Re: [patch v6 0/3] JTAG driver introduction

[Rick Altherr]

On Sun, Aug 27, 2017 at 3:30 PM, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> On Fri, Aug 25, 2017 at 6:52 PM, Rick Altherr <raltherr@xxxxxxxxxx> wrote:
>
>>> 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.

>>>>> 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