linux-kernel - Re: [PATCH v2] drivers: counter: Add Cros EC Sync counter

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Date:   Mon, 20 Apr 2020 15:08:10 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Gwendal Grignou <gwendal@...omium.org>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Benson Leung <bleung@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-iio <linux-iio@...r.kernel.org>
Subject: Re: [PATCH v2] drivers: counter: Add Cros EC Sync counter

On Mon, Apr 20, 2020 at 11:54:16AM -0700, Gwendal Grignou wrote:
> On Tue, Apr 14, 2020 at 1:48 PM William Breathitt Gray
> <vilhelm.gray@...il.com> wrote:
> >
> > On Mon, Apr 13, 2020 at 12:55:14PM -0700, Gwendal Grignou wrote:
> > > When the camera vsync pin is connected to the embedded controller (EC) of
> > > a chromebook, the EC reports a sensor with a counter that increases
> > > at each GPIO rising edge.
> > >
> > > The sensor is presented using the counter subsystem.
> > > In addition, it is also presented via the IIO subsystem with a timestamp,
> > > allowing synchronisation with sensors connected to the same EC, for
> > > image stabilisation or augmented reality applications.
> >
> > Hi Gwendal,
> >
> > Sorry for the delay. I have some changes requested below.
> >
> > > To enable the counter:
> > > via counter ABI:
> > > echo "rising edge" > counterX/count0/signal_action
> > > via iio ABI
> > > echo 1 > iio:deviceY/en
> > >
> > > To disable the counter:
> > > via counter ABI:
> > > echo "none" > counterX/count0/signal_action
> > > via iio ABI
> > > echo 0 > iio:deviceY/en
> >
> > Although in theory a user could manually disable the actions for a
> > Signal, this is a very roundabout way of actually disabling the Count.
> > It's better to expose an "enable" attribute to allow the users to
> > perform this functionality; for example:
> >
> > echo 0 > counterX/count0/enable
> > echo 1 > counterX/count0/enable
> >
> > >
> > > To read the current counter value:
> > > via counter ABI:
> > > cat counterX/count0/count
> > > via iio ABI
> > > cat iio:deviceY/in_count_raw
> >
> > I know we discussed this in the last review but it's still the same as
> > before: IIO_COUNT interface is deprecated so new drivers won't be
> > allowed to use it. You'll have to remove the IIO_COUNT code in this
> > driver and replace it with Counter subsystem equivalents.
> I understand the need of a clean separation between counter and IIO subsystems.
> I will wait for counter to offer a way to gather timestamp'ed counts.
> Do you have a plan/proposed ABI you can share?
> 
> Thanks,
> 
> Gwendal.

Hi Gwendal,

I'm working on a reimplementation of the internals of the Counter
subsystem: https://gitlab.com/vilhelmgray/iio/-/tree/counter_chardev

I'm hoping to submit it to the mailing list later this week if I don't
hit any delays; it'll include support as well for a character device
interface for userspace application.

Once those changes are merged into IIO, I'll submit a patch to add
timestamp support -- hopefully within a week or two after. Right now I
haven't yet chosen any specific format for timestamps, but I will likely
match the format IIO subsystem currently has for its timestamp support
so that migration is easier for these drivers.

William Breathitt Gray

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