Date:	Tue, 2 Sep 2014 09:00:10 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Johan Hovold <johan@...nel.org>
Cc:	Octavian Purdila <octavian.purdila@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>, wsa@...-dreams.de,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>, linux-usb@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>, linux-gpio@...r.kernel.org,
	linux-i2c@...r.kernel.org, Daniel Baluta <daniel.baluta@...el.com>,
	Laurentiu Palcu <laurentiu.palcu@...el.com>
Subject: Re: [PATCH v2 1/3] mfd: add support for Diolan DLN-2 devices

On Mon, 01 Sep 2014, Johan Hovold wrote:
> On Mon, Sep 01, 2014 at 07:22:39PM +0300, Octavian Purdila wrote:
> > On Mon, Sep 1, 2014 at 6:46 PM, Lee Jones <lee.jones@...aro.org> wrote:
> > > On Mon, 01 Sep 2014, Octavian Purdila wrote:
> > >
> > >> On Mon, Sep 1, 2014 at 2:39 PM, Lee Jones <lee.jones@...aro.org> wrote:
> > >> > On Mon, 01 Sep 2014, Octavian Purdila wrote:
> > >> >> On Mon, Sep 1, 2014 at 12:51 PM, Lee Jones <lee.jones@...aro.org> wrote:
> 
> > >> >> > You should have a small MFD driver which controls resources and
> > >> >> > registers children.  All other functionality should live in their
> > >> >> > respective drivers/X locations i.e. USB functionallity should normally
> > >> >> > live in drivers/usb.
> > >> >> >
> > >> >>
> > >> >> OK, that sounds better. I am not sure how to handle the registration
> > >> >> part though, since in this case we need to create the children at
> > >> >> runtime, from the usb probe routine.
> > >> >>
> > >> >> The only solution I see is to move the driver completely to
> > >> >> usb/drivers and continue to use the MFD infrastructure. Does that
> > >> >> sound OK to you?
> > >> >
> > >> > I have no problem with that.  If this is an MFD driver, it _should_
> > >> > live in drivers/mfd.  However, all of that USB specific stuff
> > >> > defiantly should not.
> > >> >
> > >> > >> It is a multi-function driver which is using the USB interface, so I
> > >> am not sure where it belongs. The only driver that calls
> > >> mfd_add_devices and is not in drivers/mfd is the hid sensor hub
> > >> driver.
> > >>
> > >> BTW, the mfd/viperboard.c driver is very similar with this driver. It
> > >> has less USB specific stuff because the protocol is simpler, but still
> > >> has some.
> > >
> > > Looking at viperboard.c, it seems to use some basic generic framework
> > > calls to obtain some information about the device information like
> > > version numbers.  Your driver is leaps and bounds more USB centric.
> > >
> > > Your MFD driver should know about things like; regmap, platform data,
> > > memory allocation, same-chip devices (children), etc.  Your MFD driver
> > > should not need to concern itself with; endpoints, slots, URBs, USB
> > > device IDs and the like.  The later knowledge belongs in drivers/usb.
> > >
> > > You should be calling mfd_add_devices() from within the MFD driver.
> > > At a guess, I would say that you need a new entry for the USB stuff in
> > > your mfd_cells structure.
> > >
> > 
> > Makes sense, thanks for making clearing up what the MFD part of the
> > driver should do.
> > 
> > Here is how I think it could work:
> > 
> >  * keep the usb probe routine in the MFD driver (and keep it a usb driver)
> > 
> >  * add a new cell for the usb part
> > 
> >  * pass the usb_interface via platform_data to the USB sub-driver's
> > platform_device probe routine and continue the USB setup there
> > 
> > Lee, USB folks, is this acceptable?
> 
> No, no. USB is not a function of the MFD device, it's the transport.
> Thus there should be no USB MFD-cell. No subdriver can work without it.
> 
> And the USB id belongs in the MFD-driver in the same way that an
> i2c id (address) does.
> 
> Just like an MFD device with i2c as a transport, this driver would
> function as an arbiter to a shared resource (i.e. the register space).
> The reason it seems much more USB-centric than an i2c-mfd driver is that
> that transport API is simpler and some code have also already been
> generalised (e.g. regmap), whereas we appear to have only two USB
> mfd-drivers thus far.
> 
> The viperboard is perhaps a bad example in so far that it has pushed the
> transport details down into the subdrivers (and thus into gpio, i2c and
> iio subsystems) instead of handling it one place.

Thanks for your explanation.  I take your point about the USB ID and I
did say I was guessing that the USB part should exist as a child
device.

So after your comments I decided to do a little investigation.  It
appears that this MFD driver is _just_ using the common API which all
other devices utilising USB comms are forced to use.  Is that correct?

If so, I have a question.  Is there no way to hide more of the USB
specifics inside a better, simpler API?  It looks like the drivers
which use USB are subjected to a lot (too much) of what might be
considered internals.  Or is it just that the client has to tinker
with too many dials to get anything sensible out? *shudders*

> I haven't looked at the details of the protocol for the device in
> question, but it might even be possible to use regmap here (as I
> mentioned in my comments on v1).

Obviously that would be preferred.

Octavian, did you look into that?

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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