Re: [PATCH v6 0/7] gpio: expose line bias flags to userspace

[Kent Gibson <warthog618@gmail.com>]

On Tue, Nov 05, 2019 at 10:04:22AM +0800, Kent Gibson wrote:
> Patches are against Bart's gpio/for-kent branch[1].
> 
> The patch has been successfully tested against gpio-mockup, and 
> on a Raspberry Pi, in both cases using the feature/pud_set_config 
> branch of my Go gpiod library[2], as well as with my feature/pud 
> development branch of libgpiod[3].  Patch 7 has only been tested using 
> my gpiod library as libgpiod has not yet been updated to support the 
> SET_CONFIG ioctl.
> 

I've just pushed a first pass at SET_CONFIG support into my libgpiod 
feature/pud branch.  It is causing me a bit of grief.  Due to the way
the libgpiod API is structured, with the direction flags pulled out into 
the request type, I thought it would be cleaner to keep changes to direction 
orthogonal to changes to the other handle flags.

So I've added these methods to the API:

int gpiod_line_set_config(struct gpiod_line *line, int flags)
int gpiod_line_set_direction_input(struct gpiod_line *line)
int gpiod_line_set_direction_output(struct gpiod_line *line,
				    int value)

along with their bulk equivalents.

I've coded that and started adding tests when I tripped over changing
bias.  The kernel requires a direction to be set, but I'm setting it
as-is in gpiod_line_set_config - so that wont work.
Open drain/source are in the same boat - they require output mode.

I see these options:
 1. set the direction as part of gpiod_line_set_config
 2. relax the kernel restriction.
 3. don't support changing bias or open source/drain.
 4. rethink the API.

The first option requires caching the value set for outputs which I'm a
bit hesitant to do, though I'm not sure why - I've already added caching
of the handle flags for the direction functions.

Any preferences or suggestions?

Cheers,
Kent.