Date: Tue, 09 Jan 2024 12:58:34 +0100
From: Philipp Zabel <p.zabel@...gutronix.de>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, 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 Di, 2024-01-09 at 11:59 +0100, Krzysztof Kozlowski wrote:
> On 08/01/2024 13:17, Philipp Zabel wrote:
> > On Fr, 2024-01-05 at 16:59 +0100, Krzysztof Kozlowski wrote:
> > 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.

Thank you for the clarification.
I only understood later in the mail and didn't update this properly.

> > > +	/* 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

Keeping ownership on the list is fine, the reset-gpio driver makes its
own copy of of_phandle_args anyway. I was just wondering whether it
could make this copy from platform data instead of from the
of_parse_phandle_with_args() return value.

[...]
> > 
> > > @@ -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.

Thank you. I think we could also just make that check

	if (WARN_ON(!rcdev->of_args && ...))

instead and skip the of_xlate call in that case (or implement of_xlate
in the reset-gpio driver to make this more explicit).

> > 
> > > +		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?

I propose to reorder after parsing the phandles: check/create the gpio
platform device right after parsing the gpio handle. Only then lock
reset_list_mutex look for the rcdev.

1. of_parse_phandle(resets)
1b. error? Then of_parse_phandle(reset-gpios)
2b. gpio? Then check if we created reset-gpios platform device. If not:
create new reset-gpios platform device/, defer if probe failed
3. Lock reset_list_mutex, find reset-controller based on any of above
phandles.

> 
> > 
> > >  	}
> > >  
> > > +	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.

Sorry, I meant to write

"This gets called with args as parsed. If there is a match, this will 
 overwrite args (in the gpio_fallback case) _or_ return NULL."

at least at the end, when I understood the following.

> > 
> > > +
> > >  	if (!rcdev) {

So in this non-NULL branch there was no overwriting.

> > > -		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.

Now I think we can skip the overwriting altogether and just adapt the
following of_reset_n_cells check ad of_xlate call as mentioned above.

regards
Philipp

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