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Date:   Thu, 12 Jul 2018 10:44:59 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Boris Brezillon <boris.brezillon@...tlin.com>,
        Wolfram Sang <wsa@...-dreams.de>, 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
Subject: Re: [PATCH v4 01/10] i3c: Add core I3C infrastructure

On 2018-07-12 10:08, Arnd Bergmann wrote:
> On Thu, Jul 12, 2018 at 6:41 AM, Peter Rosin <peda@...ntia.se> wrote:
>> On 2018-07-11 19:12, Boris Brezillon wrote:
>>> On Wed, 11 Jul 2018 17:39:56 +0200 Arnd Bergmann <arnd@...db.de> wrote:
>>>
>>> That's exactly the sort of discussion I wanted to trigger. Maybe we
>>> shouldn't care and expose this use case as if it was X different I3C
>>> buses (with all devices present on the bus being exposed X times to the
>>> system).
>>
>> For I2C, this multiple masters for one bus case was retrofitted in
>> the i2c-demux-pinctrl driver. It's a huge kludge with a number of
>> undesirable quirks. I don't know if the circumstances for adding
>> this I2C driver also applies for I3C, but it might be an argument
>> in favor of the proposed extra bus object...
> 
> From reading the documentation and git history on that driver,
> it seems to be used only for static configuration, i.e. you use
> one driver or the other, but don't flip between them at runtime,
> right?

There is a sysfs file that can be used to change master at runtime
(current_master). This causes all client drivers to be reprobed,
which may not be the best thing to do for every client out there...

> I'm guessing that even with i3c we may have to support something
> like that, as a likely scenario might be that the i3c controller is
> multiplexed with a traditional i2c controller and/or gpios, but you
> would not be able to perform the i3c standard secondary master
> transition with the latter two because they are (by definition) not
> i3c compatible.

i2c-demux-pinctrl should probably not be used as template for something
else, but it is a good argument for some other design IMHO...

Cheers,
Peter

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