linux-kernel - Re: [PATCH 4/4] iio: resolver: add new driver for AD2S1210

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Message-ID: <CAMknhBEVLZYQieKPCCf8QHp7H06Vsdobg_7qGm+MsbL8Kg-VCQ@mail.gmail.com>
Date:   Fri, 22 Sep 2023 09:54:57 +0200
From:   David Lechner <dlechner@...libre.com>
To:     Rob Herring <robh@...nel.org>
Cc:     linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Jonathan Cameron <jic23@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Nuno Sá <nuno.sa@...log.com>,
        Axel Haslam <ahaslam@...libre.com>,
        Philip Molloy <pmolloy@...libre.com>
Subject: Re: [PATCH 4/4] iio: resolver: add new driver for AD2S1210

On Thu, Sep 21, 2023 at 5:45 PM Rob Herring <robh@...nel.org> wrote:
>
> On Wed, Sep 20, 2023 at 12:02:53PM -0500, David Lechner wrote:
> > This adds a new driver for Analog Devices, Inc. AD2S1210 resolver to
> > digital converter. The driver is based on the staging driver with the
> > following improvements:
> >

...

> > - Use devicetree to specify resolution instead of sysfs attribute.
>
> Why? sysfs allows a user to change at run-time. DT is a firmware
> change. What/who determines the resolution? Unless it's the hardware
> design/designer, it probably doesn't belong in DT.
>

This one is a bit of a gray area since it could be either way. The chip
has two input pins for selecting the resolution. If the hardware
designer decides to connect these to gpios, then they could be changed
at runtime. But if the hardware designer decides to hard-wire these pins
then we need to supply this information via the devicetree.

The choice of the resolution is determined by the physical requirements
of the full system. This device measures rotational position and velocity.
So the main reason to pick something less than the full 16-bit resolution is
if the max rotational speed of whatever is connected is going to be greater
than 125 revolutions per second (assuming 8.192 MHz clock).