linux-kernel - Re: [RFC,v2 2/6] i2c: add I2C Address Translator (ATR) support

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Date:   Tue, 10 Sep 2019 19:46:53 +0100
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Luca Ceresoli <luca@...aceresoli.net>
Cc:     Peter Rosin <peda@...ntia.se>, jacopo mondi <jacopo@...ndi.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        Vladimir Zapolskiy <vz@...ia.com>
Subject: Re: [RFC,v2 2/6] i2c: add I2C Address Translator (ATR) support


> I still have to examine in depth all of the problems in the i2c-mux
> documented in Documentation/i2c/i2c-topology (thanks for having written
> those docs!), but at first sight it looks like the ATR is not going to
> introduce big problems because of how it works.

Assuming we are using the previously discussed NEEDS_ATR flag for the adapter
instead of the attach/detach callbacks:

Can't we then simply understand an ATR as a generic 1:1 mapping device
which can be setup when registering an adapter?

When we add an adapter using i2c_add_adapter, we have:


              .-----.  Slave X @ 0x10
  .-----.     |     |   |
  | CPU |--A--| ATR |---+---- B
  `-----'     |     |
              `-----'

When we use i2c_add_mux_adapter, we have:


                                Slave X @ 0x10
              .-----.   .-----.   |
  .-----.     |     |---| ATR |---+---- B
  | CPU |--A--| MUX |   '-----'
  `-----'     |     |   .-----.
              |     |---| ATR |---+---- C
              `-----'   '-----'   |
                                 Slave Y @ 0x10


That way we could keep the topology handling solely to the mux-core.

Am I overlooking something?


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