gpio: Ignore gpio names with empty strings

If a gpiochip sets it's gpio_chip->names, it must give every gpio a
unique name. However we may not need to name all gpios, for example if
they are unused. gpio-line-names allows this by ignoring gpios with
empty names. But gpiochip_set_desc_names() will yell at us, as the empty
name will collide with other empty names. Fix this by skipping empty
strings, so that they look like unnamed gpios.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
---
 drivers/gpio/gpiolib-devprop.c | 3 ++-
 drivers/gpio/gpiolib.c         | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

On Thu, Oct 4, 2018 at 10:18 PM Brandon Maier
<brandon.maier@rockwellcollins.com> wrote:

> If a gpiochip sets it's gpio_chip->names, it must give every gpio a
> unique name. However we may not need to name all gpios, for example if
> they are unused. gpio-line-names allows this by ignoring gpios with
> empty names. But gpiochip_set_desc_names() will yell at us, as the empty
> name will collide with other empty names. Fix this by skipping empty
> strings, so that they look like unnamed gpios.
>
> Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>

Isn't the real problem that they should be passing NULL
rather than "" empty string for line names?

Yours,
Linus Walleij

On Wed, Oct 10, 2018 at 4:12 AM, Linus Walleij <linus.walleij@linaro.org> wrote:

> Isn't the real problem that they should be passing NULL
> rather than "" empty string for line names?

In our case, we are using the gpio_chip->names in place of
gpio-line-names. Because Xilinx's drivers/gpio/gpio-xilinx allocates
multiple gpio_chips under a single devicetree node, gpio-line-names
won't work. This is the same problem mentioned in Hedges Alexander's
port of gpio-xilinx[1]. I worked around this by adding a
gpio-line-names-2 that the second chip grabs and passes in through
gpio_chip->names.

This patch seemed appropriate because the intent of an "empty string"
in gpio-line-names is to be an unset gpio name. It would also prevent
name collisions from happening if we added name collision detection to
gpio-line-names. But if it's better to leave this as-is, I could
modify our code to replace "" -> NULL in gpio-xilinx.

[1] https://www.spinics.net/lists/linux-gpio/msg31773.html

Thanks,
Brandon Maier

On Wed, Oct 10, 2018 at 5:03 PM Brandon Maier
<brandon.maier@rockwellcollins.com> wrote:
> On Wed, Oct 10, 2018 at 4:12 AM, Linus Walleij <linus.walleij@linaro.org> wrote:
>
> > Isn't the real problem that they should be passing NULL
> > rather than "" empty string for line names?
>
> In our case, we are using the gpio_chip->names in place of
> gpio-line-names. Because Xilinx's drivers/gpio/gpio-xilinx allocates
> multiple gpio_chips under a single devicetree node, gpio-line-names
> won't work. This is the same problem mentioned in Hedges Alexander's
> port of gpio-xilinx[1]. I worked around this by adding a
> gpio-line-names-2 that the second chip grabs and passes in through
> gpio_chip->names.
>
> This patch seemed appropriate because the intent of an "empty string"
> in gpio-line-names is to be an unset gpio name. It would also prevent
> name collisions from happening if we added name collision detection to
> gpio-line-names. But if it's better to leave this as-is, I could
> modify our code to replace "" -> NULL in gpio-xilinx.

I think it is better to modify callers to pass NULL rather than ""
to fix this.

The reason is that it is very clear for a reader of the code that
you do not want to name a line if it is just NULL. It has a certain
elegance to it. (IMO)

Yours,
Linus Walleij