Re: [RFC PATCH] gpiolib: Provide and export gpiod_export_name

[Alexandre Courbot <gnurou@xxxxxxxxx>]

On Thu, Jul 24, 2014 at 3:12 AM, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> gpiod_export_name is similar to gpiod_export, but lets the user
> determine the name used to export a gpio pin.
>
> Currently, the pin name is determined by the chip driver with
> the 'names' array in the gpio_chip data structure, or it is set
> to gpioX, where X is the pin number, if no name is provided by
> the chip driver.

Oh, my. I did not even know about this 'names' array. This is pretty
bad - a GPIO provider should not decide what its GPIOs are used for.

Luckily for you, this creates a precedent that makes this patch more
acceptable, in that it is not making the situation worse. Even though
I consider both solutions to be bad, I actually prefer your
gpiod_export_name() function to that 'names' array in gpio_chip...

>
> It is, however, desirable to be able to provide the pin name when
> exporting the pin, for example from platform code. In other words,
> it would be useful to move the naming decision from the pin provider
> to the pin consumer. The gpio-pca953x driver provides this capability
> as part of its platform data. Other drivers could be enhanced in a
> similar way; however, this is not always possible or easy to accomplish.
> For example, mfd client drivers such as gpio-ich already use platform
> data to pass information from the mfd master driver to the client driver.
> Overloading this platform data to also provide an array of gpio pin names
> would be a challenge if not impossible.
>
> The alternative to use gpiod_export_link is also not always desirable,
> since it only creates a named link to a different directory, meaning
> the named gpio pin is not available in /sys/class/gpio but only
> in some platform specific directory and thus not as generic as possible
> and/or useful.
>
> A specific example for a use case is a gpio pin which reports AC power
> loss to user space. Depending on the platform and platform variant,
> the pin can be provided by various gpio chip drivers and pin numbers.
> It would be very desirable to have a well defined location such as
> /sys/class/gpio/ac_power_loss for this pin, so user space knows where
> to find the attribute without knowledge of the underlying platform
> variant or oher hardware details.

As I explained on the other thread, I still encourage you to have
these GPIOs under device nodes that give a hint of who is provided the
GPIO (effectively exporting the (dev, function) tuple to user-space)
instead of having them popping out under /sys/class/gpio where nobody
knows where they come from and name collisions are much more likely.

Your message sounds like you have no choice but have the named GPIO
link under the gpiochip's device node, but this is not the case -
gpio_export_link() let's you specify the device under which the link
should appear. Make that device be your "scu" device and you can have
a consistent sysfs path to access your GPIOs.

Allowing GPIOs to pop up in the same directory with an arbitrary name
is just a recipe for a mess. But that's a recipe that is already
allowed thanks to that 'names' array. So I'm really confused about
what to do with this patch. If you can do with gpio_export_link() (and
I have not seen evidence of the contrary), please go that way instead.

>
> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> ---
> Applies to tip of linux-gpio/for-next.
>
>  Documentation/gpio/sysfs.txt  | 12 ++++++++----
>  drivers/gpio/gpiolib-sysfs.c  | 23 ++++++++++++++++-------
>  include/linux/gpio/consumer.h |  9 +++++++++
>  3 files changed, 33 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/gpio/sysfs.txt b/Documentation/gpio/sysfs.txt
> index c2c3a97..8e301b2 100644
> --- a/Documentation/gpio/sysfs.txt
> +++ b/Documentation/gpio/sysfs.txt
> @@ -125,7 +125,11 @@ requested using gpio_request():
>         /* export the GPIO to userspace */
>         int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
>
> -       /* reverse gpio_export() */
> +       /* export named GPIO to userspace */
> +       int gpiod_export_name(struct gpio_desc *desc, const char *ioname,
> +                       bool direction_may_change);
> +
> +       /* reverse gpio_export() / gpiod_export_name() */
>         void gpiod_unexport(struct gpio_desc *desc);
>
>         /* create a sysfs link to an exported GPIO node */
> @@ -136,9 +140,9 @@ requested using gpio_request():
>         int gpiod_sysfs_set_active_low(struct gpio_desc *desc, int value);
>
>  After a kernel driver requests a GPIO, it may only be made available in
> -the sysfs interface by gpiod_export(). The driver can control whether the
> -signal direction may change. This helps drivers prevent userspace code
> -from accidentally clobbering important system state.
> +the sysfs interface by gpiod_export() or gpiod_export_name(). The driver
> +can control whether the signal direction may change. This helps drivers
> +prevent userspace code from accidentally clobbering important system state.
>
>  This explicit exporting can help with debugging (by making some kinds
>  of experiments easier), or can provide an always-there interface that's
> diff --git a/drivers/gpio/gpiolib-sysfs.c b/drivers/gpio/gpiolib-sysfs.c
> index be45a92..7d36230 100644
> --- a/drivers/gpio/gpiolib-sysfs.c
> +++ b/drivers/gpio/gpiolib-sysfs.c
> @@ -523,13 +523,12 @@ static struct class gpio_class = {
>   *
>   * Returns zero on success, else an error.
>   */
> -int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
> +int gpiod_export_name(struct gpio_desc *desc, const char *ioname,
> +                     bool direction_may_change)
>  {
>         unsigned long           flags;
>         int                     status;
> -       const char              *ioname = NULL;
>         struct device           *dev;
> -       int                     offset;
>
>         /* can't export until sysfs is available ... */
>         if (!gpio_class.p) {
> @@ -560,10 +559,6 @@ int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
>                 direction_may_change = false;
>         spin_unlock_irqrestore(&gpio_lock, flags);
>
> -       offset = gpio_chip_hwgpio(desc);
> -       if (desc->chip->names && desc->chip->names[offset])
> -               ioname = desc->chip->names[offset];
> -
>         dev = device_create(&gpio_class, desc->chip->dev, MKDEV(0, 0),
>                             desc, ioname ? ioname : "gpio%u",
>                             desc_to_gpio(desc));
> @@ -600,6 +595,20 @@ fail_unlock:
>         gpiod_dbg(desc, "%s: status %d\n", __func__, status);
>         return status;
>  }
> +EXPORT_SYMBOL_GPL(gpiod_export_name);
> +
> +int gpiod_export(struct gpio_desc *desc, bool direction_may_change)
> +{
> +       const char *ioname = NULL;
> +
> +       if (desc) {
> +               int offset = gpio_chip_hwgpio(desc);
> +
> +               if (desc->chip->names && desc->chip->names[offset])
> +                       ioname = desc->chip->names[offset];

I'd add a

else
    ioname = "gpio%u";

so all the name-chosing code is grouped in the same place. Then you
can remove that same check from gpiod_export_name().

> +       }
> +       return gpiod_export_name(desc, ioname, direction_may_change);
> +}
>  EXPORT_SYMBOL_GPL(gpiod_export);
>
>  static int match_export(struct device *dev, const void *data)
> diff --git a/include/linux/gpio/consumer.h b/include/linux/gpio/consumer.h
> index 05e53cc..986da3e 100644
> --- a/include/linux/gpio/consumer.h
> +++ b/include/linux/gpio/consumer.h
> @@ -260,6 +260,8 @@ static inline int desc_to_gpio(const struct gpio_desc *desc)
>  #if IS_ENABLED(CONFIG_GPIOLIB) && IS_ENABLED(CONFIG_GPIO_SYSFS)
>
>  int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
> +int gpiod_export_name(struct gpio_desc *desc, const char *ioname,
> +                     bool direction_may_change);
>  int gpiod_export_link(struct device *dev, const char *name,
>                       struct gpio_desc *desc);
>  int gpiod_sysfs_set_active_low(struct gpio_desc *desc, int value);
> @@ -273,6 +275,13 @@ static inline int gpiod_export(struct gpio_desc *desc,
>         return -ENOSYS;
>  }
>
> +static inline int gpiod_export_name(struct gpio_desc *desc,
> +                                   const char *ioname,
> +                                   bool direction_may_change)
> +{
> +       return -ENOSYS;
> +}
> +
>  static inline int gpiod_export_link(struct device *dev, const char *name,
>                                     struct gpio_desc *desc)
>  {
> --
> 1.9.1
>
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