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

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Message-ID: <20190903090621.GB1020@kunai>
Date:   Tue, 3 Sep 2019 11:06:22 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Luca Ceresoli <luca@...aceresoli.net>
Cc:     linux-media@...r.kernel.org, linux-i2c@...r.kernel.org,
        devicetree@...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>,
        Jacopo Mondi <jacopo@...ndi.org>,
        Vladimir Zapolskiy <vz@...ia.com>,
        Peter Rosin <peda@...ntia.se>
Subject: Re: [RFC,v2 2/6] i2c: add I2C Address Translator (ATR) support


> > One huge drawback for me is the attach/detach callbacks. One year ago, I
> > removed a similar callback from the I2C core ("[PATCH 0/2] i2c: remove
> > deprecated attach_adapter callback") because some drivers did a lot of
> > crazy things there. It took years to remove all that.
> 
> Oh dear, I was completely unaware, apologies! :-)

Oh, no need to apologize. You don't have to research the whole I2C history
before implementing something. Keeping the big picture is what I happily
provide.

> > What I could imagine here: the adapter (B and C each in the picture
> > above) gets a flag like NEEDS_ATR before registering to the core. The
> > flag means all clients on that bus will have their address translated.
> > The core will figure out a free alias when a device is registered. We
> > can then have an ATR specific callback with the original and translated
> > address as arguments, so one can setup the HW as needed.
> 
> Do you mean moving the alias selection code from i2c-atr.c to the i2c
> core? And the rest of the ATR core too?

I hope for something like this in the I2C core (simplified, naming needs
to be improved etc.) in i2c_new_device:

	if (client->adapter->flag & NEEDS_ATR) {
		i2c_atr_get_alias_address();
		/* probably a wrapper around a callback */
		i2c_atr_setup_hw();
	}

with all the i2c_atr_* functions in a seperate file. It would be great
if that file could be a completely independent module, but if it turns
out that we need some simple helpers in the core, I am probably OK with
that, too.

> > Do you think that would work?
> 
> Yes.

Cool!


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