Message-ID: <20190530074819.GM4574@dell>
Date:   Thu, 30 May 2019 08:48:19 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Gwendal Grignou <gwendal@...omium.org>
Cc:     andy.shevchenko@...il.com, Guenter Roeck <groeck@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, kernel@...labora.com
Subject: Re: [PATCH v5] mfd: cros_ec_dev: Register cros_ec_accel_legacy
 driver as a subdevice

On Wed, 29 May 2019, Gwendal Grignou wrote:

> On Wed, May 29, 2019 at 4:44 AM Lee Jones <lee.jones@...aro.org> wrote:
> >
> > On Tue, 28 May 2019, Gwendal Grignou wrote:
> >
> > > On Mon, Apr 1, 2019 at 8:46 PM Lee Jones <lee.jones@...aro.org> wrote:
> > > >
> > > > On Wed, 27 Feb 2019, Gwendal Grignou wrote:
> > > >
> > > > > From: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> > > > >
> > > > > With this patch, the cros_ec_ctl driver will register the legacy
> > > > > accelerometer driver (named cros_ec_accel_legacy) if it fails to
> > > > > register sensors through the usual path cros_ec_sensors_register().
> > > > > This legacy device is present on Chromebook devices with older EC
> > > > > firmware only supporting deprecated EC commands (Glimmer based devices).
> > > > >
> > > > > Tested-by: Gwendal Grignou <gwendal@...omium.org>
> > > > > Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> > > > > Reviewed-by: Gwendal Grignou <gwendal@...omium.org>
> > > > > Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
> > > > > ---
> > > > > Changes in v5:
> > > > > - Remove unnecessary white lines.
> > > > >
> > > > > Changes in v4:
> > > > > - [5/8] Nit: EC -> ECs (Lee Jones)
> > > > > - [5/8] Statically define cros_ec_accel_legacy_cells (Lee Jones)
> > > > >
> > > > > Changes in v3:
> > > > > - [5/8] Add the Reviewed-by Andy Shevchenko.
> > > > >
> > > > > Changes in v2:
> > > > > - [5/8] Add the Reviewed-by Gwendal.
> > > > >
> > > > >  drivers/mfd/cros_ec_dev.c | 66 +++++++++++++++++++++++++++++++++++++++
> > > > >  1 file changed, 66 insertions(+)
> > > > >
> > > > > diff --git a/drivers/mfd/cros_ec_dev.c b/drivers/mfd/cros_ec_dev.c
> > > > > index d275deaecb12..64567bd0a081 100644
> > > > > --- a/drivers/mfd/cros_ec_dev.c
> > > > > +++ b/drivers/mfd/cros_ec_dev.c
> > > > > @@ -376,6 +376,69 @@ static void cros_ec_sensors_register(struct cros_ec_dev *ec)
> > > > >       kfree(msg);
> > > > >  }
> > > > >
> > > > > +static struct cros_ec_sensor_platform sensor_platforms[] = {
> > > > > +     { .sensor_num = 0 },
> > > > > +     { .sensor_num = 1 }
> > > > > +};
> > > >
> > > > I'm still very uncomfortable with this struct.
> > > >
> > > > Other than these indices, the sensors have no other distinguishing
> > > > features, thus there should be no need to identify or distinguish
> > > > between them in this way.
> > > When initializing the sensors, the IIO driver expect to find in the
> > > data  structure pointed by dev_get_platdata(dev), in field sensor_num
> > > is stored the index assigned by the embedded controller to talk to a
> > > given sensor.
> > > cros_ec_sensors_register() use the same mechanism; in that function,
> > > the sensor_num field is populated from the output of an EC command
> > > MOTIONSENSE_CMD_INFO. In case of legacy mode, that command may not be
> > > available and in any case we know the EC has only either 2
> > > accelerometers present or nothing.
> > >
> > > For instance, let's compare a legacy device with a more recent one:
> > >
> > > legacy:
> > > type                  |   id          | sensor_num   | device name
> > > accelerometer  |   0           |   0                  | cros-ec-accel.0
> > > accelerometer  |   1           |   1                  | cros-ec-accel.1
> > >
> > > Modern:
> > > type                  |   id          | sensor_num   | device name
> > > accelerometer  |   0           |   0                  | cros-ec-accel.0
> > > accelerometer  |   1           |   1                  | cros-ec-accel.1
> > > gyroscope        |    0          |    2                 | cros-ec-gyro.0
> > > magnetometer |    0          |   3                  | cros-ec-mag.0
> > > light                  |    0          |   4                  | cros-ec-light.0
> > > ...
> >
> > Why can't these numbers be assigned at runtime?
> I assume you want to know why IIO drivers need to know "sensor_num"
> ahead of time. It is because each IIO driver is independent from the
> other.
> Let assume there was 2 light sensors in the device:
> type                  |   id          | sensor_num   | device name
>  light                  |    0          |   4                  | cros-ec-light.0
>  light                  |    1          |   5                  | cros-ec-light.1
> 
> In case of sensors of the same type without sensor_num, cros-ec-light
> driver has no information at probe time if it should bind to sensors
> named by the EC 4 or 5.
> 
> We could get away with cros-ec-accel, as EC always presents
> accelerometers with sensor_num  0 and 1, but I don't want to rely on
> this property in the general case.
> Only cros_ec_dev MFD driver has the global view of all sensors available.

Well seeing as this implementation has already been accepted and you're
only *using* it, rather than creating it, I think this conversation is
moot.  It looks like the original implementation patch was not
reviewed by me, which is frustrating since I would have NACKed it.

Just so you know, pointlessly enumerating identical devices manually
is not a good practice.  It is one we reject all the time.  This
imp. should too have been rejected on submission.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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