[RFC,0/2] gpiolib: Initial, basic support for shared GPIO lines

Hi,

The initial support can replace all use of GPIOD_FLAGS_BIT_NONEXCLUSIVE if the
shared GPIO is configured to follow pass through 'strategy' for the shared GPIO
pin.

I have only implemented DT support.

With the shared gpio support one can choose between three different strategy for
managing the shared gpio:
refcounted low: Keep the line low as long as there is at least one low
		request is registered
refcounted high: Keep the line high as long as there is at least one high
		request is registered
pass through: all requests are allowed to go through without refcounting.

Few shortcomings as of now:
- can not handle different GPIO_ACTIVE_ on the user side, both the root GPIO
  (which is shared) and clients must have the same GPIO_ACTIVE_ mode.
  We are using common gpio_desc.
  Like with GPIOD_FLAGS_BIT_NONEXCLUSIVE
- refcounting counts _all_ 1/0 requests coming from the users of the shared
  GPIO. This could cause issues if clients are using the gpiod API in unbalanced
  way.
  We would need to have separate tracking for each of the clients and agregate
  the level they are asking for at any moment. Basically a new gpio-chip on top
  of the real gpio pin can solve this.

Regards,
Peter
---
Peter Ujfalusi (2):
  dt-bindings: gpio: Document shared GPIO line usage
  gpiolib: Support for (output only) shared GPIO line

 .../devicetree/bindings/gpio/gpio.txt         |  66 +++++++++
 drivers/gpio/gpiolib-of.c                     |  28 +++-
 drivers/gpio/gpiolib.c                        | 132 ++++++++++++++++--
 drivers/gpio/gpiolib.h                        |  10 ++
 4 files changed, 223 insertions(+), 13 deletions(-)

Hi,

On 20/11/2019 15.34, Peter Ujfalusi wrote:
> Hi,
> 
> The initial support can replace all use of GPIOD_FLAGS_BIT_NONEXCLUSIVE if the
> shared GPIO is configured to follow pass through 'strategy' for the shared GPIO
> pin.
> 
> I have only implemented DT support.
> 
> With the shared gpio support one can choose between three different strategy for
> managing the shared gpio:
> refcounted low: Keep the line low as long as there is at least one low
> 		request is registered
> refcounted high: Keep the line high as long as there is at least one high
> 		request is registered
> pass through: all requests are allowed to go through without refcounting.
> 
> Few shortcomings as of now:
> - can not handle different GPIO_ACTIVE_ on the user side, both the root GPIO
>   (which is shared) and clients must have the same GPIO_ACTIVE_ mode.
>   We are using common gpio_desc.
>   Like with GPIOD_FLAGS_BIT_NONEXCLUSIVE
> - refcounting counts _all_ 1/0 requests coming from the users of the shared
>   GPIO. This could cause issues if clients are using the gpiod API in unbalanced
>   way.
>   We would need to have separate tracking for each of the clients and agregate
>   the level they are asking for at any moment. Basically a new gpio-chip on top
>   of the real gpio pin can solve this.

I forgot to add an example for DT.

I have this for two pcm3168a codec, their RST pin is connected to
tca6416's p0 pin.
The codec's RST line is low active, so the gpio-shared is configured to
refcounted high to make sure that a codec is not placed in reset when it
does not want it.

&main_i2c3 {
	#address-cells = <1>;
	#size-cells = <0>;

	audio_exp: gpio@21 {
		compatible = "ti,tca6416";
		reg = <0x21>;
		gpio-controller;
		#gpio-cells = <2>;

		p00 {
			gpio-shared;
			gpios = <0 GPIO_ACTIVE_LOW>;
			output-high;
			refcounted-high;
			line-name = "CODEC RESET";
		};
	};

	pcm3168a_a: audio-codec@47 {
		compatible = "ti,pcm3168a";
		reg = <0x47>;
		#sound-dai-cells = <1>;

		reset-gpios = <&audio_exp 0 GPIO_ACTIVE_LOW>;
	};

	pcm3168a_b: audio-codec@46 {
		compatible = "ti,pcm3168a";
		reg = <0x46>;
		#sound-dai-cells = <1>;

		reset-gpios = <&audio_exp 0 GPIO_ACTIVE_LOW>;
	};
};


> 
> Regards,
> Peter
> ---
> Peter Ujfalusi (2):
>   dt-bindings: gpio: Document shared GPIO line usage
>   gpiolib: Support for (output only) shared GPIO line
> 
>  .../devicetree/bindings/gpio/gpio.txt         |  66 +++++++++
>  drivers/gpio/gpiolib-of.c                     |  28 +++-
>  drivers/gpio/gpiolib.c                        | 132 ++++++++++++++++--
>  drivers/gpio/gpiolib.h                        |  10 ++
>  4 files changed, 223 insertions(+), 13 deletions(-)
> 

- Péter

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