linux-kernel - Re: [PATCH v5 1/8] i2c: core: let adapters be notified of client attach/detach

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Message-ID: <20221219095143.4b49b447@booty>
Date:   Mon, 19 Dec 2022 09:51:43 +0100
From:   Luca Ceresoli <luca.ceresoli@...tlin.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Tomi Valkeinen <tomi.valkeinen@...asonboard.com>,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Wolfram Sang <wsa@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Matti Vaittinen <Matti.Vaittinen@...rohmeurope.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Peter Rosin <peda@...ntia.se>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Michael Tretter <m.tretter@...gutronix.de>,
        Shawn Tu <shawnx.tu@...el.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Mike Pagano <mpagano@...too.org>,
        Krzysztof Hałasa <khalasa@...p.pl>,
        Marek Vasut <marex@...x.de>,
        Luca Ceresoli <luca@...aceresoli.net>
Subject: Re: [PATCH v5 1/8] i2c: core: let adapters be notified of client
 attach/detach

Hello Laurent,

thanks for the feedback and apologies for the delayed reply.

On Sun, 11 Dec 2022 18:55:39 +0200
Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:

> Hi Tomi and Luca,
> 
> Thank you for the patch.
> 
> On Thu, Dec 08, 2022 at 12:39:59PM +0200, Tomi Valkeinen wrote:
> > From: Luca Ceresoli <luca@...aceresoli.net>
> > 
> > An adapter might need to know when a new device is about to be
> > added. This will soon bee needed to implement an "I2C address
> > translator" (ATR for short), a device that propagates I2C transactions
> > with a different slave address (an "alias" address). An ATR driver
> > needs to know when a slave is being added to find a suitable alias and
> > program the device translation map.
> > 
> > Add an attach/detach callback pair to allow adapter drivers to be
> > notified of clients being added and removed.  
> 
> This may be a stupid question, but couldn't you instead use the
> BUS_NOTIFY_ADD_DEVICE and BUS_NOTIFY_DEL_DEVICE bus notifiers ?

I'm not sure they would be the correct tool for this task. Bus
notifiers inform about new events on the 'struct bus_type, i.e. any
event on the global i2c bus type. In the i2c world this means being
notified about new _adapters_, which is exactly what
drivers/i2c/i2c-dev.c does.

Here, however, we need to be informed about new _clients_ being added
under a specific adapter. I'm not sure whether the bus notifiers can
inform about new clients in addition of new adapters, but they at least
seem unable to provide per-adapter notification.

Does that seem correct?

Best regards,
-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com