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

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Date:   Mon, 28 Aug 2017 00:25:14 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Stuart Longland <stuartl@...glandclan.id.au>
Cc:     Rick Altherr <raltherr@...gle.com>,
        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>,
        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 10:50 AM, Stuart Longland
<stuartl@...glandclan.id.au> wrote:
> [Note: dropping vadimp@...llanox.com 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