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

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Message-ID: <20180122170545.GD3055@rfolt0960.corp.atmel.com>
Date:   Mon, 22 Jan 2018 18:05:45 +0100
From:   Ludovic Desroches <ludovic.desroches@...rochip.com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     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 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 Interface is:
Drive Strength:
0 Low drive
1 Low drive
2 Medium drive
3 High drive

In the Electrical Characteristics, you have:
GPIO 1.8V Low: max 1 mA
GPIO 1.8V Medium: max 10 mA
GPIO 1.8V High: max 18 mA
GPIO 3.3V Low: max 2 mA
GPIO 3.3V Medium: max 20 mA
GPIO 3.3V High: max 32 mA

> > 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"?

Yes

> 
> > The low-drive
> > value won't be advertised through pinconf-pins file excepted if it
> 
> except?

Yes

> 
> > 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.
> 
> 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.
> 

Ok

> > @@ -814,6 +834,19 @@ static void atmel_conf_pin_config_dbg_show(struct pinctrl_dev *pctldev,
> >                 seq_printf(s, "%s ", "open-drain");
> >         if (conf & ATMEL_PIO_SCHMITT_MASK)
> >                 seq_printf(s, "%s ", "schmitt");
> > +       if (conf & ATMEL_PIO_DRVSTR_MASK) {
> > +               switch ((conf & ATMEL_PIO_DRVSTR_MASK) >> ATMEL_PIO_DRVSTR_OFFSET) {
> > +               case ATMEL_PIO_DRVSTR_LO:
> > +                       seq_printf(s, "%s ", "low-drive");
> > +                       break;
> > +               case ATMEL_PIO_DRVSTR_ME:
> > +                       seq_printf(s, "%s ", "medium-drive");
> > +                       break;
> > +               case ATMEL_PIO_DRVSTR_HI:
> > +                       seq_printf(s, "%s ", "high-drive");
> > +                       break;
> 
> But here you know what VDDIOP is, right?
> 

No, I have no way to retrieve the power for the several VDDIOP power
rails.

> So in debugfs you could actually print the real drive strength
> in mA, or both "medium-drive, %u mA"?
> 
> Yours,
> Linus Walleij

Regards

Ludovic