linux-kernel - Re: [PATCH v6 00/10] Add the I3C subsystem

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Date:   Fri, 20 Jul 2018 13:13:49 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Arnd Bergmann <arnd@...db.de>, Wolfram Sang <wsa@...-dreams.de>
Cc:     Boris Brezillon <boris.brezillon@...tlin.com>,
        linux-i2c@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        "open list:DOCUMENTATION" <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>,
        Cyprian Wronka <cwronka@...ence.com>,
        Suresh Punnoose <sureshp@...ence.com>,
        Rafal Ciepiela <rafalc@...ence.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.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>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vitor Soares <Vitor.Soares@...opsys.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Xiang Lin <Xiang.Lin@...aptics.com>,
        linux-gpio@...r.kernel.org, Sekhar Nori <nsekhar@...com>,
        Przemyslaw Gaj <pgaj@...ence.com>
Subject: Re: [PATCH v6 00/10] Add the I3C subsystem

On 2018-07-20 12:57, Arnd Bergmann wrote:
> * What I understand from reading i2c-demux-pinctrl.c, a slave device
>   will only ever be observable from one master at a time, when you
>   switch over, all children get removed on one master and added to
>   the other one, to be probed again by their respective drivers.
>   I can see this as a useful feature on i3c as well, in particular to
>   deal with the situation where we have i2c slaves connected to a
>   pinmux that can switch them between an i3c master and an
>   i2c-only master (possibly a gpio based one). That particular use
>   case however doesn't seem to fix well in the current code, which
>   is structure around i3c buses.

It's pretty easy to come up with examples where this reprobing is
not desirable at all. E.g. if one of the involved I2C devices is
a HDMI encoder (I have a TDA19988 here) sitting in the middle of the
graphics pipeline. Blink-blink on the screen because some *other*
unrelated device needed to be accessed by an alternative master. Not
pretty.

(No, I don't suffer from this since I don't need the demuxer. Which is
fortunate. This was just an example, I'm sure there are others.)

Cheers,
Peter