linux-kernel - Re: [PATCH RESEND v2 3/7] mfd: cros_ec: Add cros_ec

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Date:	Tue, 20 Jan 2015 16:58:00 +0100
From:	Javier Martinez Canillas <javier.martinez@...labora.co.uk>
To:	Lee Jones <lee.jones@...aro.org>
CC:	Olof Johansson <olof@...om.net>,
	Doug Anderson <dianders@...omium.org>,
	Bill Richardson <wfrichar@...omium.org>,
	Simon Glass <sjg@...gle.com>,
	Gwendal Grignou <gwendal@...gle.com>,
	Jonathan Corbet <corbet@....net>,
	linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v2 3/7] mfd: cros_ec: Add cros_ec_lpc driver for
 x86 devices

Hello Lee,

On 01/20/2015 09:11 AM, Lee Jones wrote:
> On Fri, 02 Jan 2015, Javier Martinez Canillas wrote:
> 
>> From: Bill Richardson <wfrichar@...omium.org>
>> 
>> This adds the LPC interface to the Chrome OS EC. Like the
>> I2C and SPI drivers, this allows userspace access to the EC.
> 
> I'm fairly certain that this is _not_ an MFD device.  Please locate it
> to the proper subsystem (input?).
> 

Sorry, it wasn't my intention to use the mfd subsystem as a place to dump
random drivers. Is that I still find hard to understand what is the line
between what falls under mfd and what doesn't.

For example, I see that mfd drivers are for devices which have multiple
functions and the mfd driver is the one that spawns the platform devices
and provide an interface to access the I/O registers used by the different
platform drivers of the sub-devices.

So, the Embedded Controller driver (drivers/mfd/cros_ec.c) falls into that
category and in fact has been in the mfd driver for a long time. Now, if
an mfd device support different type of buses (e.g: i2c, spi, etc) I see
that both the core driver and the driver for the transport method are
in the drivers/mfd directory. As an example:

drivers/mfd/arizona-{core,i2c,spi}.c
drivers/mfd/da9052-{core,i2c,spi}.c
drivers/mfd/mc13xxx-{core,i2c,spi}.c
drivers/mfd/tps65912-{core,i2c,spi}.c
drivers/mfd/wm831x-{core,i2c,spi,otp}.c

In the cros_ec case, we already have drivers/mfd/cros_ec_{i2c,spi}.c so
since the Low Pin Count is another transport method I thought that this
driver belonged to the drivers/mfd directory.

Now, all those drivers may be wrong and the buses don't belong to the mfd
subsystem but then I think we need to document that since it seems that is
the correct way to do it just by looking at the other drivers.

Best regards,
Javier
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