On Sun, Aug 27, 2017 at 3:30 PM, Linus Walleij [off-list ref] wrote:

On Fri, Aug 25, 2017 at 6:52 PM, Rick Altherr [off-list ref] wrote:

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Incidentally, people are sending patches to expose the FTDI
expanders as common GPIO chips under Linux, so we can
internally in the kernel or from the usersapce character device
access them as "some GPIOs".

I know my team at Google has an internal patch for exactly that.  FTDI
expanders are complicated as they can be used as UART, GPIO, I2C, SPI
depending on configuration.  Our project was using a mix of I2C and
GPIO so I directly my team to approach it as an MFD.  I'd like to see
all of these use cases handled by the kernel but I understand the
other viewpoint of relying on libusb for cross-platform compatiblity.

Hm. I see. But I see people pushing the in-kernel method so I think
it will eventually win out.

SGTM.  I'll see if my team can clean up the MFD-based FTDI driver and
submit it upstream.

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In my worst nightmare they export GPIO lines using
the horrid ABI in /sys/gpio/*

https://sourceforge.net/p/openocd/code/ci/v0.10.0/tree/src/jtag/drivers/sysfsgpio.c

Gnah!
Whoever writes a slot-in replacement making the character device
take precendence wins lots of karma.

If they show up at Linux Plumbers or visit San Jose, I'll take them to
dinner.  I didn't see any docs for the chardev in Documentation.  I
_think_ I understand how it works from reading the relevant sections
of gpiolib.c but I can see how users end up using sysfs instead.

I intended tools/gpio/* to be the documentation:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/gpio

If we need more written documentation we can do it I guess,

I haven't been in the habit of looking in tools/.  Guess I should be.

Yours,
Linus Walleij