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Date:	Mon, 1 Sep 2014 19:22:39 +0300
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	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>, Johan Hovold <johan@...nel.org>,
	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, 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:
>> >> > On Mon, 01 Sep 2014, Octavian Purdila wrote:
>> >> >
>> >> >> On Mon, Sep 1, 2014 at 11:37 AM, Lee Jones <lee.jones@...aro.org> wrote:
>> >> >> > On Sat, 30 Aug 2014, Octavian Purdila wrote:
>> >> >> >
>> >> >> >> This patch implements the USB part of the Diolan USB-I2C/SPI/GPIO
>> >> >> >> Master Adapter DLN-2. Details about the device can be found here:
>> >> >> >>
>> >> >> >> https://www.diolan.com/i2c/i2c_interface.html.
>> >> >> >>
>> >> >> >> Information about the USB protocol can be found in the Programmer's
>> >> >> >> Reference Manual [1], see section 1.7.
>> >> >> >>
>> >> >> >> Because the hardware has a single transmit endpoint and a single
>> >> >> >> receive endpoint the communication between the various DLN2 drivers
>> >> >> >> and the hardware will be muxed/demuxed by this driver.
>> >> >> >>
>> >> >> >> Each DLN2 module will be identified by the handle field within the DLN2
>> >> >> >> message header. If a DLN2 module issues multiple commands in parallel
>> >> >> >> they will be identified by the echo counter field in the message header.
>> >> >> >>
>> >> >> >> The DLN2 modules can use the dln2_transfer() function to issue a
>> >> >> >> command and wait for its response. They can also register a callback
>> >> >> >> that is going to be called when a specific event id is generated by
>> >> >> >> the device (e.g. GPIO interrupts). The device uses handle 0 for
>> >> >> >> sending events.
>> >> >> >>
>> >> >> >> [1] https://www.diolan.com/downloads/dln-api-manual.pdf
>> >> >> >
>> >> >> > MFD is not a dumping ground for misfit h/w.  Almost all of this code
>> >> >> > looks like it belongs in drivers/usb.  Please move it there.
>> >> >> >
>> >> >>
>> >> >> We initially submitted this driver as a pure USB driver, with our own
>> >> >> module registration mechanism, but during the first round of reviews
>> >> >> people pointed out that a MFD driver is the better approach, and I
>> >> >> agree. I also see that there are already a couple of USB drivers
>> >> >> implemented as MFD drivers.
>> >> >
>> >> > Can you link me to your previous submission please?
>> >>
>> >> Sure, here it is:
>> >>
>> >> https://lkml.org/lkml/2014/8/20/228
>> >>
>> >> >
>> >> >> Do you see a better approach?
>> >> >
>> >> > 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?
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