Re: [PATCH v2 3/3] gpiolib: Add GPIO initialization

[Lothar Waßmann <LW@xxxxxxxxxxxxxxxxxxx>]

Hi,

On Tue, 7 Feb 2017 12:09:50 +0100 Uwe Kleine-König wrote:
> Hello,
> 
> (Markus left Pengutronix, so I dropped his non-working address from the
> recipients and pick up his discussion.)
> 
> as far as I see, this topic isn't closed yet. I want something similar,
> maybe even a bit more than this patch achieves:
> 
> Some gpios should get a fixed direction. If such a gpio is configured as
> output, a value should be specified. This is already working today with
> gpio-hogs.
> 
> Now additionally I want to initialize some gpios but allow them to be
> grabbed later. IMHO there are the following new cases:
> 
> It should be possible to:
> 
>  a) change the value of a gpio initially configured as output
>  b) change the direction of a gpio initially configured as output
>  c) change the direction of a gpio initially configured as input
> 
> IMHO the dts should describe which case should be applied to a given
> gpio.
> 
> Is it a valid assumption that a gpio that can change direction is also
> allowed to change value when configured as output? I assume this in the
> following discussion, some details need to change if this shouldn't be
> implied. (I think we need a d) then, not sure how this should look
> though.)
> 
> I'd suggest the following description for these cases:
> 
>  a)
>  	/*
> 	 * initially configured as low output. Consumer can do
> 	 * gpio_set_value(..., 1); but not gpio_direction_input(...);
> 	 */
>  	nodename {
> 		gpio-hog;
> 		gpios = <...>;
> 		output-low-init;
> 	};
> 
>  b)
>  	/*
> 	 * initially configured as low output. Consumer can do
> 	 * gpio_set_value(..., 1); and gpio_direction_input(...);
> 	 */
>  	nodename {
> 		gpio-hog;
> 		gpios = <...>;
> 		output-init-low;
> 	};
> 
>  c)
>  	/*
> 	 * initially configured as input. Consumer can do
> 	 * gpio_direction_output(..., ...) and then set the value
> 	 * freely.
> 	 */
> 	 nodename {
> 	 	gpio-hog;
> 		gpios = <...>;
> 		input-init;
> 	};
> 
> I don't like that "output-low-init" and "output-init-low" are so
> similar, so if someone suggests a better naming scheme, that would be
> great.
> 
The position of the 'output' or 'input' within the word could
imply whether the direction can be changed lateron or not. E.g.:
output-init-low => an output whose state is initially low, but can be
                   reconfigured to high lateron.

init-low-output => a gpio that is initially configured as output low,  
                   but can subsequently be reconfigured as input or to
                   a different state

init-input      => your case c)
input           => a GPIO input that cannot be reconfigured

IOW: if the pin direction is mentioned in front of the 'init', it
cannot be changed lateron, if it is mentioned after the 'init' it can
be changed. If nothing follows 'init', the 'init' can be omitted.
This would also allow an 'output-low' as a GPIO that is permanently
grounded...

Lothar Waßmann
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