Re: [PATCH 1/2] dt-bindings: input: goodix: Add powered-in-suspend property

[Doug Anderson]

Hi,

On Wed, Apr 19, 2023 at 8:18 AM Jeff LaBundy <jeff@xxxxxxxxxxx> wrote:
>
> Hi Doug and Fei,
>
> Thank you for the informative discussion; I can empathize with the pain
> these issues bring.
>
> On Wed, Apr 19, 2023 at 07:38:13AM -0700, Doug Anderson wrote:
> > Hi,
> >
> > On Wed, Apr 19, 2023 at 3:44 AM Fei Shao <fshao@xxxxxxxxxxxx> wrote:
> > >
> > > Hi Jeff,
> > >
> > > On Wed, Apr 19, 2023 at 8:21 AM Jeff LaBundy <jeff@xxxxxxxxxxx> wrote:
> > > >
> > > > Hi Fei,
> > > >
> > > > On Tue, Apr 18, 2023 at 08:49:51PM +0800, Fei Shao wrote:
> > > > > We observed that on Chromebook device Steelix, if Goodix GT7375P
> > > > > touchscreen is powered in suspend (because, for example, it connects to
> > > > > an always-on regulator) and with the reset GPIO asserted, it will
> > > > > introduce about 14mW power leakage.
> > > > >
> > > > > This property is used to indicate that the touchscreen is powered in
> > > > > suspend. If it's set, the driver will stop asserting the reset GPIO in
> > > > > power-down, and it will do it in power-up instead to ensure that the
> > > > > state is always reset after resuming.
> > > > >
> > > > > Signed-off-by: Fei Shao <fshao@xxxxxxxxxxxx>
> > > > > ---
> > > >
> > > > This is an interesting problem; were you able to root-cause why the silicon
> > > > exhibits this behavior? Simply asserting reset should not cause it to draw
> > > > additional power, let alone 14 mW. This almost sounds like a back-powering
> > > > problem during suspend.
> > > >
> > > There was a fix for this behavior before so I didn't dig into it on
> > > the silicon side.
> > > I can ask internally and see if we can have Goodix to confirm this is
> > > a known HW erratum.
> >
> > Certainly it doesn't hurt to check, but it's not really that shocking
> > to me that asserting reset could cause a power draw on some hardware.
> > Reset puts hardware into a default state and that's not necessarily
> > low power. I guess ideally hardware would act like it's "off" when
> > reset is asserted and then then init to the default state on the edge
> > as reset was deasserted, but I not all hardware is designed in an
> > ideal way.
>
> While that is true in theory, I have never, ever seen that to be the case
> when there is not some other underlying problem.
>
> What I have seen, however, is that asserting reset actually causes the GPIO
> to sink current from some other supply and through the IC. I loosely suspect
> that if you probe the IC's rails and digital I/O during the failure condition,
> you may find one of them resting at some mid-rail voltage or diode drop. It
> seems you have a similar suspicion.
>
> In that case, it may mean that some other supply in the system should actually
> be kept on, or that supplies are being brought down out of order. In which
> case, the solution should actually be a patch to the affected platform(s) dts
> and not the mainline driver.

I agree that it's a bandaid, but I'm not hopeful that a better
solution will be found.

Specifically, I'd expect a current leak in the system when you turn a
supply off and then assert a GPIO high. That's when the device can
start backpowering itself from a GPIO. In this case, it's the
opposite. We're keeping the supply on and asserting the (active low)
reset GPIO to cause the higher power draw. In another design it was
confirmed that the power draw went away when we were able to turn the
regulator off (but still keep the active low reset GPIO asserted).
We've also confirmed that power is good if we keep the supply on and
_don't_ assert the reset GPIO. Both of these two experiments provide
some evidence that the system is configured properly and we're not
backpowering something.

I guess I should revise the above, though. I could believe that there
is a current leak but on the touchscreen controller board itself,
which is a black box to us. I have certainly been involved in products
in the past where the default state of the system at reset caused a
minor current leak (I remember an EE telling me that as soon as
software started running I should quickly change the direction of a
GPIO) and it wouldn't shock me if the touchscreen controller board had
a problem like this. If there is a problem like this on the
touchscreen controller board there's not much we can do to workaround
it.

Unfortunately, if the problem ends up needing a hardware change to fix
more correctly (which I suspect it does), our hands are tied a bit.
This is not prototype hardware but is final hardware.

I guess one further note is that, at least on the project I was
involved in that had a similar problem, folks in China did a bunch of
analysis on this and went as far as adding an extra regulator to the
main board schematic to "fix" it. Had the issue just been something
where we were misconfiguing GPIOs or leaving a regulator in the wrong
state then they (probably) would have identified it rather than
spinning the board.

-Doug