linux-kernel - Re: [PATCH] pinctrl: at91-pio4: add support for drive-strength property

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Message-ID: <20180201095021.GA2580@rfolt0960.corp.atmel.com>
Date:   Thu, 1 Feb 2018 10:50:21 +0100
From:   Ludovic Desroches <ludovic.desroches@...rochip.com>
To:     Rob Herring <robh@...nel.org>
CC:     Linus Walleij <linus.walleij@...aro.org>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        <linux-gpio@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pinctrl: at91-pio4: add support for drive-strength
 property

On Mon, Jan 29, 2018 at 01:01:30PM -0600, Rob Herring wrote:
> On Mon, Jan 22, 2018 at 09:37:38AM +0100, Linus Walleij wrote:
> > On Thu, Jan 18, 2018 at 5:02 PM, Ludovic Desroches
> > <ludovic.desroches@...rochip.com> wrote:
> > 
> > > Add support for the drive-strength property. Usually its value is
> > > expressed in mA. Since the numeric value depends on VDDIOP voltage,
> > > the controller uses low, medium and high to define the drive-strengh.
> > 
> > Aha I see. That's complex. It certainly results in a certain mA drive
> > strength in the end, but what you're saying is that this is not usually
> > what we configure.
> > 
> > > The PIO controller accepts two values for the low drive: 0 or 1. Most
> > > of the time, we don't care about the drive strength, there is no need
> > > to change it, so 0 is considered as the default value.
> > 
> > Do you mean default value as in "whatever the hardware was set
> > up as at boot time"?
> > 
> > > The low-drive
> > > value won't be advertised through pinconf-pins file excepted if it
> > 
> > except?
> > 
> > > has been set explicitly in the device tree ie if its value is
> > > different from 0.
> > >
> > > Signed-off-by: Ludovic Desroches <ludovic.desroches@...rochip.com>
> > 
> > OK I think I get it.
> > 
> > >  Optional properties:
> > >  - GENERIC_PINCONFIG: generic pinconfig options to use, bias-disable,
> > > -bias-pull-down, bias-pull-up, drive-open-drain, input-schmitt-enable,
> > > -input-debounce, output-low, output-high.
> > > +bias-pull-down, bias-pull-up, drive-open-drain, drive-strength,
> > > +input-schmitt-enable, input-debounce, output-low, output-high.
> > (...)
> > > +                       drive-strength = <ATMEL_PIO_DRVSTR_LO>;
> > 
> > So you say you support this argument and it will be something like
> > 
> > include/dt-bindings/pinctrl/at91.h:#define ATMEL_PIO_DRVSTR_LO  1
> > include/dt-bindings/pinctrl/at91.h:#define ATMEL_PIO_DRVSTR_ME  2
> > include/dt-bindings/pinctrl/at91.h:#define ATMEL_PIO_DRVSTR_HI  3
> > 
> > But the definition if generic drive strength is actually in mA.
> 
> Yes, and the reason we put unit suffixes on properties is to avoid 
> differing units.
>  
> > I think it is OK to deviate from stating it in mA, but you should
> > write this in the DT bindings so people do not get confused.
> 
> I don't think it is okay. If "drive-strength" doesn't work, then use 
> a vendor specific property ("atmel,drive-strength"). Of course, I might 
> forget this in the next version and tell you to use a standard property 
> in mA. 

Ok, so I am going to send the next version! The purpose is to keep using the
generic pinconf parsing function.
If I need to use "atmel,drive-strength" property, I assume I could
manage it with custom_params, isn't it?

Regards

Ludovic