linux-kernel - Re: [RFC PATCH v2 0/6] i2c: of: reserve unknown and ancillary addresses

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Date:   Wed, 15 Apr 2020 10:27:12 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-i2c@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-i3c@...ts.infradead.org,
        Kieran Bingham <kieran@...uared.org.uk>,
        Niklas Söderlund <niklas.soderlund@...natech.se>,
        Luca Ceresoli <luca@...aceresoli.net>,
        Jacopo Mondi <jacopo@...ndi.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Vladimir Zapolskiy <vz@...ia.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/6] i2c: of: reserve unknown and ancillary
 addresses

Status update on this series:

> TODO: make sure there are no concurrency issues in patch 6 when handling
> the struct i2c_client.

This turns out to be annoying. How to make sure that we don't modify the
i2c_client while the adapter it is sitting on just gets removed. AFAICS
we need a new locking scheme just for that and I am not convinced this
is the way forward.

Also, there is still this small room for regressing when there are DTs
having multiple addresses specified in the DT and the drivers use
i2c_new_dummy_client on these addresses. I have verified that no in-tree
users of i2c_new_dummy (and friends) do work on extra addresses but
still I'd like to completely avoid this potential regression.

One solution to both problems would be to unregister the reserved device
when its address is requested. I am working on this prototype currently.
However, I am not sure yet if one issue might make this approach messy:
re-registering the reserved device when the probe of the requested
address fails.

We will see...

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