Re: [PATCH 0/4] USB: ftdio_sio: GPIO validity fixes

[Johan Hovold <johan@kernel.org>]

On Fri, Dec 04, 2020 at 04:47:35PM +0000, Marc Zyngier wrote:
> Having recently tried to use the CBUS GPIOs that come thanks to the
> ftdio_sio driver, it occurred to me that the driver has a couple of
> usability issues:
> 
> - it advertises potential GPIOs that are reserved to other uses (LED
>   control, or something else)

Consider the alternative, that the gpio offsets (for CBUS0, CBUS1, CBUS2
or CBUS4) varies depending on how the pins have been muxed. Hardly very
user friendly.

> - it returns an odd error (-ENODEV), instead of the expected -EINVAL
>   when a line is unavailable, leading to a difficult diagnostic

Hmm, maybe. Several gpio driver return -ENODEV when trying to request
reserved pins. Even gpiolib returns -ENODEV when a pins is not yet
available due to probe deferal.

-EBUSY could also be an alternative, but that's used to indicate that a
line is already in use as a gpio.

> We address the issues in a number of ways:
> 
> - Stop reporting invalid GPIO lines as valid to userspace. It
>   definitely seems odd to do so. Instead, report the line as being
>   used, making the userspace interface a bit more consistent.
> 
> - Implement the init_valid_mask() callback in the ftdi_sio driver,
>   allowing it to report which lines are actually valid.
> 
> - As suggested by Linus, give an indication to the user of why some of
>   the GPIO lines are unavailable, and point them to a useful tool
>   (once per boot). It is a bit sad that there next to no documentation
>   on how to use these CBUS pins.

Don't be sad, Marc; write some documentation. ;)

Johan