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

[Linus Walleij]

On Fri, Aug 25, 2017 at 10:50 AM, Stuart Longland
<stuartl@xxxxxxxxxxxxxxxxxx> wrote:
> [Note: dropping vadimp@xxxxxxxxxxxxx as SMTP server complained about the
> DNS server returning NXDOMAIN.  Apologies.]
> On 25/08/17 18:32, Linus Walleij wrote:
>> Gnah!
>> Whoever writes a slot-in replacement making the character device
>> take precendence wins lots of karma.
>
> What would such a replacement look like though?

Something that looks for /dev/gpiochipN and if it exists open the GPIOs
from there and make that take precedence over any /sys/gpio/*
poking.

> Some sort of system whereby you can read/write single-line commands as
> if talking to a GPIO expander over a UART?

I don't really understand the question. All GPIO expanders become
a gpiochip, and have their own character device in /dev.

> Would you access the GPIOs one by one, or would you perhaps map them
> into a bitmap (maybe arbitrarily, up to 64-bits wide) and perform masked
> operations on the bitmap?

The character device supports up to 64bits of simultaneous line
switches, but the in-kernel API can only handle 32bits
in a single register write.

> I'm no fan of the sysfs GPIO interface, but it beats poking around at
> registers behind the kernel's back.

Have a look at libgpiod and tools/gpio/* in the kernel.

Yours,
Linus Walleij