RE: [PATCH v6 01/13] pinctrl: core: Add pinctrl_get_device()

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Hi Linus and Andy,

> Subject: Re: [PATCH v6 01/13] pinctrl: core: Add pinctrl_get_device()
> 
> On Tue, Mar 7, 2023 at 9:13 AM Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> > > Mon, Mar 06, 2023 at 09:00:02AM +0000, Biju Das kirjoitti:
> 
> > > > Add pinctrl_get_device() to find a device handle associated with a
> > > > pincontrol group(i.e. by matching function name and group name for
> > > > a device). This device handle can then be used for finding match
> > > > for the pin output disable device that protects device against
> > > > short circuits on the pins.
> > >
> > > Not sure I understand the use case. Please, create a better commit
> message.
> >
> > OK, Basically pinmux_enable_setting allows exclusive access of pin to a
> device.
> > It won't allow multiple devices to claim a pin.
> 
> So what is the use case? Which two devices need to use the same pin at the
> same time?

Andy asked a question Can a pin be used by multiple devices at same time
My answer is no. 

The reason is,

There will be a single owner claiming a pin at given time.
setting->data.mux will be unique for a pin. So there won't be a collision when
looking by the same pair of function and group name from different device.

> 
> You can already:
> 
> 1) Use the same pin with different devices at different times, because
>    pin configs can be changed arbitrarily at runtime, see for example:
>   drivers/i2c/muxes/i2c-demux-pinctrl.c

Agreed.

> 
> 2) Mux a pin to a certain device *and* use it for GPIO at the same time,
>   all that is needed is to set .strict to false in struct pinmux_ops.
>   This should be false if e.g. the GPIO can be used to "sample" the
>   output of a I2C block connected to the same pins, so the two
>   functions (I2C and GPIO) are not electrically decoupled.
> 
> So do you really have a use case where two devices need to use the same pin
> at the same time? I've seen much but I haven't seen this before!
> Which two devices are that?

I don't have a use case like that.

Cheers,
Biju




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