Date:   Fri, 12 Oct 2018 13:58:15 +0300
From:   Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
To:     <kieran.bingham@...asonboard.com>,
        Lee Jones <lee.jones@...aro.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     Vladimir Zapolskiy <vz@...ia.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Wolfram Sang <wsa@...-dreams.de>, <devicetree@...r.kernel.org>,
        <linux-gpio@...r.kernel.org>, <linux-media@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] mfd: ds90ux9xx: add TI DS90Ux9xx de-/serializer MFD
 driver

Hi Kieran,

On 10/12/2018 12:20 PM, Kieran Bingham wrote:
> Hi Vladimir,
> 
> On 12/10/18 09:39, Lee Jones wrote:
>> On Fri, 12 Oct 2018, Vladimir Zapolskiy wrote:
>>> On 10/12/2018 09:03 AM, Lee Jones wrote:
>>>> On Tue, 09 Oct 2018, Vladimir Zapolskiy wrote:
>>>>
>>>>> From: Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
>>>>>
>>>>> The change adds I2C device driver for TI DS90Ux9xx de-/serializers,
>>>>> support of subdevice controllers is done in separate drivers, because
>>>>> not all IC functionality may be needed in particular situations, and
>>>>> this can be fine grained controlled in device tree.
>>>>>
>>>>> The development of the driver was a collaborative work, the
>>>>> contribution done by Balasubramani Vivekanandan includes:
>>>>> * original implementation of the driver based on a reference driver,
>>>>> * regmap powered interrupt controller support on serializers,
>>>>> * support of implicitly or improperly specified in device tree ICs,
>>>>> * support of device properties and attributes: backward compatible
>>>>>   mode, low frequency operation mode, spread spectrum clock generator.
>>>>>
>>>>> Contribution by Steve Longerbeam:
>>>>> * added ds90ux9xx_read_indirect() function,
>>>>> * moved number of links property and added ds90ux9xx_num_fpd_links(),
>>>>> * moved and updated ds90ux9xx_get_link_status() function to core driver,
>>>>> * added fpd_link_show device attribute.
>>>>>
>>>>> Sandeep Jain added support of pixel clock edge configuration.
>>>>>
>>>>> Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
>>>>> ---
>>>>>  drivers/mfd/Kconfig           |  14 +
>>>>>  drivers/mfd/Makefile          |   1 +
>>>>>  drivers/mfd/ds90ux9xx-core.c  | 879 ++++++++++++++++++++++++++++++++++
>>>>>  include/linux/mfd/ds90ux9xx.h |  42 ++
>>>>>  4 files changed, 936 insertions(+)
>>>>>  create mode 100644 drivers/mfd/ds90ux9xx-core.c
>>>>>  create mode 100644 include/linux/mfd/ds90ux9xx.h
>>>>>
>>>>> diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
>>>>> index 8c5dfdce4326..a969fa123f64 100644
>>>>> --- a/drivers/mfd/Kconfig
>>>>> +++ b/drivers/mfd/Kconfig
>>>>> @@ -1280,6 +1280,20 @@ config MFD_DM355EVM_MSP
>>>>>  	  boards.  MSP430 firmware manages resets and power sequencing,
>>>>>  	  inputs from buttons and the IR remote, LEDs, an RTC, and more.
>>>>>  
>>>>> +config MFD_DS90UX9XX
>>>>> +	tristate "TI DS90Ux9xx FPD-Link de-/serializer driver"
>>>>> +	depends on I2C && OF
>>>>> +	select MFD_CORE
>>>>> +	select REGMAP_I2C
>>>>> +	help
>>>>> +	  Say yes here to enable support for TI DS90UX9XX de-/serializer ICs.
>>>>> +
>>>>> +	  This driver provides basic support for setting up the de-/serializer
>>>>> +	  chips. Additional functionalities like connection handling to
>>>>> +	  remote de-/serializers, I2C bridging, pin multiplexing, GPIO
>>>>> +	  controller and so on are provided by separate drivers and should
>>>>> +	  enabled individually.
>>>>
>>>> This is not an MFD driver.
>>>
>>> Why do you think so? The representation of the ICs into device tree format
>>> of hardware description shows that this is a truly MFD driver with multiple
>>> IP subcomponents naturally mapped into MFD cells.
>>
>> This driver does too much real work ('stuff') to be an MFD driver.
>> MFD drivers should not need to care of; links, gates, modes, pixels,
>> frequencies maps or properties.  Nor should they contain elaborate
>> sysfs structures to control the aforementioned 'stuff'.
>>
>> Granted, there may be some code in there which could be appropriate
>> for an MFD driver.  However most of it needs moving out into a
>> function driver (or two).
>>
>>> Basically it is possible to replace explicit of_platform_populate() by
>>> adding a "simple-mfd" compatible, if it is desired.
>>>
>>>> After a 30 second Google of what this device actually does, perhaps
>>>> drivers/media might be a better fit?
>>>
>>> I assume it would be quite unusual to add a driver with NO media functions
>>> and controls into drivers/media.
>>
>> drivers/media may very well not be the correct place for this.  In my
>> 30 second Google, I saw that this device has a lot to do with cameras,
>> hence my media association.
>>
>> If *all* else fails, there is always drivers/misc, but this should be
>> avoided if at all possible.
> 
> The device as a whole is FPD Link for camera devices I believe, but it

I still don't understand (I could be biased though) why there is such
a strong emphasis on cameras and media stuff in the discussion.

No, "the device as a whole is FPD Link for camera devices" is a wrong
statement. On hand I have a number of boards with serializers/deserializers
from the TI DS90Ux9xx IC series and sensors are NOT connected to them.

> certainly has different functions which are broken out in this
> implementation.

No, there is absolutely nothing broken out from the presented MFD drivers,
the drivers are completely integral and basically I don't expect any.

If you are concerned about media functionality, the correspondent MFD
*cell* drivers will be added into drivers/media, drivers/gpu/drm or
whatever is to be a proper place.

> I think it might be quite awkward having the i2c components in
> drivers/i2c and the media components in drivers/media/i2c, so what about
> creating drivers/media/i2c/fpd-link (or such) as a container?

I open drivers/media/i2c/Kconfig and all entries with no exception are
under from 'if VIDEO_V4L2'. The MFD drivers do NOT require on depend on
VIDEO_V4L2 or any other multimedia frameworks, nor the MFD drivers export
any multimedia controls.

> Our GMSL implementation is also a complex camera(s) device - but does
> not yet use the MFD framework, perhaps that's something to add to my
> todo list.
> 

Okay, but the TI DS90Ux9xx is NOT a camera device, and it is NOT a multimedia
device, but it is a pure MFD device so the argument is not applicable.

> We currently keep all of the complexity within the max9286.c driver, but
> I could foresee that being split further if more devices add to the
> complexity of managing the bus. At which point we might want an
> equivalent drivers/media/i2c/gmsl/ perhaps?
> 

--
Best wishes,
Vladimir

>>> Laurent, can you please share your opinion?
> 

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