Date: Tue, 1 Aug 2017 16:48:26 +0200
From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Wolfram Sang <wsa@the-dreams.de>
Cc: Arnd Bergmann <arnd@arndb.de>, linux-i2c@vger.kernel.org,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Przemyslaw Sroka <psroka@cadence.com>,
        Arkadiusz Golec <agolec@cadence.com>,
        Alan Douglas <adouglas@cadence.com>,
        Bartosz Folta <bfolta@cadence.com>, Damian Kos <dkos@cadence.com>,
        Alicja Jurasik-Urbaniak <alicja@cadence.com>,
        Jan Kotas <jank@cadence.com>, Cyprian Wronka <cwronka@cadence.com>,
        Alexandre Belloni <alexandre.belloni@free-electrons.com>,
        Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
        Nishanth Menon <nm@ti.com>, Rob Herring <robh+dt@kernel.org>,
        Pawel Moll <pawel.moll@arm.com>, Mark Rutland <mark.rutland@arm.com>,
        Ian Campbell <ijc+devicetree@hellion.org.uk>,
        Kumar Gala <galak@codeaurora.org>, devicetree@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 2/5] i3c: Add core I3C infrastructure
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On Tue, 1 Aug 2017 16:12:18 +0200
Wolfram Sang <wsa@the-dreams.de> wrote:

> > > The second way is to have a number of #ifdef and complex
> > > Kconfig dependencies for the driver to only register the
> > > device_driver objects for the buses that are enabled. This
> > > is also doable, but everyone gets the logic wrong the first time.  
> > 
> > Hm, I understand now why you'd prefer to have a single bus. Can't we
> > solve this problem with a module_i3c_i2c_driver() macro that would hide
> > all this complexity from I2C/I3C drivers?  
> 
> Do you know of devices speaking both i3c and i2c as of today?

I do not know of any real devices as of today (all my tests have been
done with a dummy/fake I3C slaves emulated with a slave IP), but the
spec clearly describe what legacy/static addresses are for and one of
their use case is to connect an I3C device on an I2C bus and let it act
as an I2C device.

> 
> I think I3C/I2C is a bit different than I2C/SPI. For the latter, it
> might happen that you have only this or that bus on the board, so it
> makes sense to support both. But if you have I3C, you can simply attach
> the I2C device onto it. I guess you would only implement I3C in the
> device if you explicitly need its feature set. And then, a I2C fallback
> doesn't make much sense? Or am I missing something?

Unless you want your device (likely a sensor) to be compatible with both
I3C and I2C so that you can target even more people.

> 
> OK, now I know that those I3C+I2C devices will exist, even if only for
> Murphy's law. However, my assumptions would be that those devices are
> not common and so we could live with the core plus bus_drivers
> seperation we have for SPI/I2C already (although I would love a common
> regmap-based I2C/SPI abstraction).
> 

I'm perfectly fine with the I3C / I2C framework separation. The only
minor problem I had with that was the inaccuracy of the
sysfs/device-model representation: we don't have one i2c and one i3c
bus, we just have one i3c bus with a mix of i2c and i3c devices.

Apart from that, I'm happy with the current approach.