linux-kernel - Re: [RFC 2/5] i3c: Add core I3C infrastructure

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Message-ID: <20170801171434.2bc2aa84@bbrezillon>
Date:   Tue, 1 Aug 2017 17:14:34 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Wolfram Sang <wsa@...-dreams.de>, linux-i2c@...r.kernel.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Przemyslaw Sroka <psroka@...ence.com>,
        Arkadiusz Golec <agolec@...ence.com>,
        Alan Douglas <adouglas@...ence.com>,
        Bartosz Folta <bfolta@...ence.com>,
        Damian Kos <dkos@...ence.com>,
        Alicja Jurasik-Urbaniak <alicja@...ence.com>,
        Jan Kotas <jank@...ence.com>,
        Cyprian Wronka <cwronka@...ence.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Nishanth Menon <nm@...com>, Rob Herring <robh+dt@...nel.org>,
        Pawel Moll <pawel.moll@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ian Campbell <ijc+devicetree@...lion.org.uk>,
        Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/5] i3c: Add core I3C infrastructure

On Tue, 1 Aug 2017 16:22:21 +0200
Arnd Bergmann <arnd@...db.de> wrote:

> On Tue, Aug 1, 2017 at 3:58 PM, Boris Brezillon
> <boris.brezillon@...e-electrons.com> wrote:
> > On Tue, 1 Aug 2017 15:34:14 +0200
> > Boris Brezillon <boris.brezillon@...e-electrons.com> wrote:  
> >> On Tue, 1 Aug 2017 15:11:44 +0200
> >> Arnd Bergmann <arnd@...db.de> wrote:  
> >> > On Tue, Aug 1, 2017 at 2:29 PM, Boris Brezillon
> >> > <boris.brezillon@...e-electrons.com> wrote:  
> > I just realized I forgot to add a "depends on I2C" in the I3C Kconfig
> > entry. Indeed, I'm unconditionally calling functions provided by the
> > I2C framework which have no dummy wrapper when I2C support is disabled.
> > I could of course conditionally compile some portion of the I3C
> > framework so that it still builds when I2C is disabled but I'm not sure
> > it's worth the trouble.
> >
> > This "depends on I2C" should also solve the I2C+I3C driver issue, since
> > I2C is necessarily enabled when I3C is.
> >
> > Am I missing something?  
> 
> That should solve another part of the problem, as a combined driver then
> just needs 'depends on I3C'.
> 
> On top of that, the i3c_driver structure could also contain callback
> pointers for the i2c subsystem, e.g. i2c_probe(), i2c_remove() etc.
> When the i2c_probe() callback exists, the i3c layer could construct
> a 'struct i2c_driver' with those callbacks and register that under the
> cover. This would mean that combined drivers no longer need to
> register two driver objects.

That should work. Actually, i2c_driver contains a few more hooks, like
->alert(), ->command() and ->detect(). Of course we could assume that
I3C/I2C drivers do not need them, but I'm wondering if it's not easier
to just add an i2c_driver pointer inside the i3c_driver struct and let
the driver populate it if it needs to supports both protocols.

Something like:

	struct i3c_driver {
		...
		struct i2c_driver *i2c_compat;
		...
	};


and then in I3C/I2C drivers:

	static struct i2c_driver my_i2c_driver = {
		...
	};

	static struct i3c_driver my_i3c_driver = {
		...
		.i2c_compat = &my_i2c_driver,
		...
	};
	module_i3c_driver(my_i3c_driver);



Of course, you'll have a few fields of ->i2c_compat that would be
filled by the core (like the driver name which can be extracted from
my_i3c_driver->driver.name).