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

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Date:   Mon, 31 Jul 2017 22:46:58 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Wolfram Sang <wsa@...-dreams.de>
Cc:     linux-i2c@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        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@...r.kernel.org
Subject: Re: [RFC 2/5] i3c: Add core I3C infrastructure

Le Mon, 31 Jul 2017 21:17:21 +0200,
Wolfram Sang <wsa@...-dreams.de> a écrit :

> > +This document is just a brief introduction to the I3C protocol and the concepts
> > +it brings on the table. If you need more information, please refer to the MIPI
> > +I3C specification.  
> 
> I wish I could.
> 
> > +
> > +Introduction
> > +============
> > +
> > +The I3C (I-Cube-C) is a MIPI standardized protocol designed to overcome I2C  
> 
> "Eye-three-See", according to:
> http://eecatalog.com/sensors/2017/07/05/after-35-years-of-i2c-i3c-improves-capability-and-performance/

I remember hearing eye-cube-see during the discussion we had with
Cadence engineers but I might wrong. I'll double check (or maybe I'll
just drop any mention of the pronunciation).

> 
> > +Backward compatibility with I2C devices
> > +=======================================
> > +
> > +The I3C protocol has been designed to be backward compatible with I2C devices.
> > +This backward compatibility allows one to connect a mix of I2C and I3C devices
> > +on the same bus, though, in order to be really efficient, I2C devices should
> > +be equipped with 50 ns spike filters.  
> 
> I just found a slide which says I3C does not support clock stretching.
> That should be mentioned here, too.
> 

You're right, clock stretching is not allowed, and if devices without a
50ns spike filter are connected to the bus, it lowers the maximum
speed for all devices, which renders I3C kind of useless.