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

From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Wolfram Sang <wsa@the-dreams.de>,
	linux-i2c@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	linux-doc@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Przemyslaw Sroka <psroka@cadence.com>,
	Arkadiusz Golec <agolec@cadence.com>,
	Alan Douglas <adouglas@cadence.com>,
	Bartosz Folta <bfolta@cadence.com>, Damian Kos <dkos@cadence.com>,
	Alicja Jurasik-Urbaniak <alicja@cadence.com>,
	Jan Kotas <jank@cadence.com>,
	Cyprian Wronka <cwronka@cadence.com>,
	Alexandre Belloni <alexandre.belloni@free-electrons.com>,
	Thomas Petazzoni <thomas.petazzoni@free-electrons.com>,
	Nishanth Menon <nm@ti.com>, Rob Herring <robh+dt@kernel.org>,
	Pawel Moll <pawel.moll@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Ian Campbell <ijc+devicetree@hellion.org.uk>,
	Ku
Subject: Re: [RFC 2/5] i3c: Add core I3C infrastructure
Date: Tue, 1 Aug 2017 15:34:14 +0200	[thread overview]
Message-ID: <20170801153414.6ce34ee8@bbrezillon> (raw)
In-Reply-To: <CAK8P3a0SH4kWcvXK+_iPt9LWuNnpjr+omp+ZpyTLjT=H602n+w@mail.gmail.com>

On Tue, 1 Aug 2017 15:11:44 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> On Tue, Aug 1, 2017 at 2:29 PM, Boris Brezillon
> <boris.brezillon@free-electrons.com> wrote:
> > On Tue, 1 Aug 2017 14:00:05 +0200
> > Arnd Bergmann <arnd@arndb.de> wrote:  
> 
> >> Another argument for a combined bus would be devices that
> >> can be attached to either i2c and i3c, depending on the host
> >> capabilities.  
> >
> > Hm, that's already the case, isn't it? And you'll anyway need to
> > develop specific code for both cases in the I2C/I3C device driver
> > because I2C and I3C transfers are different. So I don't see how it
> > would help to have a single bus here.
> >  
> >> We have discussed whether i2c and spi should be
> >> merged into a single bus_type in the past, as a lot of devices
> >> can be attached to either of them.  
> >
> > Oh, really? What's the rational behind that? I mean, I2C and SPI are
> > quite different, and even if some devices provide both interfaces, I
> > don't see why we should merge them. But you probably had good reasons
> > to do so.  
> 
> Well, we never changed it, so at least the work required to merge
> the two was considered too much to justify any advantages.
> 
> The main problem with having one driver that can operate on
> different bus types (i2c plus either spi or i3c) is the handling for
> the various combinations in configurations (e.g. I2C=m, SPI=y).
> 
> The easy case is having a module_init function that registers two
> device drivers, but that requires having a Kconfig dependency
> on both subsystems, and you can't use the module_i2c_driver()
> helper.
> 
> The second way is to have a number of #ifdef and complex
> Kconfig dependencies for the driver to only register the
> device_driver objects for the buses that are enabled. This
> is also doable, but everyone gets the logic wrong the first time.

Hm, I understand now why you'd prefer to have a single bus. Can't we
solve this problem with a module_i3c_i2c_driver() macro that would hide
all this complexity from I2C/I3C drivers?

> 
> What we end up doing to work around this for other drivers is
> to have the base driver in one library module, and separate
> modules for the bus-specific portions, which can then
> use module_i2c_driver again. There are many instances
> for combined i2c/spi drivers in the kernel, and it works fine,
> but it adds a fair bit of overhead compared to having one
> driver that would e.g. use regmap to abstract the differences
> in the probe() function and otherwise keeps everything in
> one place.
> 
>        Arnd