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Subject: Re: [PATCH/RFC 2/2] gpio: of: Add DT overlay support for GPIO hogs
To:     Geert Uytterhoeven <geert+renesas@glider.be>,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
        Rob Herring <robh+dt@kernel.org>
Cc:     Peter Ujfalusi <peter.ujfalusi@ti.com>,
        Chris Brandt <chris.brandt@renesas.com>,
        linux-gpio@vger.kernel.org, devicetree@vger.kernel.org,
        linux-renesas-soc@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20191230133852.5890-1-geert+renesas@glider.be>
 <20191230133852.5890-3-geert+renesas@glider.be>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <41e1c51e-bc17-779e-8c68-bf2e652871eb@gmail.com>
Date:   Mon, 6 Jan 2020 17:34:31 -0600
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On 12/30/19 7:38 AM, Geert Uytterhoeven wrote:
> As GPIO hogs are configured at GPIO controller initialization time,
> adding/removing GPIO hogs in DT overlays does not work.
> 
> Add support for GPIO hogs described in DT overlays by registering an OF
> reconfiguration notifier, to handle the addition and removal of GPIO hog
> subnodes to/from a GPIO controller device node.
> 
> Note that when a GPIO hog device node is being removed, its "gpios"
> properties is no longer available, so we have to keep track of which
> node a hog belongs to, which is done by adding a pointer to the hog's
> device node to struct gpio_desc.

If I have read the patches and the existing overlay source correctly,
then some observations:

- A gpio hog node added in an overlay will be properly processed.

- A gpio hog node already existing in the live devicetree, but with a
  non-active status will be properly processed if the status of the
  gpio hog node is changed to "ok" in the overlay.

- If a gpio hog node already exists in the live devicetree with an
  active status, then any updated or added properties in that gpio
  hog node in the overlay will have no effect.

  There is a scenario where the updated property would have an effect:
  apply a second overlay that sets the status to inactive, then apply
  a third overlay that sets the status back to active.  This is a
  rather contrived example and I think it should be documented as
  not supported and the result undefined.

It would be good to document this explicitly.


> 
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> ---
>  drivers/gpio/gpiolib-of.c | 84 +++++++++++++++++++++++++++++++++++++++
>  drivers/gpio/gpiolib-of.h |  2 +
>  drivers/gpio/gpiolib.c    | 14 +++++--
>  drivers/gpio/gpiolib.h    |  3 ++
>  4 files changed, 100 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpio/gpiolib-of.c b/drivers/gpio/gpiolib-of.c
> index dfae797846bb746b..89a6138ac0a4b506 100644
> --- a/drivers/gpio/gpiolib-of.c
> +++ b/drivers/gpio/gpiolib-of.c
> @@ -649,6 +649,10 @@ static int of_gpiochip_add_hog(struct gpio_chip *chip, struct device_node *hog)
>  		ret = gpiod_hog(desc, name, lflags, dflags);
>  		if (ret < 0)
>  			return ret;
> +
> +#ifdef CONFIG_OF_DYNAMIC
> +		desc->hog = hog;
> +#endif
>  	}
>  
>  	return 0;
> @@ -676,11 +680,91 @@ static int of_gpiochip_scan_gpios(struct gpio_chip *chip)
>  			of_node_put(np);
>  			return ret;
>  		}
> +
> +		of_node_set_flag(np, OF_POPULATED);
>  	}
>  
>  	return 0;
>  }
>  
> +#ifdef CONFIG_OF_DYNAMIC
> +/**
> + * of_gpiochip_remove_hog - Remove all hogs in a hog device node
> + * @chip:	gpio chip to act on
> + * @hog:	device node describing the hogs
> + */
> +static void of_gpiochip_remove_hog(struct gpio_chip *chip,
> +				   struct device_node *hog)
> +{
> +	struct gpio_desc *descs = chip->gpiodev->descs;
> +	unsigned int i;
> +
> +	for (i = 0; i < chip->ngpio; i++) {
> +		if (test_bit(FLAG_IS_HOGGED, &descs[i].flags) &&
> +		    descs[i].hog == hog)
> +			gpiochip_free_own_desc(&descs[i]);
> +	}
> +}
> +
> +static int of_gpiochip_match_node(struct gpio_chip *chip, void *data)
> +{
> +	return chip->gpiodev->dev.of_node == data;
> +}
> +
> +static struct gpio_chip *of_find_gpiochip_by_node(struct device_node *np)
> +{
> +	return gpiochip_find(np, of_gpiochip_match_node);
> +}
> +
> +static int of_gpio_notify(struct notifier_block *nb, unsigned long action,
> +			  void *arg)
> +{
> +	struct of_reconfig_data *rd = arg;
> +	struct gpio_chip *chip;
> +	int ret;
> +
> +	switch (of_reconfig_get_state_change(action, arg)) {
> +	case OF_RECONFIG_CHANGE_ADD:
> +		if (!of_property_read_bool(rd->dn, "gpio-hog"))
> +			return NOTIFY_OK;	/* not for us */
> +
> +		if (of_node_test_and_set_flag(rd->dn, OF_POPULATED))
> +			return NOTIFY_OK;

I don't think OF_POPULATED could be already set.  It seems to be a
bug if it is.


> +
> +		chip = of_find_gpiochip_by_node(rd->dn->parent);
> +		if (chip == NULL)
> +			return NOTIFY_OK;	/* not for us */

If I understand correctly, "not for us" is a misleading comment.
The notification is for the node rd->dn->parent, but the device
does not exist, so we can't do the hogging at the moment.  (If the
device is created later, then the gpio hog child node will exist,
and the init will "do the right thing" with the hog node -- so
not a problem.)

 
> +
> +		ret = of_gpiochip_add_hog(chip, rd->dn);
> +		if (ret < 0) {
> +			pr_err("%s: failed to add hogs for %pOF\n", __func__,
> +			       rd->dn);
> +			of_node_clear_flag(rd->dn, OF_POPULATED);
> +			return notifier_from_errno(ret);
> +		}
> +		break;
> +
> +	case OF_RECONFIG_CHANGE_REMOVE:
> +		if (!of_node_check_flag(rd->dn, OF_POPULATED))
> +			return NOTIFY_OK;	/* already depopulated */

I don't think OF_POPULATED could be already cleared.  It seems to be a
bug if it is.

> +
> +		chip = of_find_gpiochip_by_node(rd->dn->parent);
> +		if (chip == NULL)
> +			return NOTIFY_OK;	/* not for us */

Again, a misleading comment.

> +
> +		of_gpiochip_remove_hog(chip, rd->dn);
> +		of_node_clear_flag(rd->dn, OF_POPULATED);
> +		break;
> +	}
> +
> +	return NOTIFY_OK;
> +}
> +
> +struct notifier_block gpio_of_notifier = {
> +	.notifier_call = of_gpio_notify,
> +};
> +#endif /* CONFIG_OF_DYNAMIC */
> +
>  /**
>   * of_gpio_simple_xlate - translate gpiospec to the GPIO number and flags
>   * @gc:		pointer to the gpio_chip structure
> diff --git a/drivers/gpio/gpiolib-of.h b/drivers/gpio/gpiolib-of.h
> index 9768831b1fe2f25b..ed26664f153782fc 100644
> --- a/drivers/gpio/gpiolib-of.h
> +++ b/drivers/gpio/gpiolib-of.h
> @@ -35,4 +35,6 @@ static inline bool of_gpio_need_valid_mask(const struct gpio_chip *gc)
>  }
>  #endif /* CONFIG_OF_GPIO */
>  
> +extern struct notifier_block gpio_of_notifier;
> +
>  #endif /* GPIOLIB_OF_H */
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index bff5ac774d870b67..ef12cfcaf0962c1c 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -2952,6 +2952,9 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
>  		clear_bit(FLAG_PULL_DOWN, &desc->flags);
>  		clear_bit(FLAG_BIAS_DISABLE, &desc->flags);
>  		clear_bit(FLAG_IS_HOGGED, &desc->flags);
> +#ifdef CONFIG_OF_DYNAMIC
> +		desc->hog = NULL;
> +#endif
>  		ret = true;
>  	}
>  
> @@ -5145,10 +5148,15 @@ static int __init gpiolib_dev_init(void)
>  	if (ret < 0) {
>  		pr_err("gpiolib: failed to allocate char dev region\n");
>  		bus_unregister(&gpio_bus_type);
> -	} else {
> -		gpiolib_initialized = true;
> -		gpiochip_setup_devs();
> +		return ret;
>  	}
> +
> +	gpiolib_initialized = true;
> +	gpiochip_setup_devs();
> +
> +	if (IS_ENABLED(CONFIG_OF_DYNAMIC))
> +		WARN_ON(of_reconfig_notifier_register(&gpio_of_notifier));
> +
>  	return ret;
>  }
>  core_initcall(gpiolib_dev_init);
> diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
> index a4a759920faa48ab..7af9931e8572304a 100644
> --- a/drivers/gpio/gpiolib.h
> +++ b/drivers/gpio/gpiolib.h
> @@ -118,6 +118,9 @@ struct gpio_desc {
>  	const char		*label;
>  	/* Name of the GPIO */
>  	const char		*name;
> +#ifdef CONFIG_OF_DYNAMIC
> +	struct device_node	*hog;
> +#endif
>  };
>  
>  int gpiod_request(struct gpio_desc *desc, const char *label);
>