linux-kernel - Re: [PATCH v2 2/2] dt-bindings: iio: dac: Add docs for AD5758 DAC

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Date:   Mon, 11 Jun 2018 10:42:35 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Stefan Popa <stefan.popa@...log.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald <pmeerw@...erw.net>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, linux-iio@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] dt-bindings: iio: dac: Add docs for AD5758 DAC

On Thu, Jun 7, 2018 at 3:11 PM, Stefan Popa <stefan.popa@...log.com> wrote:

> + - adi,slew: Array of slewrate settings should contain 3 fields:
> +       1: Should be either 0 or 1 in order to enable or disable slewrate.

We have a standard binding in pin control for "slew-rate".

It doesn't have a unit today though.

> +       2: Slew rate clock:
> +               Valid values for the slew rate update frequency [Hz]:
> +               * 240000
> +               * 200000
> +               * 150000
> +               * 128000
> +               * 64000
> +               * 32000
> +               * 16000
> +               * 8000
> +               * 4000
> +               * 2000
> +               * 1000
> +               * 512
> +               * 256
> +               * 128
> +               * 64
> +               * 16
> +       3: Slew rate step:
> +               Defines by how much the output value changes at each update.
> +               Valid values for the step size LSBs:
> +               * 4
> +               * 12
> +               * 64
> +               * 120
> +               * 256
> +               * 500
> +               * 1820
> +               * 2048

This is pretty idiomatic and just encodes the configuration fields from the
hardware into the device tree.

The normal slew rate unit is Volts per microsecond.

I get the feeling that we could define that simply, and then calculate
the slew rate clock and step using that to find the best match for
a certain slew rate.

Yours,
Linus Walleij