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From: Linus Walleij <linus.walleij@linaro.org>
Date: Wed, 24 Aug 2016 14:14:23 +0200
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Subject: Re: [RFC PATCH 0/3] UART slave device bus
To: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>, Rob Herring <robh@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Marcel Holtmann <marcel@holtmann.org>, Jiri Slaby <jslaby@suse.com>,
        Sebastian Reichel <sre@kernel.org>, Pavel Machek <pavel@ucw.cz>,
        Peter Hurley <peter@hurleysoftware.com>, NeilBrown <neil@brown.name>,
        "Dr . H . Nikolaus Schaller" <hns@goldelico.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Mon, Aug 22, 2016 at 5:45 PM, One Thousand Gnomes
<gnomes@lxorguk.ukuu.org.uk> wrote:

> and if I look at the usermode crapfest on a lot of Android systems it
> looks similar but with the notion of things like being able to describe
>
> -       Use GPIO mode sleeping and assume first char is X to save power

It's really nasty hardware design, or a software hack to solve
a hardware problem: what should have been done is
of course create a UART with an asynchronous low-power mode
that can recieve a character and wake up the system at any time,
handing over the wakeup character(s) to the driver. That is
obviously the usecase they were designing for.

But yeah, I guess we have to contain hacks like that.

> -       Power up, wait n ms, write, read, wait n ms, power down (which
>         has to be driven at the ldisc/user level as only the ldisc
>         understands transactions, or via ioctls (right now Android user
>         space tends to do hardcoded writes to /sys.. gpio to drive power

This kind of abominational abuse of the GPIO sysfs ABI is
partly why I've obsoleted it. The right abstraction is the
fixed regulator with a GPIO line obviously, then some
sequencing along the lines of what you can find in
drivers/mmc/core/pwrseq*

Unfortunately that sysfs ABI crept in during a window of time
when GPIO was unmaintained and I am trying my best to
contain and improve the situation.

Yours,
Linus Walleij