Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v6 00/10] Add the I3C subsystem
To:     Arnd Bergmann <arnd@arndb.de>, Wolfram Sang <wsa@the-dreams.de>
Cc:     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>,
        Kumar Gala <galak@codeaurora.org>,
        DTML <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Vitor Soares <Vitor.Soares@synopsys.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Xiang Lin <Xiang.Lin@synaptics.com>,
        linux-gpio@vger.kernel.org, Sekhar Nori <nsekhar@ti.com>,
        Przemyslaw Gaj <pgaj@cadence.com>
References: <20180719152930.3715-1-boris.brezillon@bootlin.com>
 <CAK8P3a2MP5+oFz--e+BM24D8Ahw7H_RKgN1S6wY1nQE6JL0dmw@mail.gmail.com>
 <2ab0ab75-2df0-2714-f007-c33b25481016@axentia.se>
 <20180720101206.tv7nsoanwo5ftnia@ninjato>
 <CAK8P3a2Pqo5DuYBb09+w4nGWGOw+e_tZPxnA06qjNO_6Hw6NvQ@mail.gmail.com>
From:   Peter Rosin <peda@axentia.se>
Organization: Axentia Technologies AB
Message-ID: <21b269c5-a3a7-c5de-c81e-c9c9301ae13e@axentia.se>
Date:   Fri, 20 Jul 2018 13:13:49 +0200
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.0
MIME-Version: 1.0
In-Reply-To: <CAK8P3a2Pqo5DuYBb09+w4nGWGOw+e_tZPxnA06qjNO_6Hw6NvQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Received-SPF: None (protection.outlook.com: axentia.se does not designate
 permitted sender hosts)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Jul 2018 11:13:54.4704 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: a56ad5f8-722c-4e77-72b0-08d5ee31e280
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 4ee68585-03e1-4785-942a-df9c1871a234
X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM5PR0201MB2452
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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