Message-ID: <767645a7-835d-4ac8-ac70-a701cea6df30@linaro.org>
Date: Tue, 9 Jan 2024 11:59:13 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Philipp Zabel <p.zabel@...gutronix.de>,
 Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
 Banajit Goswami <bgoswami@...cinc.com>,
 Bjorn Andersson <andersson@...nel.org>,
 Konrad Dybcio <konrad.dybcio@...aro.org>, Liam Girdwood
 <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>,
 Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
 Conor Dooley <conor+dt@...nel.org>, Jaroslav Kysela <perex@...ex.cz>,
 Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
 linux-arm-msm@...r.kernel.org, linux-sound@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Bartosz Golaszewski <brgl@...ev.pl>,
 Sean Anderson <sean.anderson@...o.com>
Subject: Re: [PATCH v2 2/4] reset: Instantiate reset GPIO controller for
 shared reset-gpios

On 08/01/2024 13:17, Philipp Zabel wrote:
> On Fr, 2024-01-05 at 16:59 +0100, Krzysztof Kozlowski wrote:
>> Devices sharing a reset GPIO could use the reset framework for
>> coordinated handling of that shared GPIO line.  We have several cases of
>> such needs, at least for Devicetree-based platforms.
>>
>> If Devicetree-based device requests a reset line, which is missing but
>                                                     ^^^^^^^^^^^^^^^^
> Nitpick: the "resets" property is missing, not the reset line.
> 
> "If Devicetree-based device requests a reset line, but there only is a
>  reset-gpios property instead of a "resets" property, ..." maybe?

Ack

> 
>> there is a reset-gpios property, instantiate a new "reset-gpio" platform
>> device which will handle such reset line.  This allows seamless handling
>> of such shared reset-gpios without need of changing Devicetree binding [1].
>>
>> All newly registered "reset-gpio" platform devices will be stored on
>> their own list to avoid any duplicated devices.
> 
> That's not strictly true. The reset_gpio_device_list only contains the
> of_phandle_args for lookup.

Ack, I will re-phrase it.

> 
>> The key to find each of
>> such platform device is the entire Devicetree GPIO specifier: phandle to
>> GPIO controller, GPIO number and GPIO flags.  If two devices have
>> conflicting "reset-gpios" property, e.g. with different ACTIVE_xxx
>> flags, this would spawn two separate "reset-gpio" devices, where the
>> second would fail probing on busy GPIO reques
> 
> request.

Ack

> 
> Is that true? 

It should be true and my tests confirmed it.

The code below looks like overwrites of_phandle_args so
> that only one reset-gpio device is spawned for each gpio node.

The code will iterate over list of of_node and of_phandle_args and
compare them with __reset_gpios_args_match(). If all match: do not
create new platform device.

If they do not match, e.g. ACTIVE_LOW -> ACTIVE_HIGH, create new
platform device. This will be the second device for the same GPIO.
Probing of that device in reset-gpio driver will fail:

[   19.198775] reset-gpio reset-gpio.2.auto: error -EBUSY: Could not get
reset gpios

because GPIO is used by reset-gpio.1.auto already.

> 
>> Link: https://lore.kernel.org/all/YXi5CUCEi7YmNxXM@robh.at.kernel.org/ [1]
>> Cc: Bartosz Golaszewski <brgl@...ev.pl>
>> Cc: Sean Anderson <sean.anderson@...o.com>
>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
>> ---
>>  drivers/reset/core.c             | 176 ++++++++++++++++++++++++++++---
>>  include/linux/reset-controller.h |   4 +
>>  2 files changed, 167 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/reset/core.c b/drivers/reset/core.c
>> index 4d5a78d3c085..ec9b3ff419cf 100644
>> --- a/drivers/reset/core.c
>> +++ b/drivers/reset/core.c
>> @@ -13,6 +13,7 @@
>>  #include <linux/module.h>
>>  #include <linux/of.h>
>>  #include <linux/acpi.h>
>> +#include <linux/platform_device.h>
>>  #include <linux/reset.h>
>>  #include <linux/reset-controller.h>
>>  #include <linux/slab.h>
>> @@ -23,6 +24,10 @@ static LIST_HEAD(reset_controller_list);
>>  static DEFINE_MUTEX(reset_lookup_mutex);
>>  static LIST_HEAD(reset_lookup_list);
>>  
>> +/* Protects reset_gpio_device_list */
>> +static DEFINE_MUTEX(reset_gpio_device_mutex);
>> +static LIST_HEAD(reset_gpio_device_list);
> 
> I would call this reset_gpio_lookup_list or
> reset_gpio_phandle_args_list.

Ack

> 
>> +
>>  /**
>>   * struct reset_control - a reset control
>>   * @rcdev: a pointer to the reset controller device
>> @@ -63,6 +68,16 @@ struct reset_control_array {
>>  	struct reset_control *rstc[] __counted_by(num_rstcs);
>>  };
>>  
>> +/**
>> + * struct reset_gpio_device - ad-hoc created reset-gpio device
>> + * @of_args: phandle to the reset controller with all the args like GPIO number
>> + * @list: list entry for the reset_lookup_list
>> + */
>> +struct reset_gpio_device {
> 
> Similarly, I would call this reset_gpio_lookup or
> reset_gpio_phandle_args.

Ack

> 
>> +	struct of_phandle_args of_args;
>> +	struct list_head list;
>> +};
>> +
>>  static const char *rcdev_name(struct reset_controller_dev *rcdev)
>>  {
>>  	if (rcdev->dev)
>> @@ -813,13 +828,119 @@ static void __reset_control_put_internal(struct reset_control *rstc)
>>  	kref_put(&rstc->refcnt, __reset_control_release);
>>  }
>>  
>> +static bool __reset_gpios_args_match(const struct of_phandle_args *a1,
>> +				     const struct of_phandle_args *a2)
>> +{
>> +	unsigned int i;
>> +
>> +	if (!a2)
>> +		return false;
>> +
>> +	if (a1->args_count != a2->args_count)
>> +		return false;
>> +
>> +	for (i = 0; i < a1->args_count; i++)
>> +		if (a1->args[i] != a2->args[i])
>> +			break;
> 
> Just return false in the loop and simplify the following to return
> true.

Ack
> 
>> +
>> +	/* All args matched? */
>> +	if (i == a1->args_count)
>> +		return true;
>> +
>> +	return false;
>> +}
>> +
>> +/*
>> + * @node:	node of the device requesting reset
>> + * @reset_args:	phandle to the reset controller with all the args like GPIO number
>> + */
>> +static int __reset_add_reset_gpio_device(struct device_node *node,
>> +					 struct of_phandle_args *args)
>> +{
>> +	struct reset_gpio_device *rgpio_dev;
>> +	struct platform_device *pdev;
>> +	int ret;
>> +
>> +	lockdep_assert_not_held(&reset_list_mutex);
>> +
>> +	mutex_lock(&reset_gpio_device_mutex);
>> +
>> +	list_for_each_entry(rgpio_dev, &reset_gpio_device_list, list) {
>> +		if (args->np == rgpio_dev->of_args.np) {
>> +			if (__reset_gpios_args_match(args,
>> +						     &rgpio_dev->of_args)) {
>> +				ret = 0;
>> +				goto out_unlock;
>> +			}
>> +		}
>> +	}
>> +
>> +	/* Not freed in normal path, persisent subsyst data */
>> +	rgpio_dev = kzalloc(sizeof(*rgpio_dev), GFP_KERNEL);
> 
> Since this is persistent, instead of letting the reset-gpio driver call
> of_parse_phandle_with_args() again, this could be passed in via
> platform data. Is there a reason not to do that instead?

We can pass it as read only platform data, but we cannot pass the
ownership. This is associated with registered platform device, not with
bound one device->driver.

Imagine case:
1. modprobe reset-gpio,
2. Driver is bound to the device,
3. unbind  (echo > unbind)
4. rmmod
5. goto 1

> 
>> +	if (!rgpio_dev) {
>> +		ret = -ENOMEM;
>> +		goto out_unlock;
>> +	}
>> +
>> +	rgpio_dev->of_args = *args;
>> +	pdev = platform_device_register_data(NULL, "reset-gpio",
>> +					     PLATFORM_DEVID_AUTO, &node,
>> +					     sizeof(node));
>> +	ret = PTR_ERR_OR_ZERO(pdev);
>> +	if (!ret)
>> +		list_add(&rgpio_dev->list, &reset_gpio_device_list);
>> +	else
>> +		kfree(rgpio_dev);
>> +
>> +out_unlock:
>> +	mutex_unlock(&reset_gpio_device_mutex);
>> +
>> +	return ret;
>> +}
>> +
>> +static struct reset_controller_dev *__reset_find_rcdev(struct of_phandle_args *args,
>> +						       bool gpio_fallback,
>> +						       const void *cookie)
> 
> Unused cookie.

Ack
> 
>> +{
>> +	struct reset_controller_dev *r, *rcdev;
>> +
>> +	lockdep_assert_held(&reset_list_mutex);
>> +
>> +	rcdev = NULL;
>> +	list_for_each_entry(r, &reset_controller_list, list) {
>> +		if (args->np == r->of_node) {
>> +			if (gpio_fallback) {
>> +				if (__reset_gpios_args_match(args, r->of_args)) {
>> +					/*
>> +					 * Fake args (take first reset) and
>> +					 * args_count (to matcg reset-gpio
> 
> match

Ack
> 
>> +					 * of_reset_n_cells) because reset-gpio
>> +					 * has only one reset and does not care
>> +					 * about reset of GPIO specifier.
>> +					 */
>> +					args->args[0] = 0;
>> +					args->args_count = 1;
> 
> I'd expect args to be an input-only argument, but here its contents are
> overwritten after a match. Why?
> 
> This has an effect in __of_reset_control_get(), that I find hard to
> follow. See below.
> 
>> +					rcdev = r;
>> +					break;
>> +				}
>> +			} else {
>> +				rcdev = r;
>> +				break;
>> +			}
>> +		}
>> +	}
>> +
>> +	return rcdev;
>> +}
>> +
>>  struct reset_control *
>>  __of_reset_control_get(struct device_node *node, const char *id, int index,
>>  		       bool shared, bool optional, bool acquired)
>>  {
>> +	struct of_phandle_args args = {0};
>> +	bool gpio_fallback = false;
>>  	struct reset_control *rstc;
>> -	struct reset_controller_dev *r, *rcdev;
>> -	struct of_phandle_args args;
>> +	struct reset_controller_dev *rcdev;
>>  	int rstc_id;
>>  	int ret;
>>  
>> @@ -839,21 +960,50 @@ __of_reset_control_get(struct device_node *node, const char *id, int index,
>>  					 index, &args);
>>  	if (ret == -EINVAL)
>>  		return ERR_PTR(ret);
>> -	if (ret)
>> -		return optional ? NULL : ERR_PTR(ret);
>> +	if (ret) {
>> +		/*
>> +		 * There can be only one reset-gpio for regular devices, so
>> +		 * don't bother with GPIO index.
>> +		 */
> 
> I don't understand this comment. The GPIO index should be checked as
> part of __reset_gpios_args_match(), or should it not?

This and earlier comment are result of a bit hacky approach to the
problem: how to find reset controllers for that GPIO?

The point is that our reset gpio controller has only 1 reset, thus
of_reset_n_cells=1. However args_count from of_parse_handle is >0, which
later is compared in reset core:

https://elixir.bootlin.com/linux/latest/source/drivers/reset/core.c#L859

That part we need to match.

I could make the reset-gpio driver to have of_reset_n_cells=2, but what
would be the point? The rest of the cells are not really relevant,
because you cannot refer to this reset gpio controller with any other
arguments.

To remind: my solution spawns one reset-gpio controller for one GPIO.

> 
>> +		ret = of_parse_phandle_with_args(node, "reset-gpios", "#gpio-cells",
>> +						 0, &args);
>> +		if (ret)
>> +			return optional ? NULL : ERR_PTR(ret);
>>  
>> -	mutex_lock(&reset_list_mutex);
>> -	rcdev = NULL;
>> -	list_for_each_entry(r, &reset_controller_list, list) {
>> -		if (args.np == r->of_node) {
>> -			rcdev = r;
>> -			break;
>> -		}
>> +		gpio_fallback = true;
> 
> Is there a reason not just call __reset_add_reset_gpio_device() here?
> With that, there should be no need to call __reset_find_rcdev() twice.

Hm, could be, although not sure if code would be simpler.

This entire function handles two cases:
1. Get normal reset controller ("resets" OF property),
2. If above fails, get reset-gpio controller ("reset-gpios" OF property)

Therefore the entire solution is following approach:
1. of_parse_phandle(resets)
1b. error? Then of_parse_phandle(reset-gpios)
2. Find reset-controller based on any of above phandles.
3. error? Check if we created reset-gpios platform device. If not:
create new reset-gpios platform device/
3b. Platform device could probe, so lookup again for reset controller or
defer probe.

What type of flow do you propose?

> 
>>  	}
>>  
>> +	mutex_lock(&reset_list_mutex);
>> +	rcdev = __reset_find_rcdev(&args, gpio_fallback, NULL);
> 
> This gets called with args as parsed. If there is a match, this will 
> overwrite args (in the gpio_fallback case) and return NULL.

Overwrite not complete. It will only overwrite args_count and return a
valid rcdev.
I do not see overwriting in case of returning NULL.

> 
>> +
>>  	if (!rcdev) {
>> -		rstc = ERR_PTR(-EPROBE_DEFER);
>> -		goto out;
>> +		if (gpio_fallback) {
>> +			/*
>> +			 * Registering reset-gpio device might cause immediate
>> +			 * bind, thus taking reset_list_mutex lock via
>> +			 * reset_controller_register().
>> +			 */
>> +			mutex_unlock(&reset_list_mutex);
>> +			ret = __reset_add_reset_gpio_device(node, &args);
> 
> So this will also be called with args as parsed.
> 
>> +			mutex_lock(&reset_list_mutex);
>> +			if (ret) {
>> +				rstc = ERR_PTR(ret);
>> +				goto out;
>> +			}
>> +			/*
>> +			 * Success: reset-gpio could probe immediately, so
>> +			 * re-check the lookup.
>> +			 */
>> +			rcdev = __reset_find_rcdev(&args, gpio_fallback, NULL);
> 
> And this will again be called with args as parsed and overwrite args
> again.>
>> +			if (!rcdev) {
>> +				rstc = ERR_PTR(-EPROBE_DEFER);
>> +				goto out;
>> +			}
>> +			/* Success, rcdev is valid thus do not bail out */
>> +		} else {
>> +			rstc = ERR_PTR(-EPROBE_DEFER);
>> +			goto out;
>> +		}
>>  	}
> 
> So at this point args is overwritten in the gpio_fallback case. I would
> find it much clearer to just overwrite args here and make the first
> parameter to __reset_find_rcdev() const.

I think I get your point. Overwriting happens after we store the
original of_args, but the code is indeed not intuitive. I think I can
move it further, as you suggested.


Best regards,
Krzysztof

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