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

[Corey Minyard <cminyard@mvista.com>]

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. 

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.

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.

-corey

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