Re: [PATCH 02/12] watchdog: Add the ability to provide data to read

[Guenter Roeck <linux@roeck-us.net>]

On 8/20/19 8:58 AM, Corey Minyard wrote:
> On Tue, Aug 20, 2019 at 06:53:40AM -0700, Guenter Roeck wrote:
>> On 8/20/19 5:12 AM, Corey Minyard wrote:
>>> On Mon, Aug 19, 2019 at 07:09:46PM -0600, Jerry Hoemann wrote:
>>>> On Mon, Aug 19, 2019 at 07:23:09PM -0500, Corey Minyard wrote:
>>>>> On Mon, Aug 19, 2019 at 03:43:45PM -0700, Guenter Roeck wrote:
>>>>>> On Mon, Aug 19, 2019 at 03:37:01PM -0500, minyard@acm.org wrote:
>>>>>>> From: Corey Minyard <cminyard@mvista.com>
>>>>>>>
>>>>>>> This is for the read data pretimeout governor.
>>>>>>>
>>>>>>> Signed-off-by: Corey Minyard <cminyard@mvista.com>
>>>>>>
>>>>>> On further thought, I think it would be a bad idea to add this
>>>>>> functionality: It changes the userspace ABI for accessing the watchdog
>>>>>> device. Today, when a watchdog device is opened, it does not provide
>>>>>> read data, it does not hang, and returns immediately. A "cat" from it
>>>>>> is an easy and quick means to test if a watchdog works.
>>>>>
>>>>> Umm, why would a "cat" from a watchdog tell you if a watchdog works?
>>>>
>>>> cat /dev/watchdog starts the watchdog running.
>>>>
>>>> Then one can do useful things like monitor /sys/class/watchdog/watchdogN and see
>>>> time ticking down, etc..,
>>>>
>>>> echo V > /dev/watchdog stops the watchdog assuming driver supports WDIOF_MAGICCLOSE.
>>>>
>>>> So I can test without having to reboot.
>>>>
>>>> One can't test magic close with the proposed change as /dev/watchdog
>>>> is exclusive open.
>>>
>>> Sure you can:
>>>
>>> # echo "" >/dev/watchdog0
>>> [   92.390649] watchdog: watchdog0: watchdog did not stop!
>>> # sleep 2
>>> # cat /sys/class/watchdog/watchdog0/timeleft
>>> 8
>>> # echo "V" >/dev/watchdog0
>>>
>>> Works just fine.  But I can make it so that reading returns an error
>>> unless the governor is the read one.
>>>
>>> The question is if this is required to transfer the IPMI watchdog
>>> over to the standard interface.  It currently has this function,
>>> do we do an API change to move it over?
>>>
>> Having to change the standard watchdog API to accommodate a non-standard driver
>> is most definitely not the right approach. If it was, anyone could use it to
>> force standard API/ABI changes. Just implement driver X outside its subsystem
>> and then claim you need to change the subsystem to accommodate it.
> 
> I'm not advocating anything of the sort.  I think it can be done in
> a way that keeps the API the same unless you enable a new pretimeout
> governor.  I would not suggest that the API be changed, and I should
> have handled that in the original design.
> 
>>
>> On a side note, a standard watchdog driver can implement its own ioctl functions.
> 
> I am aware of that, but you can't provide read data on a file descriptor
> through that interface.  The actions and preactions could be done that
> way, but that seemed a more general function that could benefit other
> drivers.
> 
That comment was more directed towards the ioctls you are adding to the
watchdog core, which I think would require more discussion.

> The function to provide read data might be useful, I don't know, but
> it could be used with any driver that did a normal interrupt pretimeout.
> I can't remember why it was originally done.  I vaguely remember someone
> asking for it, but that was 17 years ago.
> 

I find it odd. Only one driver can have the watchdog device open,
and that open file should be used to ping the watchdog. It can't do that
while waiting for a pretimeout. This almost sounds like the user wrote
an application which waited for the pretimeout to happen before pinging
the watchdog.

Talking about a standardized ABI to inform userspace if a pretimeout
occurred... if there is a use case for this, I'd rather trigger a udev
event on the "pretimeout" sysfs attribute. That would make much more
sense to me than sending a random data byte to the "read" function.
The WDIOC_GETSTATUS ioctl could then report that a pretimeout occurred.
Or, depending on the use case, just report the fact that a pretimeout
occurred with WDIOC_GETSTATUS. That would be really simple to add,
and I would support it.

> It could just be left out and added back if someone complains.  That's
> not very friendly since it's an API change, but then we would know if
> anyone was using it.

It is still better than an API change for standard watchdog drivers,
and a somewhat awkward interface to inform userspace about pretimeout
events.

Thanks,
Guenter

> 
> -corey
> 
>>
>> Guenter
>>
>>> -corey
>>>
>>>>
>>>>
>>>>
>>>>
>>>> -- 
>>>>
>>>> -----------------------------------------------------------------------------
>>>> Jerry Hoemann                  Software Engineer   Hewlett Packard Enterprise
>>>> -----------------------------------------------------------------------------
>>>
>>
>