Message-ID: <CAD=FV=U980_xkfMt-x9cv6LgyK0qfRt81HYEtu_sW3c+PqFN9A@mail.gmail.com>
Date:   Tue, 20 Jul 2021 09:09:59 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] HID: i2c-hid: goodix: Tie the reset line to true state
 of the regulator

Hi,

On Tue, Jul 20, 2021 at 1:01 AM Benjamin Tissoires
<benjamin.tissoires@...hat.com> wrote:
>
> On Thu, Jul 15, 2021 at 9:50 PM Jiri Kosina <jikos@...nel.org> wrote:
> >
> > On Fri, 25 Jun 2021, Douglas Anderson wrote:
> >
> > > The regulator for the touchscreen could be:
> > > * A dedicated regulator just for the touchscreen.
> > > * A regulator shared with something else in the system.
> > > * An always-on regulator.
> > >
> > > How we want the "reset" line to behave depends a bit on which of those
> > > three cases we're in. Currently the code is written with the
> > > assumption that it has a dedicated regulator, but that's not really
> > > guaranteed to be the case.
> > >
> > > The problem we run into is that if we leave the touchscreen powered on
> > > (because someone else is requesting the regulator or it's an always-on
> > > regulator) and we assert reset then we apparently burn an extra 67 mW
> > > of power. That's not great.
> > >
> > > Let's instead tie the control of the reset line to the true state of
> > > the regulator as reported by regulator notifiers. If we have an
> > > always-on regulator our notifier will never be called. If we have a
> > > shared regulator then our notifier will be called when the touchscreen
> > > is truly turned on or truly turned off.
> > >
> > > Using notifiers like this nicely handles all the cases without
> > > resorting to hacks like pretending that there is no "reset" GPIO if we
> > > have an always-on regulator.
> > >
> > > NOTE: if the regulator is on a shared line it's still possible that
> > > things could be a little off. Specifically, this case is not handled
> > > even after this patch:
> > > 1. Suspend goodix (send "sleep", goodix stops requesting regulator on)
> > > 2. Other regulator user turns off (regulator fully turns off).
> > > 3. Goodix driver gets notified and asserts reset.
> > > 4. Other regulator user turns on.
> > > 5. Goodix driver gets notified and deasserts reset.
> > > 6. Nobody resumes goodix.
> > >
> > > With that set of steps we'll have reset deasserted but we will have
> > > lost the results of the I2C_HID_PWR_SLEEP from the suspend path. That
> > > means we might be in higher power than we could be even if the goodix
> > > driver thinks things are suspended. Presumably, however, we're still
> > > in better shape than if we were asserting "reset" the whole time. If
> > > somehow the above situation is actually affecting someone and we want
> > > to do better we can deal with it when we have a real use case.
> > >
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> >
> > Applied, thanks Doug.
>
> Thanks Jiri for taking this one in.
>
> FWIW, I am really glad Doug made the effort of splitting i2c-hid-core
> and i2c-hid-goodix, because this is the kind of patch that would have
> been a nightmare to make it generic :)

Yeah, it was a bunch of work to split it but I think it worked out
pretty well overall. :-)

Looking at this patch today with fresh eyes, I just spotted a bug
though! :( I should have used the "devm" variant of
regulator_register_notifier() since otherwise nothing is
unregistering. Oops. I'll quickly send a fixup.

OK, it's should show up here:

https://lore.kernel.org/r/20210720090736.1.Idc6db7d0f2c2ecc6e533e5b918a651a66f337b2f@changeid

-Doug

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