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

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Date:   Wed, 20 Nov 2019 15:34:07 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     <linus.walleij@...aro.org>, <bgolaszewski@...libre.com>,
        <robh+dt@...nel.org>
CC:     <linux-gpio@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <m.szyprowski@...sung.com>, <broonie@...nel.org>,
        <mripard@...nel.org>, <p.zabel@...gutronix.de>,
        <devicetree@...r.kernel.org>
Subject: [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(-)

-- 
Peter

Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki