DEFAULT_TIME    (15)
>>> @@ -198,6 +203,14 @@ do {                            \
>>>
>>>   /* functions */
>>>
>>> +static inline unsigned int s3c2410wdt_max_timeout(struct clk *clock)
>>> +{
>>> +    unsigned long freq = clk_get_rate(clock);
>>> +
>>> +    return S3C2410_WTCNT_MAXCNT / (freq /
>>> (S3C2410_WTCON_PRESCALE_MAX + 1)
>>> +                       / S3C2410_WTCON_MAXDIV);
>>> +}
>>> +
>>>   static inline struct s3c2410_wdt *freq_to_wdt(struct notifier_block
>>> *nb)
>>>   {
>>>       return container_of(nb, struct s3c2410_wdt, freq_transition);
>>> @@ -567,6 +580,9 @@ static int s3c2410wdt_probe(struct
>>> platform_device *pdev)
>>>           return ret;
>>>       }
>>>
>>> +    wdt->wdt_device.min_timeout = 1;
>>> +    wdt->wdt_device.max_timeout = s3c2410wdt_max_timeout(wdt->clock);
>>
>> Can the frequency of clock change? E.g. with devfreq? No problem if it
>> goes lower but if it gets higher than initial, then the problem will
>> appear again.
>>
> 
> That's a very good question. As Guenter said we will be in deep troubles
> if that ever happens since the driver doesn't take that into account.
> 
> The .set_timeout handler just sets the counter according to the current
> frequency and that's never updated, unless a new timeout is set of course.
> 
> So in other words, I just made the same assumptions that the driver is
> currently doing.

Not entirely. Change of clock frequency will affect currently set
timeout. But the next timeout will be using new frequency.

However you are setting the maximum timeout once. It will never change.

> At least the Exynos SoCs manual don't mention frequency
> scaling for the watchdog timer source clock and AFAICT none of the CLK_WDT
> parents scale their frequencies but I don't know if that's true for all
> the machines using this driver (i.e: out-of-tree boards).

I looked at Exynos4 family because the devfreq was tested there. The WDT
clock goes from ACLK100 (or ACLK66 on different socs).

1. Existing devfreq for Exynos4 does not change ACLK100 frequency.
2. New patches from Chanwoo (Cc) add scaling of ACLK100 also to 50 MHz:
http://lkml.iu.edu/hypermail/linux/kernel/1512.1/04828.html

The problem will be more severe if the watchdog got configured on 50 MHz
and then devfreq bumps the clock to 100 MHz...

Best regards,
Krzysztof