linux-kernel - Re: [PATCH 2/6] staging:iio:ad2s90: Remove spi setup that should be done via dt

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Message-ID: <CAHd-oW5ax5uj=xtpOD3HYHT+TM8eAZ6kJ91mHpFVf+VFX6NoeQ@mail.gmail.com>
Date:   Thu, 15 Nov 2018 12:44:39 -0200
From:   Matheus Tavares Bernardino <matheus.bernardino@....br>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, linux-iio@...r.kernel.org,
        devel@...verdev.osuosl.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        kernel-usp@...glegroups.com,
        Victor Colombo <victorcolombo@...il.com>, broonie@...nel.org
Subject: Re: [PATCH 2/6] staging:iio:ad2s90: Remove spi setup that should be
 done via dt

On Sun, Nov 11, 2018 at 9:42 AM Jonathan Cameron <jic23@...nel.org> wrote:
>
> On Fri,  9 Nov 2018 20:00:40 -0200
> Matheus Tavares <matheus.bernardino@....br> wrote:
>
> > The ad2s90 driver currently sets some spi settings (max_speed_hz and
> > mode) at ad2s90_probe. This should, instead, be handled via device tree.
> > This patch removes these configurations from the probe function.
> >
> > Note: The way in which the mentioned spi settings need to be specified
> > on the ad2s90's node of a device tree will be documented in the future
> > patch "dt-bindings:iio:resolver: Add docs for ad2s90".
> >
> > Signed-off-by: Matheus Tavares <matheus.bernardino@....br>
> I'd actually like to get Rob and Mark's views on this one.  Previously
> I would just have applied it without thinking on the basis these can
> be easily specified from devicetree.
>
> Recent discussions with Rob have suggested that the settings in devicetree
> should perhaps be concerned with specifying constraints about the device
> that are not visible to the driver.  The driver itself should apply
> the device constraints, but there are others such as wiring that
> might reduce the maximum frequency for example...
>
> So should a driver be clamping an over specified value from DT for
> example?  Or given that is optional in DT, should it be making sure
> that a controller max frequency isn't too high for the sensor?
>

First of all, thanks for the review and comments.

By what you've said here and in the reviews for patches 3 and 4 of
this patch-set, it seems to me that the most reasonable thing would be
to keep the SPI mode 3 settings at the driver but the max frequency
setting at DT and check if it exceeds the maximum at the driver (as
patch 3 does). This makes sense to me, based on what you've said,
because mode 3 is a device constraint visible to the driver (as it
won't change) but max frequency is not (because of things such as
wiring, as you said).

What do you think, Jonathan, Rob, and Mark?

Matheus

> It seems to be unusual to do this, but to my mind it would make
> sense and might be worth pushing out into more drivers.
>
> Jonathan
> > ---
> >  drivers/staging/iio/resolver/ad2s90.c | 11 -----------
> >  1 file changed, 11 deletions(-)
> >
> > diff --git a/drivers/staging/iio/resolver/ad2s90.c b/drivers/staging/iio/resolver/ad2s90.c
> > index ff32ca76ca00..95c118c48400 100644
> > --- a/drivers/staging/iio/resolver/ad2s90.c
> > +++ b/drivers/staging/iio/resolver/ad2s90.c
> > @@ -77,7 +77,6 @@ static int ad2s90_probe(struct spi_device *spi)
> >  {
> >       struct iio_dev *indio_dev;
> >       struct ad2s90_state *st;
> > -     int ret;
> >
> >       indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
> >       if (!indio_dev)
> > @@ -94,16 +93,6 @@ static int ad2s90_probe(struct spi_device *spi)
> >       indio_dev->num_channels = 1;
> >       indio_dev->name = spi_get_device_id(spi)->name;
> >
> > -     /* need 600ns between CS and the first falling edge of SCLK */
> > -     spi->max_speed_hz = 830000;
> > -     spi->mode = SPI_MODE_3;
> > -     ret = spi_setup(spi);
> > -
> > -     if (ret < 0) {
> > -             dev_err(&spi->dev, "spi_setup failed!\n");
> > -             return ret;
> > -     }
> > -
> >       return devm_iio_device_register(indio_dev->dev.parent, indio_dev);
> >  }
> >
>