Date:   Mon, 7 Sep 2020 12:45:56 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kent Gibson <warthog618@...il.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-doc <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-acpi@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 23/23] Documentation: gpio: add documentation for gpio-mockup

On Sat, Sep 5, 2020 at 5:16 AM Randy Dunlap <rdunlap@...radead.org> wrote:
>
> Hi,
>
> On 9/4/20 8:45 AM, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bgolaszewski@...libre.com>
> >
> > There's some documentation for gpio-mockup's debugfs interface in the
> > driver's source but it's not much. Add proper documentation for this
> > testing module.
> >
> > Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>
> > ---
> >  .../admin-guide/gpio/gpio-mockup.rst          | 87 +++++++++++++++++++
> >  1 file changed, 87 insertions(+)
> >  create mode 100644 Documentation/admin-guide/gpio/gpio-mockup.rst
> >
> > diff --git a/Documentation/admin-guide/gpio/gpio-mockup.rst b/Documentation/admin-guide/gpio/gpio-mockup.rst
> > new file mode 100644
> > index 000000000000..1d452ee55f8d
> > --- /dev/null
> > +++ b/Documentation/admin-guide/gpio/gpio-mockup.rst
> > @@ -0,0 +1,87 @@
> > +.. SPDX-License-Identifier: GPL-2.0-only
> > +
> > +GPIO Testing Driver
> > +===================
> > +
> > +The GPIO Testing Driver (gpio-mockup) provides a way to create simulated GPIO
> > +chips for testing purposes. There are two ways of configuring the chips exposed
> > +by the module. The lines can be accessed using the standard GPIO character
> > +device interface as well as manipulated using the dedicated debugfs directory
> > +structure.
>
> Could configfs be used for this instead of debugfs?
> debugfs is ad hoc.
>
> > +
> > +Creating simulated chips using debugfs
> > +--------------------------------------
> > +
> > +When the gpio-mockup module is loaded (or builtin) it creates its own directory
> > +in debugfs. Assuming debugfs is mounted at /sys/kernel/debug/, the directory
> > +will be located at /sys/kernel/debug/gpio-mockup/. Inside this directory there
> > +are two attributes: new_device and delete_device.
> > +
> > +New chips can be created by writing a single line containing a number of
> > +options to "new_device". For example:
> > +
> > +.. code-block:: sh
> > +
> > +    $ echo "label=my-mockup num_lines=4 named_lines" > /sys/kernel/debug/gpio-mockup/new_device
> > +
> > +Supported options:
> > +
> > +    num_lines=<num_lines> - number of GPIO lines to expose
> > +
> > +    label=<label> - label of the dummy chip
> > +
> > +    named_lines - defines whether dummy lines should be named, the names are
> > +                  of the form X-Y where X is the chip's label and Y is the
> > +                  line's offset
> > +
> > +Note: only num_lines is mandatory.
> > +
> > +Chips can be dynamically removed by writing the chip's label to
> > +"delete_device". For example:
> > +
> > +.. code-block:: sh
> > +
> > +    echo "gpio-mockup.0" > /sys/kernel/debug/gpio-mockup/delete_device
> > +
> > +Creating simulated chips using module params
> > +--------------------------------------------
> > +
> > +Note: this is an older, now deprecated method kept for backward compatibility
> > +for user-space tools.
> > +
> > +When loading the gpio-mockup driver a number of parameters can be passed to the
> > +module.
> > +
> > +    gpio_mockup_ranges
> > +
> > +        This parameter takes an argument in the form of an array of integer
> > +        pairs. Each pair defines the base GPIO number (if any) and the number
> > +        of lines exposed by the chip. If the base GPIO is -1, the gpiolib
> > +        will assign it automatically.
> > +
> > +        Example: gpio_mockup_ranges=-1,8,-1,16,405,4
> > +
> > +        The line above creates three chips. The first one will expose 8 lines,
> > +        the second 16 and the third 4. The base GPIO for the third chip is set
> > +        to 405 while for two first chips it will be assigned automatically.
> > +
> > +    gpio_named_lines
> > +
> > +        This parameter doesn't take any arguments. It lets the driver know that
> > +        GPIO lines exposed by it should be named.
> > +
> > +        The name format is: gpio-mockup-X-Y where X is the letter associated
> > +        with the mockup chip and Y is the line offset.
>
> Where does this 'X' letter associated with the mockup chip come from?
>
> > +
> > +Manipulating simulated lines
> > +----------------------------
> > +
> > +Each mockup chip creates its own subdirectory in /sys/kernel/debug/gpio-mockup/.
> > +The directory is named after the chip's label. A symlink is also created, named
> > +after the chip's name, which points to the label directory.
> > +
> > +Inside each subdirectory, there's a separate attribute for each GPIO line. The
> > +name of the attribute represents the line's offset in the chip.
> > +
> > +Reading from a line attribute returns the current value. Writing to it (0 or 1)
> > +changes its pull.
>
> What does "pull" mean here?
>

Yeah I should probably clarify this. "Pull" here means a simulated
pull-down/up resistor basically.

Bart

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