Re: [PATCH v7 1/3] clocksource/drivers/timer-tegra186: add WDIOC_GETTIMELEFT support

[Jon Hunter <jonathanh@nvidia.com>]

On 02/05/2025 12:29, Daniel Lezcano wrote:
> On 02/05/2025 13:06, Jon Hunter wrote:
>>
>>
>> On 02/05/2025 11:51, Daniel Lezcano wrote:
>>> On Fri, May 02, 2025 at 11:16:17AM +0100, Jon Hunter wrote:
>>>>
>>>>
>>>> On 02/05/2025 10:19, Daniel Lezcano wrote:
>>>>> On Fri, May 02, 2025 at 12:37:25PM +0800, Robert Lin wrote:
>>>>>> From: Pohsun Su <pohsuns@nvidia.com>
>>>>>>
>>>>>> This change adds support for WDIOC_GETTIMELEFT so userspace
>>>>>> programs can get the number of seconds before system reset by
>>>>>> the watchdog timer via ioctl.
>>>>>>
>>>>>> Signed-off-by: Pohsun Su <pohsuns@nvidia.com>
>>>>>> Signed-off-by: Robert Lin <robelin@nvidia.com>
>>>>>> ---
>>>>>>    drivers/clocksource/timer-tegra186.c | 64 +++++++++++++++++++++ 
>>>>>> + +++++-
>>>>>>    1 file changed, 63 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/clocksource/timer-tegra186.c b/drivers/ 
>>>>>> clocksource/timer-tegra186.c
>>>>>> index ea742889ee06..8d5698770cbd 100644
>>>>>> --- a/drivers/clocksource/timer-tegra186.c
>>>>>> +++ b/drivers/clocksource/timer-tegra186.c
>>>>>> @@ -1,8 +1,9 @@
>>>>>>    // SPDX-License-Identifier: GPL-2.0-only
>>>>>>    /*
>>>>>> - * Copyright (c) 2019-2020 NVIDIA Corporation. All rights reserved.
>>>>>> + * Copyright (c) 2019-2025 NVIDIA Corporation. All rights reserved.
>>>>>>     */
>>>>>> +#include <linux/bitfield.h>
>>>>>>    #include <linux/clocksource.h>
>>>>>>    #include <linux/module.h>
>>>>>>    #include <linux/interrupt.h>
>>>>>> @@ -30,6 +31,7 @@
>>>>>>    #define TMRSR 0x004
>>>>>>    #define  TMRSR_INTR_CLR BIT(30)
>>>>>> +#define  TMRSR_PCV GENMASK(28, 0)
>>>>>>    #define TMRCSSR 0x008
>>>>>>    #define  TMRCSSR_SRC_USEC (0 << 0)
>>>>>> @@ -46,6 +48,9 @@
>>>>>>    #define  WDTCR_TIMER_SOURCE_MASK 0xf
>>>>>>    #define  WDTCR_TIMER_SOURCE(x) ((x) & 0xf)
>>>>>> +#define WDTSR 0x004
>>>>>> +#define  WDTSR_CURRENT_EXPIRATION_COUNT GENMASK(14, 12)
>>>>>> +
>>>>>>    #define WDTCMDR 0x008
>>>>>>    #define  WDTCMDR_DISABLE_COUNTER BIT(1)
>>>>>>    #define  WDTCMDR_START_COUNTER BIT(0)
>>>>>> @@ -235,12 +240,69 @@ static int tegra186_wdt_set_timeout(struct 
>>>>>> watchdog_device *wdd,
>>>>>>        return 0;
>>>>>>    }
>>>>>> +static unsigned int tegra186_wdt_get_timeleft(struct 
>>>>>> watchdog_device *wdd)
>>>>>> +{
>>>>>> +    struct tegra186_wdt *wdt = to_tegra186_wdt(wdd);
>>>>>> +    u32 expiration, val;
>>>>>> +    u64 timeleft;
>>>>>> +
>>>>>> +    if (!watchdog_active(&wdt->base)) {
>>>>>> +        /* return zero if the watchdog timer is not activated. */
>>>>>> +        return 0;
>>>>>> +    }
>>>>>> +
>>>>>> +    /*
>>>>>> +     * Reset occurs on the fifth expiration of the
>>>>>> +     * watchdog timer and so when the watchdog timer is configured,
>>>>>> +     * the actual value programmed into the counter is 1/5 of the
>>>>>> +     * timeout value. Once the counter reaches 0, expiration count
>>>>>> +     * will be increased by 1 and the down counter restarts.
>>>>>> +     * Hence to get the time left before system reset we must
>>>>>> +     * combine 2 parts:
>>>>>> +     * 1. value of the current down counter
>>>>>> +     * 2. (number of counter expirations remaining) * (timeout/5)
>>>>>> +     */
>>>>>> +
>>>>>> +    /* Get the current number of counter expirations. Should be a
>>>>>> +     * value between 0 and 4
>>>>>> +     */
>>>>>> +    val = readl_relaxed(wdt->regs + WDTSR);
>>>>>> +    expiration = FIELD_GET(WDTSR_CURRENT_EXPIRATION_COUNT, val);
>>>>>> +    if (WARN_ON(expiration > 4))
>>>>>> +        return 0;
>>>>>
>>>>> Each call will generate a big warning in the message. May be simpler
>>>>> to add a pr_err() with an explicit message.
>>>>
>>>> I prefer the WARN. This should never happen. If we do change this, then
>>>> dev_WARN() might be more appropriate, but I think that this is fine 
>>>> too.
>>>
>>> The function tegra186_wdt_get_timeleft() is triggered from userspace
>>> via an ioctl or sysfs. The documentation process/coding-style.rst says:
>>>
>>> """
>>> Do not WARN lightly
>>> *******************
>>>
>>> WARN*() is intended for unexpected, this-should-never-happen situations.
>>> WARN*() macros are not to be used for anything that is expected to 
>>> happen
>>> during normal operation. These are not pre- or post-condition 
>>> asserts, for
>>> example. Again: WARN*() must not be used for a condition that is 
>>> expected
>>> to trigger easily, for example, by user space actions. pr_warn_once() 
>>> is a
>>> possible alternative, if you need to notify the user of a problem.
>>> """
>>>
>>> I understand it is important to check the return value in order to
>>> have a consistent result when computing the remaining time but that
>>> should not trigger a warning each time.
>>
>> Yes so WARN sounds appropriate. It should never happen. I don't see 
>> the issue.
> 
> The issue is if there is an userspace application reading the ioctl and 
> or the sysfs, then the warning will be emitted each time if the never- 
> happen condition exists. Preferably replace the WARN_ON by 
> pr_warn_once() as suggested if the bug must be reported.

Sounds a bit funny 'if the never-happen condition exists' :-)

However, I will be fine with WARN_ON_ONCE(). I think that this warrants 
more of a large WARN splat than pr_warn() because it should never happen.

Jon

-- 
nvpublic