linux-kernel - Re: [PATCH v5 2/2] i2c: sunxi: add P2WI (Push/Pull 2 Wire Interface) controller support

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Message-ID: <13219561.hrRmVFjmAv@wuerfel>
Date:	Tue, 10 Jun 2014 16:47:55 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Boris BREZILLON <boris.brezillon@...e-electrons.com>,
	Wolfram Sang <wsa@...-dreams.de>,
	Randy Dunlap <rdunlap@...radead.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Shuge <shuge@...winnertech.com>, kevin@...winnertech.com,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-i2c@...r.kernel.org
Subject: Re: [PATCH v5 2/2] i2c: sunxi: add P2WI (Push/Pull 2 Wire Interface) controller support

On Tuesday 10 June 2014 16:36:04 Maxime Ripard wrote:
> On Tue, Jun 10, 2014 at 03:54:56PM +0200, Arnd Bergmann wrote:
> > On Tuesday 10 June 2014 15:47:16 Boris BREZILLON wrote:
> > > 
> > > +config I2C_SUN6I_P2WI
> > > +       tristate "Allwinner sun6i internal P2WI controller"
> > > +       depends on ARCH_SUNXI
> > > +       help
> > > +         If you say yes to this option, support will be included for the
> > > +         P2WI (Push/Pull 2 Wire Interface) controller embedded in some sunxi
> > > +         SOCs.
> > > +         The P2WI looks like an SMBus controller (which supports only byte
> > > +         accesses), except that it only supports one slave device.
> > > +         This interface is used to connect to specific PMIC devices (like the
> > > +         AXP221).
> > > +
> > 
> > Sorry for the stupid question, but why is this an i2c driver if the
> > hardware protocol is completely different?
> 
> It's not completely different. It deviates, but still looks very
> similar to i2c, and to be precise, SMBus.
> 
> You'll have the full discussion that led to do this in i2c here:
> http://www.spinics.net/lists/linux-i2c/msg15066.html
> 
> Also, one significant thing to take into account is that the
> communication with a device starts as I2C, only to switch to this
> protocol after some initialization sequence.

Ok, sounds good.

> > I understand that a lot of devices can be driven using either spi or
> > i2c, and we have two sets of {directories,maintainers,bus_types,...}
> > for them. Your description sounds like this is a separate option
> > that isn't any closer to i2c than it is to spi.
> 
> That's not true. It's *much* closer from I2C than it is from SPI.

Ok.

> > Would it perhaps be better to expose it only as a regmap rather than
> > an i2c host?
> 
> That could be a solution, but is it a common practice to define a bus
> adapter driver in a regmap driver?

No, not yet.

Maybe Boris can just put an explanation into the changeset description
of the driver so other people are able to find it more easily.

	Arnd
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