linux-kernel - Re: [RFC 2/5] i3c: Add core I3C infrastructure

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Message-ID: <20170801164826.423183ff@bbrezillon>
Date:   Tue, 1 Aug 2017 16:48:26 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Wolfram Sang <wsa@...-dreams.de>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-i2c@...r.kernel.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Przemyslaw Sroka <psroka@...ence.com>,
        Arkadiusz Golec <agolec@...ence.com>,
        Alan Douglas <adouglas@...ence.com>,
        Bartosz Folta <bfolta@...ence.com>,
        Damian Kos <dkos@...ence.com>,
        Alicja Jurasik-Urbaniak <alicja@...ence.com>,
        Jan Kotas <jank@...ence.com>,
        Cyprian Wronka <cwronka@...ence.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Nishanth Menon <nm@...com>, 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>, devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/5] i3c: Add core I3C infrastructure

On Tue, 1 Aug 2017 16:12:18 +0200
Wolfram Sang <wsa@...-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.