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Date:   Fri, 20 Jul 2018 17:41:32 +0200
From:   Wolfram Sang <wsa@the-dreams.de>
To:     Peter Rosin <peda@axentia.se>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Boris Brezillon <boris.brezillon@bootlin.com>,
        linux-i2c@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
        "open list:DOCUMENTATION" <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>,
        Cyprian Wronka <cwronka@cadence.com>,
        Suresh Punnoose <sureshp@cadence.com>,
        Rafal Ciepiela <rafalc@cadence.com>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.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>,
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        Xiang Lin <Xiang.Lin@synaptics.com>,
        linux-gpio@vger.kernel.org, Sekhar Nori <nsekhar@ti.com>,
        Przemyslaw Gaj <pgaj@cadence.com>
Subject: Re: [PATCH v6 00/10] Add the I3C subsystem
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> I don't have an actual example for I2C, maybe Wolfram does? But I can
> invent a case. E.g. the speedy DMA-enabled master cannot generate
> RESTART, which is a must for (re-)configuration, but not for passing
> data to the device.

DMA capable controllers may also not react adequate to the slave doing
clock stretching (which is forbidden in I3C).

Renesas R-Car Gen2 has two I2C IP cores. One can do DMA and automatic
transfers to the PMIC, the other has I2C slave functionality. One cannot
do I2C_SMBUS_QUICK, the other can. And some more kind of quirks.
Sometimes you can mux the pins to GPIO, so you have a third option.

This setup is the reason the demux driver exists.

> Also consider some future HW that has several I3C blocks, but they
> are not identical. There's one beefy kind and one slim kind (I'm sure
> you can find HW with different flavors of I2C blocks). Even if the
> HW designers intended for one type of block to be superior in every
> aspect, they might have made a mistake? This HW also has a pinmux, so
> the SW is free to route different I3C blocks to the actual I3C bus.

So, basically this is what happened with R-Car. Now, I tend to think
that I3C is much more complex and noone would put two I3C IP cores into
on SoC. But it was not too long ago that I wouldn't believe someone put
two different I2C IP cores into a SoC. Then again, it happened when I2C
was around for 35 years...


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