Date: Mon, 31 Jul 2017 21:17:45 +0200
From: Wolfram Sang <wsa@the-dreams.de>
To: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: linux-i2c@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
        linux-doc@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>, 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@vger.kernel.org
Subject: Re: [RFC 0/5] Add I3C subsystem
Message-ID: <20170731191745.GB1542@katana>
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Hi Boris,

> This patch series is a proposal for a new I3C [1] subsystem.

Nice. Good luck with that!

Some hi-level comments from me related to I2C. I can't say a lot more
because the specs are not public :(

> - the bus element is a separate object and is not implicitly described
>   by the master (as done in I2C). The reason is that I want to be able
>   to handle multiple master connected to the same bus and visible to
>   Linux.
>   In this situation, we should only have one instance of the device and
>   not one per master, and sharing the bus object would be part of the
>   solution to gracefully handle this case.
>   I'm not sure if we will ever need to deal with multiple masters
>   controlling the same bus and exposed under Linux, but separating the
>   bus and master concept is pretty easy, hence the decision to do it
>   now, just in case we need it some day.

=46rom my experience, it is a good thing to have this separation.

> - I2C backward compatibility has been designed to be transparent to I2C
>   drivers and the I2C subsystem. The I3C master just registers an I2C
>   adapter which creates a new I2C bus. I'd say that, from a
>   representation PoV it's not ideal because what should appear as a
>   single I3C bus exposing I3C and I2C devices here appears as 2
>   different busses connected to each other through the parenting (the
>   I3C master is the parent of the I2C and I3C busses).
>   On the other hand, I don't see a better solution if we want something
>   that is not invasive.

I agree this is the least invasive and also the most compatible
approach. The other solution would probably be to have some kind of
emulation layer?

> I'd also like to get feedback on the doc. Should I detail a bit more
> the protocol or the framework API? Is this the kind of things you
> expect in a subsystem doc?

Since the spec is not public, details about the protocol will be
especially useful, I'd say.

Regards,

   Wolfram


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