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Subject: Re: [PATCH v4 01/10] i3c: Add core I3C infrastructure
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Boris Brezillon <boris.brezillon@bootlin.com>,
        Wolfram Sang <wsa@the-dreams.de>, linux-i2c@vger.kernel.org,
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        Alan Douglas <adouglas@cadence.com>,
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        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Nishanth Menon <nm@ti.com>, Rob Herring <robh+dt@kernel.org>,
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From:   Peter Rosin <peda@axentia.se>
Organization: Axentia Technologies AB
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Date:   Thu, 12 Jul 2018 10:44:59 +0200
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On 2018-07-12 10:08, Arnd Bergmann wrote:
> On Thu, Jul 12, 2018 at 6:41 AM, Peter Rosin <peda@axentia.se> wrote:
>> On 2018-07-11 19:12, Boris Brezillon wrote:
>>> On Wed, 11 Jul 2018 17:39:56 +0200 Arnd Bergmann <arnd@arndb.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