Re: [Patch v3 1/2] gpio: add GPIO hogging mechanism

[Benoit Parrot <bparrot@ti.com>]

Thanks for the quick feedback.

Alexandre Courbot <gnurou@gmail.com> wrote on Fri [2014-Dec-12 17:54:06 +0900]:
> On Thu, Dec 11, 2014 at 8:48 AM, Benoit Parrot <bparrot@ti.com> wrote:
> > Alexandre Courbot <gnurou@gmail.com> wrote on Wed [2014-Dec-10 20:19:51 +0900]:
> >> On Fri, Dec 5, 2014 at 6:02 AM, Benoit Parrot <bparrot@ti.com> wrote:
> >> > Based on Boris Brezillion's work this is a reworked patch
> >> > of his initial GPIO hogging mechanism.
> >> > This patch provides a way to initally configure specific GPIO
> >> > when the gpio controller is probed.
> >> >
> >> > The actual DT scanning to collect the GPIO specific data is performed
> >> > as part of the gpiochip_add().
> >> >
> >> > The purpose of this is to allows specific GPIOs to be configured
> >> > without any driver specific code.
> >> > This particularly useful because board design are getting
> >> > increasingly complex and given SoC pins can now have upward
> >> > of 10 mux values a lot of connections are now dependent on
> >> > external IO muxes to switch various modes and combination.
> >> >
> >> > Specific drivers should not necessarily need to be aware of
> >> > what accounts to a specific board implementation. This board level
> >> > "description" should be best kept as part of the dts file.
> >> >
> >> > Signed-off-by: Benoit Parrot <bparrot@ti.com>
> >> > ---
> >> > Changes since v2:
> >> >  * Refactor the gpio-hog mechanism to split the DT related action
> >> >    from the actual "hogging" operation.
> >> >  * This allows non-DT providers to implement hogs as well.
> >> >  * Added FLAG_IS_HOGGED to mark hogged gpio and make gpiochip removal
> >> >    able to release hogged gpio.
> >> >  * Similarly to the hogging, the cleanup is performed as part of
> >> >    of_gpiochip_remove
> >> >  * Refactor the gpio-hog mechanism as private functions meant to
> >> >    be to invoked from of_gpiochip_add().
> >> >
> >> > Changes since v1:
> >> >  * Refactor the gpio-hog mechanism as private functions meant to
> >> >    be to invoked from of_gpiochip_add().
> >> >
> >> >  drivers/gpio/gpiolib-of.c     | 127 ++++++++++++++++++++++++++++++++++++++++++
> >> >  drivers/gpio/gpiolib.c        | 118 ++++++++++++++++++++++++++++++---------
> >> >  drivers/gpio/gpiolib.h        |   1 +
> >> >  include/linux/gpio/consumer.h |   9 +++
> >> >  4 files changed, 229 insertions(+), 26 deletions(-)
> >> >
> >> > diff --git a/drivers/gpio/gpiolib-of.c b/drivers/gpio/gpiolib-of.c
> >> > index 604dbe6..e13134d 100644
> >> > --- a/drivers/gpio/gpiolib-of.c
> >> > +++ b/drivers/gpio/gpiolib-of.c
> >> > @@ -22,6 +22,7 @@
> >> >  #include <linux/of_gpio.h>
> >> >  #include <linux/pinctrl/pinctrl.h>
> >> >  #include <linux/slab.h>
> >> > +#include <linux/gpio/machine.h>
> >> >
> >> >  #include "gpiolib.h"
> >> >
> >> > @@ -111,6 +112,128 @@ int of_get_named_gpio_flags(struct device_node *np, const char *list_name,
> >> >  EXPORT_SYMBOL(of_get_named_gpio_flags);
> >> >
> >> >  /**
> >> > + * of_get_gpio_hog() - Get a GPIO hog descriptor, names and flags for GPIO API
> >> > + * @np:                device node to get GPIO from
> >> > + * @name:      GPIO line name
> >> > + * @flags:     a flags pointer to fill in
> >> > + *
> >> > + * Returns GPIO descriptor to use with Linux GPIO API, or one of the errno
> >> > + * value on the error condition.
> >> > + */
> >> > +
> >> > +static struct gpio_desc *of_get_gpio_hog(struct device_node *np,
> >> > +                                 const char **name,
> >> > +                                 enum gpio_lookup_flags *lflags,
> >> > +                                 enum gpiod_flags *dflags)
> >> > +{
> >> > +       struct device_node *chip_np;
> >> > +       enum of_gpio_flags xlate_flags;
> >> > +       struct gpio_desc *desc;
> >> > +       const char *dir_val;
> >> > +       struct gg_data gg_data = {
> >> > +               .flags = &xlate_flags,
> >> > +               .out_gpio = NULL,
> >> > +       };
> >> > +       u32 tmp;
> >> > +       int i, ret;
> >> > +
> >> > +       chip_np = np->parent;
> >> > +       if (!chip_np)
> >> > +               return ERR_PTR(-EINVAL);
> >> > +
> >> > +       xlate_flags = 0;
> >> > +       *lflags = 0;
> >> > +       *dflags = 0;
> >> > +
> >> > +       ret = of_property_read_u32(chip_np, "#gpio-cells", &tmp);
> >> > +       if (ret)
> >> > +               return ERR_PTR(ret);
> >> > +
> >> > +       if (tmp > MAX_PHANDLE_ARGS)
> >> > +               return ERR_PTR(-EINVAL);
> >> > +
> >> > +       gg_data.gpiospec.args_count = tmp;
> >> > +       gg_data.gpiospec.np = chip_np;
> >> > +       for (i = 0; i < tmp; i++) {
> >> > +               ret = of_property_read_u32_index(np, "gpios", i,
> >> > +                                          &gg_data.gpiospec.args[i]);
> >> > +               if (ret)
> >> > +                       return ERR_PTR(ret);
> >> > +       }
> >> > +
> >> > +       gpiochip_find(&gg_data, of_gpiochip_find_and_xlate);
> >> > +       if (!gg_data.out_gpio) {
> >> > +               if (np->parent == np)
> >> > +                       return ERR_PTR(-ENXIO);
> >> > +               else
> >> > +                       return ERR_PTR(-EPROBE_DEFER);
> >> > +       }
> >> > +
> >> > +       if (xlate_flags & OF_GPIO_ACTIVE_LOW)
> >> > +               *lflags |= GPIO_ACTIVE_LOW;
> >> > +
> >> > +       if (!of_property_read_string(np, "direction", &dir_val)) {
> >> > +               if (!strcmp(dir_val, "input"))
> >> > +                       *dflags |= GPIOD_IN;
> >> > +               else if (!strcmp(dir_val, "output-low"))
> >> > +                       *dflags |= GPIOD_OUT_LOW;
> >> > +               else if (!strcmp(dir_val, "output-high"))
> >> > +                       *dflags |= GPIOD_OUT_HIGH;
> >> > +       }
> >>
> >> ... else?
> >>
> >> We should probably return an error if the property is not specified -
> >> is there a point in hogging a GPIO without a direction? E.g:
> >>
> >> if (of_property_read_string(np, "direction", &dir_val))
> >>     return ERR_PTR(-EINVAL);
> >>
> >> if (!strcmp(...
> >>
> >> to use the nice pattern that errors (and not normal behavior) are the exception.
> >
> > Bah, I was going for compartmentalization.
> > It make sense if you don't think about it ..... :)
> >
> >>
> >> > +
> >> > +       if (name && of_property_read_string(np, "line-name", name))
> >> > +               *name = np->name;
> >> > +
> >> > +       desc = gg_data.out_gpio;
> >> > +
> >> > +       return desc;
> >> > +}
> >> > +
> >> > +/**
> >> > + * _gpiochip_hog - Scan gpio-controller and apply GPIO hog as requested
> >> > + * @chip:      gpio chip to act on
> >> > + *
> >> > + * This is only used by of_gpiochip_add to request/set GPIO initial
> >> > + * configuration.
> >> > + */
> >> > +static void _gpiochip_hog(struct gpio_chip *chip)
> >>
> >> Rename to of_gpio_scan_hogs() maybe?
> >
> > Given that it is meant for gpiochip_add, how about
> > _gpiochip_scan_hogs()?
> 
> of_gpiochip_scan_hogs(), and this is my last offer. :P (why do you
> want to prefix it with __ btw?)

Not sure really the _ prefix just made them look more like private functions.
Not stuck up on it, though.

of_gpiochip_scan_hogs() it is.

> 
> >
> >>
> >> > +{
> >> > +       struct gpio_desc *desc = NULL;
> >> > +       struct device_node *np;
> >> > +       const char *name;
> >> > +       enum gpio_lookup_flags lflags;
> >> > +       enum gpiod_flags dflags;
> >> > +
> >> > +       for_each_child_of_node(chip->dev->of_node, np) {
> >> > +               if (!of_property_read_bool(np, "gpio-hog"))
> >> > +                       continue;
> >> > +
> >> > +               desc = of_get_gpio_hog(np, &name, &lflags, &dflags);
> >> > +               if (IS_ERR(desc))
> >> > +                       continue;
> >> > +
> >> > +               __gpiod_hog(desc, name, lflags, dflags);
> >>
> >> You are not propagating any error returned by __gpiod_hog here.
> >
> > _gpiochip_hog is a void function given that __gpiod_hog() is the last call of that loop
> > there is nothing to propagate.
> > You would still want to scan all of the child node regardless of errors, no?
> 
> You're right. Besides hogging failure should probably not be a fatal
> error. In this case please make sure that all possible errors related
> to hogging are at least reported accordingly in the log.
> 
> >
> >>
> >> > +       }
> >> > +}
> >> > +
> >> > +/**
> >> > + * _gpiochip_unhog - Scan gpio-controller and apply GPIO hog as requested
> >> > + * @chip:      gpio chip to act on
> >> > + *
> >> > + * This is only used by of_gpiochip_remove to free hogged gpios
> >> > + *
> >> > + */
> >> > +static void _gpiochip_unhog(struct gpio_chip *chip)
> >> > +{
> >> > +       int id;
> >> > +
> >> > +       for (id = 0; id < chip->ngpio; id++) {
> >> > +               if (test_bit(FLAG_IS_HOGGED, &chip->desc[id].flags))
> >> > +                       gpiod_put(&chip->desc[id]);
> >> > +       }
> >> > +}
> >>
> >> This function is not DT-specific. It should be included in gpiolib.c
> >> and called from there before of_gpiochip_remove().
> >
> > Agreed, any name request while I am at it or tis this fine as is?
> 
> Name looks good, although I don't know why the '_' prefix?

I will change it to gpiochip_free_hogs()
> 
> >
> >>
> >> > +
> >> > +/**
> >> >   * of_gpio_simple_xlate - translate gpio_spec to the GPIO number and flags
> >> >   * @gc:                pointer to the gpio_chip structure
> >> >   * @np:                device node of the GPIO chip
> >> > @@ -302,10 +425,14 @@ void of_gpiochip_add(struct gpio_chip *chip)
> >> >
> >> >         of_gpiochip_add_pin_range(chip);
> >> >         of_node_get(chip->of_node);
> >> > +
> >> > +       _gpiochip_hog(chip);
> >> >  }
> >> >
> >> >  void of_gpiochip_remove(struct gpio_chip *chip)
> >> >  {
> >> >         gpiochip_remove_pin_ranges(chip);
> >> >         of_node_put(chip->of_node);
> >> > +
> >> > +       _gpiochip_unhog(chip);
> >> >  }
> >> > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> >> > index e8e98ca..4ef6eb8 100644
> >> > --- a/drivers/gpio/gpiolib.c
> >> > +++ b/drivers/gpio/gpiolib.c
> >> > @@ -849,6 +849,7 @@ static bool __gpiod_free(struct gpio_desc *desc)
> >> >                 clear_bit(FLAG_REQUESTED, &desc->flags);
> >> >                 clear_bit(FLAG_OPEN_DRAIN, &desc->flags);
> >> >                 clear_bit(FLAG_OPEN_SOURCE, &desc->flags);
> >> > +               clear_bit(FLAG_IS_HOGGED, &desc->flags);
> >> >                 ret = true;
> >> >         }
> >> >
> >> > @@ -1631,6 +1632,58 @@ struct gpio_desc *__must_check __gpiod_get_optional(struct device *dev,
> >> >  }
> >> >  EXPORT_SYMBOL_GPL(__gpiod_get_optional);
> >> >
> >> > +
> >> > +/**
> >> > + * __gpiod_get_helper - helper function to request and configure a given GPIO
> >> > + * @desc:      gpio whose value will be assigned
> >> > + * @con_id:    unction within the GPIO consumer
> >> > + * @lflags:    gpio_lookup_flags - returned from of_find_gpio() or
> >> > + *             of_get_gpio_hog()
> >> > + * @dflags:    gpiod_flags - optional GPIO initialization flags
> >> > + *
> >> > + * Return 0 on success, -ENOENT if no GPIO has been assigned to the
> >> > + * requested function and/or index, or another IS_ERR() code if an error
> >> > + * occurred while trying to acquire the GPIO.
> >> > + */
> >> > +static int __gpiod_get_helper(struct gpio_desc *desc, const char *con_id,
> >> > +               unsigned long lflags, enum gpiod_flags dflags)
> >> > +{
> >> > +       int status;
> >> > +
> >> > +       status = gpiod_request(desc, con_id);
> >>
> >> As I mentioned in the previous revision, this will prevent the module
> >> from being unloaded with hogged GPIOs. You need to use
> >> gpiochip_request_own_desc() here and gpiochip_free_own_desc() instead
> >> of gpiod_put() to free hogged GPIOs. Therefore the call to
> >> gpiod_request/gpiochip_request_own_gpio should be taken out of this
> >> (very nice otherwise!) helper.
> >
> > I can split the functionality out but I do not understand why in this case using
> > gpiod_request would prevent module from being unloaded?
> > Isn't gpiochip_remove() part of a gpio module unload sequence?
> >
> > Because then the _gpiochip_unhog() would release these descriptors. Am I missing something?
> 
> This is because gpiod_request() does a try_module_get(), which will
> cause an error when someone tries to unload the module with, say,
> rmmod. The corresponding calls to gpiod_put() that would decrease the
> module usage count are typically done at module unload time, and thus
> never get a chance to be called.

Ok no problem I'll change that too.

> 
> > Also would using gpiochip_request_own_desc() basically allow the very same hogged GPIO to be
> > requested later on by a consumer.
> 
> No, both gpiod_request() and gpiochip_request_own_desc() call
> __gpiod_request(), which sets the FLAG_REQUESTED flag on the
> descriptor, ensuring it cannot be requested again later.

Ok