Date:	Thu, 11 Dec 2014 18:04:15 +0800
From:	Flora Fu <flora.fu@...iatek.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Rob Herring <robh+dt@...nel.org>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>, <arm@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	srv_heupstream <srv_heupstream@...iatek.com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	HenryC Chen <henryc.chen@...iatek.com>,
	Menghui Lin <menghui.lin@...iatek.com>,
	Olof Johansson <olof@...om.net>,
	Sascha Hauer <kernel@...gutronix.de>,
	Kumar Gala <galak@...eaurora.org>,
	Grant Likely <grant.likely@...aro.org>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>,
	Eddie Huang <eddie.huang@...iatek.com>,
	Dongdong Cheng <dongdong.cheng@...iatek.com>
Subject: Re: [PATCH v3 1/8] soc: mediatek: Add PMIC wrapper for MT8135 and
 MT6397 SoC

Hi,

On Tue, 2014-12-09 at 12:20 +0100, Arnd Bergmann wrote:
> On Tuesday 09 December 2014 11:30:15 Matthias Brugger wrote:
> > 2014-12-09 11:13 GMT+01:00 Sascha Hauer <s.hauer@...gutronix.de>:
> > > On Tue, Dec 09, 2014 at 09:23:18AM +0100, Arnd Bergmann wrote:
> > >>
> > >> I think we have had a similar case recently where a controller wasn't
> > >> actually using I2C, but the sofware protocol was close enough so we decided
> > >> to make it appear as i2c in Linux.
> > >>
> > >> Would that work for you, i.e. register the pmic wrapper as a fake spi
> > >> master driver in drivers/spi/ and register the rtc/regulator/codec
> > >> as SPI clients from DT?
> > >
> > > I don't think that's appropriate. I mean technically that could even
> > > work, but in software you really don't see anything from the underlying
> > > SPI bus. The SoC and the PMIC are really tightly coupled via the PMIC
> > > wrapper. This goes to the point where pins of the SoCs internal I2C and
> > > keypad controllers are routed over the SPI bus out of the PMIC. In
> > > software you do this by setting a bit in the I2C controller. If it's
> > > set, the signals are routed out of the PMIC instead of the main die.
> > > As said, technically we probably could create a fake SPI master, but
> > > that wouldn't really fit to this situation.
> 
> Ok, I see.
> 
> > I agree with Sascha. Although from the hardware point of view, the
> > communication between the PMIC and the SOC is done through SPI from
> > the point of view of the software everything looks like I2C commands
> > which will be "transalted" into SPI messages by the PMIC wrapper.
> 
> If it looks like i2c messages, would it be more appropriate to make
> it appear as an i2c controller then?


Although the message looks like I2C command, it is not I2C.
Form source code, the software does not touch any I2C i/o or protocols.
It depends SoC and has specific initial flow, read and write transfer
state. It is not able to an i2c controller.
That's why we consider its a proprietary hardware with specific
protocols. How about let it appear in driver/soc?

Thanks,
Flora

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