Re: [char-misc-next, v4, 5/7] watchdog: mei_wdt: register wd device only if required

[Guenter Roeck]

On 01/18/2016 05:19 AM, Winkler, Tomas wrote:
> > only
> > > > if required
> > > >
> > > > Hi Tomas,
> > > >
> > > > On Fri, Jan 08, 2016 at 12:49:25AM +0200, Winkler, Tomas wrote:
> > > > > From: Alexander Usyskin <alexander.usyskin@xxxxxxxxx>
> > > > >
> > > > > For Intel Broadwell and newer platforms, the ME device can inform
> > > > > the host whether the watchdog functionality is activated or not.
> > > > > If the watchdog functionality is not activated then the watchdog interface
> > > > > can be not registered and eliminate unnecessary pings and hence lower the
> > > > > power consumption by avoiding waking up the device.
> > > > > The feature can be deactivated also without reboot
> > > > > in that case the watchdog device should be unregistered at runtime.
> > > > >
> > > > > Signed-off-by: Alexander Usyskin <alexander.usyskin@xxxxxxxxx>
> > > > > Signed-off-by: Tomas Winkler <tomas.winkler@xxxxxxxxx>
> > > > > ---
> > > > > V2: rework unregistration
> > > > > V3: rebase; implement unregistraion also at runtime
> > > > > V4: Rebase the code over patchset : "watchdog: Replace driver based
> > > > refcounting"
> > > > >    drivers/watchdog/mei_wdt.c | 196
> > > > ++++++++++++++++++++++++++++++++++++++++++---
> > > > >    1 file changed, 187 insertions(+), 9 deletions(-)
> > > > >
> > > > > diff --git a/drivers/watchdog/mei_wdt.c b/drivers/watchdog/mei_wdt.c
> > > > > index e7e3f144f2b0..85b27fc5d4ec 100644
> > > > > --- a/drivers/watchdog/mei_wdt.c
> > > > > +++ b/drivers/watchdog/mei_wdt.c
> > > > [ ... ]
> > > >
> > > > > +static void mei_wdt_unregister_work(struct work_struct *work)
> > > > > +{
> > > > > +	struct mei_wdt *wdt = container_of(work, struct mei_wdt, unregister);
> > > > > +
> > > > > +	mei_wdt_unregister(wdt);
> > > > > +}
> > > >
> > > > Registration is synchronous, unregistration is asynchronous.
> > > >
> > > > Assuming that is on purpose, I think it warrants an explanation.
> > > The unregistration is detected on response from the  ping, which is run under
> > same mutex as unregistration.
> > > Tomas
> > And that explains why registration can be synchronous and unregistration
> > has to be asynchronous ?
>
> You need to connect the dots but yes.
> The registration is run from the internal ping request (in probe) or from an internal event (in runtime), so the flow is not already locked by the watchdog mutex.
> Hope it helps.

What I was asking for is a comment such as

"We can not unregister directly because a ping operation (triggered
through the watchdog subsystem) is pending and must be completed first."

Guenter