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

Maxime Ripard maxime.ripard at free-electrons.com
Thu Mar 5 10:28:17 PST 2015

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 at csie.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?


Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
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