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SubjectRe: [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@microchip.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@microchip.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

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