linux-kernel - Re: [PATCH 1/4 v4] drivers: create a pin control subsystem

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Message-Id: <201108191636.29161.arnd@arndb.de>
Date:	Fri, 19 Aug 2011 16:36:28 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Jamie Iles <jamie@...ieiles.com>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Grant Likely <grant.likely@...retlab.ca>,
	Stephen Warren <swarren@...dia.com>,
	Russell King <linux@....linux.org.uk>,
	Joe Perches <joe@...ches.com>,
	Linaro Dev <linaro-dev@...ts.linaro.org>,
	Lee Jones <lee.jones@...aro.org>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH 1/4 v4] drivers: create a pin control subsystem

On Friday 19 August 2011, Linus Walleij wrote:
> On Fri, Aug 19, 2011 at 12:48 PM, Jamie Iles <jamie@...ieiles.com> wrote:
> 
> >> +static struct class pinctrl_class = {
> >> +     .name = "pinctrl",
> >> +     .dev_release = pinctrl_dev_release,
> >> +     .dev_attrs = pinctrl_dev_attrs,
> >> +};
> >
> > Greg K-H has mentioned in the past that class is now deprecated for new
> > use and that a bus_type should be used instead.
> 
> Can you provide a reference with some detail?
> The pin control devices are usually aleady on a bus like the
> platform_bus or amba_bus or i2c_bus, then they register a
> class device in this case.
> 
> The kerneldoc documentation says
> "A bus is a channel between the processor and one or more devices."
> 
> This isn't the case here.
> 
> Anyhthing that help me understand this is appreciated, Arnd?

Taking Greg on Cc as well.

The main difference between a normal device and a class device is
that one is linked from /sys/bus/*/devices/* and the other is linked
from /sys/class/*/*. However, they both live in /sys/devices/.../*
as directories.

I always liked the separation between the two, although there are
a few cases where there is a grey area (e.g. /sys/bus/hid or
/sys/class/mmc_host) and the abstraction doesn't really fit.

IIRC Greg would prefer now to never have had the distinction
and wants to make all future uses use a bus_type.

	Arnd
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