linux-kernel - Re: [PATCH 1/3] gpiolib: convert 'devprop_gpiochip_set_names' to support multiple gpiochip baks per device

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Date:   Mon, 19 Jul 2021 00:57:23 -0700
From:   Gregory Fong <gregory.0xf0@...il.com>
To:     Sergio Paracuellos <sergio.paracuellos@...il.com>
Cc:     linux-gpio@...r.kernel.org, linus.walleij@...aro.org,
        bgolaszewski@...libre.com, f.fainelli@...il.com,
        matthias.bgg@...il.com, opensource@...rst.com,
        andy.shevchenko@...il.com, git@...nthomson.fastmail.com.au,
        neil@...wn.name, hofrat@...dl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] gpiolib: convert 'devprop_gpiochip_set_names' to
 support multiple gpiochip baks per device

Hi Sergio,

On Thu, Jul 08, 2021 at 09:04:27AM +0200, Sergio Paracuellos wrote:
> The default gpiolib-of implementation does not work with the multiple
> gpiochip banks per device structure used for example by the gpio-mt7621
> and gpio-brcmstb drivers. To fix these kind of situations driver code
> is forced to fill the names to avoid the gpiolib code to set names
> repeated along the banks. Instead of continue with that antipattern
> fix the gpiolib core function to get expected behaviour for every
> single situation adding a field 'offset' in the gpiochip structure.
> Doing in this way, we can assume this offset will be zero for normal
> driver code where only one gpiochip bank per device is used but
> can be set explicitly in those drivers that really need more than
> one gpiochip.

This is a nice improvement, thanks for putting this together!  A few
remarks below:

> 
> Signed-off-by: Sergio Paracuellos <sergio.paracuellos@...il.com>
> ---
>  drivers/gpio/gpiolib.c      | 34 ++++++++++++++++++++++++++++------
>  include/linux/gpio/driver.h |  4 ++++
>  2 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index 27c07108496d..f3f45b804542 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -382,11 +382,16 @@ static int devprop_gpiochip_set_names(struct gpio_chip *chip)
>  	if (count < 0)
>  		return 0;
>  
> -	if (count > gdev->ngpio) {
> -		dev_warn(&gdev->dev, "gpio-line-names is length %d but should be at most length %d",
> -			 count, gdev->ngpio);
> -		count = gdev->ngpio;
> -	}
> +	/*
> +	 * When offset is set in the driver side we assume the driver internally
> +	 * is using more than one gpiochip per the same device. We have to stop
> +	 * setting friendly names if the specified ones with 'gpio-line-names'
> +	 * are less than the offset in the device itself. This means all the
> +	 * lines are not present for every single pin within all the internal
> +	 * gpiochips.
> +	 */
> +	if (count <= chip->offset)
> +		return 0;

This case needs a descriptive warning message.  Silent failure to assign
names here will leave someone confused about what they're doing wrong.

>  
>  	names = kcalloc(count, sizeof(*names), GFP_KERNEL);
>  	if (!names)
> @@ -400,8 +405,25 @@ static int devprop_gpiochip_set_names(struct gpio_chip *chip)
>  		return ret;
>  	}
>  
> +	/*
> +	 * When more that one gpiochip per device is used, 'count' can
> +	 * contain at most number gpiochips x chip->ngpio. We have to
> +	 * correctly distribute all defined lines taking into account
> +	 * chip->offset as starting point from where we will assign
> +	 * the names to pins from the 'names' array. Since property
> +	 * 'gpio-line-names' cannot contains gaps, we have to be sure
> +	 * we only assign those pins that really exists since chip->ngpio
> +	 * can be different of the chip->offset.
> +	 */
> +	count = (count > chip->offset) ? count - chip->offset : count;
> +	if (count > chip->ngpio) {

In the multiple gpiochip case, if there are 3+ gpiochips this seems like
it will yield an invalid warning. For example, if there are 3 gpiochips
(banks 0, 1, and 2), and all gpios are given names in gpio-line-names,
isn't this condition going to always evaluate to true for bank 1,
resulting in an invalid warning?  In that case I would think setting
count to chip->ngpio is the *expected* behavior.

Since that's a "normal" behavior in the multiple gpiochip case, I'm not
sure there's a simple way to detect an over-long gpio-line-names here
in this function anymore.

> +		dev_warn(&gdev->dev, "gpio-line-names is length %d but
> should be at most length %d", +			 count,
> chip->ngpio);
> +		count = chip->ngpio; +	} + for (i = 0; i < count; i++)
> -		gdev->descs[i].name = names[i]; +
> gdev->descs[i].name = names[chip->offset + i];
>  
>  	kfree(names);
>  
> [snip]

Best regards,
Gregory