linux-kernel - Re: [PATCH v2 1/4] i2c: sunxi: Add Reduced Serial Bus (RSB) support

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Date:	Thu, 5 Mar 2015 19:28:17 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Wolfram Sang <wsa@...-dreams.de>
Cc:	Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, linux-i2c@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-sunxi@...glegroups.com,
	Hans de Goede <hdegoede@...hat.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 1/4] i2c: sunxi: Add Reduced Serial Bus (RSB) support

Hi Wolfram,

On Wed, Mar 04, 2015 at 06:27:11PM +0100, Wolfram Sang wrote:
> On Mon, Mar 02, 2015 at 04:24:43PM +0800, Chen-Yu Tsai wrote:
> > The RSB controller looks like an SMBus controller which only supports byte
> > and word data transfers. It can also do double-word data transfers, but the
> > I2C subsystem does not support this, nor have we seen devices using this.
> > 
> > The RSB differs from standard SMBus protocol on several aspects:
> > - it uses addresses set at runtime to address slaves. Runtime addresses
> >   are sent to slaves using their 12bit hardware addresses. Up to 15
> >   runtime addresses are available.
> > - it adds a parity bit every 8bits of data and address for read and
> >   write accesses; this replaces the ack bit
> > - only one read access is required to read a byte (instead of a write
> >   followed by a read access in standard SMBus protocol)
> > - there's no Ack bit after each read access
> > 
> > This means this bus cannot be used to interface with standard SMBus
> > devices (known devices supporting this interface are the AXP223, AXP806,
> > AXP809 PMICs and the AC100 codec/RTC). However the RSB protocol is an
> > extension of P2WI, which was close enough to SMBus to be integrated into
> > the I2C subsystem in commit 3e833490fae5 ("i2c: sunxi: add P2WI (Push/Pull
> > 2 Wire Interface) controller support").
> > 
> > Signed-off-by: Chen-Yu Tsai <wens@...e.org>
> 
> I don't have the bandwidth for a full review right now. However, I
> already wanted to tell you guys that my gut feeling is that this
> protocol is quite far away from I2C. P2WI was already at the edge.
> Maybe there is a better place for such custom stuff? I dunno yet.

That's unfortunate, especially since it looks closer to SPI than what
P2WI even was.

What would be your suggestion?

Thanks,
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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