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

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Date:   Mon, 28 Aug 2017 00:30:05 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Rick Altherr <raltherr@...gle.com>
Cc:     Oleksandr Shamray <oleksandrs@...lanox.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Jiří Pírko <jiri@...nulli.us>,
        Arnd Bergmann <arnd@...db.de>,
        system-sw-low-level@...lanox.com,
        Greg KH <gregkh@...uxfoundation.org>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        openocd-devel-owner@...ts.sourceforge.net, mec@...ut.net,
        Rob Herring <robh+dt@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        vadimp@...llanox.com, Tobias Klauser <tklauser@...tanz.ch>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [patch v6 0/3] JTAG driver introduction

On Fri, Aug 25, 2017 at 6:52 PM, Rick Altherr <raltherr@...gle.com> 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.

>>>> 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,

Yours,
Linus Walleij