linux-kernel - Re: [PATCH] gpio: Document the 'valid

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Message-ID: <a7ab9d47-cd17-4098-b2ba-d53dfc19dbed@gmail.com>
Date: Thu, 27 Feb 2025 10:24:18 +0200
From: Matti Vaittinen <mazziesaccount@...il.com>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
 Bartosz Golaszewski <brgl@...ev.pl>, linux-gpio@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gpio: Document the 'valid_mask' being internal

On 26/02/2025 13:42, Matti Vaittinen wrote:
> On 26/02/2025 12:18, Linus Walleij wrote:
>> On Wed, Feb 26, 2025 at 7:09 AM Matti Vaittinen
>> <mazziesaccount@...il.com> wrote:
>>> On 25/02/2025 23:36, Linus Walleij wrote:
>>>> we can maybe move it to struct gpio_device in
>>>> drivers/gpio/gpiolib.h?
>>>>
>>>> This struct exist for every gpio_chip but is entirely gpiolib-internal.
>>>>
>>>> Then it becomes impossible to do it wrong...
>>>
>>> True. I can try seeing what it'd require to do that. But ... If there
>>> are any drivers out there altering the valid_mask _after_ registering
>>> the driver to the gpio-core ... Then it may be a can of worms and I may
>>> just keep the lid closed :)
>>
>> That's easy to check with some git grep valid_mask
> 
> True. I just tried. It seems mostly Ok, but...
> For example the drivers/gpio/gpio-rcar.c uses the contents of the 
> 'valid_mask' in it's set_multiple callback to disallow setting the value 
> of masked GPIOs.
> 
> For uneducated person like me, it feels this check should be done and 
> enforced by the gpiolib and not left for untrustworthy driver writers 
> like me! (I am working on BD79124 driver and it didn't occur to me I 
> should check for the valid_mask in driver :) If gpiolib may call the 
> driver's set_multiple() with masked lines - then the bd79124 driver just 
> had one unknown bug less :rolleyes:) )
> 
> I tried looking at the gpiolib to see how this works... It seems to me:
> 
> gpio_chip_set_multiple() does not seem to check for valid_mask. This is 
> called from the gpiod_set_array_value_complex() - which gave me a 
> headache as it is, as name says, complex. Well, I didn't spot valid_mask 
> check but I may have missed a thing or 2...
> 
> If someone remembers straight away how this is supposed to work - I 
> appreciate any guidance. If not, then I try doing some testing when I 
> wire the BD79124 to my board for the next version of the BD79124 series.

I did some quick testing. I used:

adc: adc@10 {

...

	channel@0 {
		reg = <0>;
	};
	channel@1 {
		reg = <1>;
	};
	/* ... up to the channel@6. */

	gpio-controller;
};

which left GPIO0 ... GPIO6 masked (pins used for ADC) and only GPIO7 
unmasked.

Then I added:
gpiotst {
	compatible = "rohm,foo-bd72720-gpio";
	rohm,dvs-vsel-gpios = <&adc 5 0>, <&adc 6 0>;
};

and a dummy driver which does:
gpio_array = devm_gpiod_get_array(&pdev->dev, "rohm,dvs-vsel",
				  GPIOD_OUT_LOW);

...

ret = gpiod_set_array_value_cansleep(gpio_array->ndescs,
		gpio_array->desc, gpio_array->info, values);

As a result the bd79124 gpio driver got it's set_multiple called with 
masked pins. (Oh, and I had accidentally prepared to handle this as I 
had added a sanity check for pinmux register in the set_multiple()).

I suppose one can think this is a result of invalid DT and that drivers 
shouldn't need to be prepared for that. But ... After supporting 
customers who try to integrate IC drivers to their products ... I think 
it's still better to be prepared. I definitely wouldn't blame the rcar 
driver authors for their valid_mask sanity check :)

After all this babbling, my point is that having the valid mask visible 
for drivers is useful. Especially because there are cases where the 
'valid_mask' can be directly compared to the 'mask' parameter in the 
set_multiple. It's clear and efficient check, and I could assume the 
set_multiple() is an optimized call, and thus being efficient makes sense.

So... Long story short - I would still suggest keeping the valid_mask 
visible and either taking just the doc update (as was done in the 
original patch) - or skipping the valid_mask initialization in gpiolib 
if driver provides non NULL value.

What do you think?

Yours,
	-- Matti