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Subject: Re: [PATCH 4/7] mfd: ds90ux9xx: add TI DS90Ux9xx de-/serializer MFD
 driver
To:     Lee Jones <lee.jones@linaro.org>,
        Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Cc:     kieran.bingham@ideasonboard.com,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Marek Vasut <marek.vasut@gmail.com>,
        Wolfram Sang <wsa@the-dreams.de>, devicetree@vger.kernel.org,
        linux-gpio@vger.kernel.org, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20181008211205.2900-1-vz@mleia.com>
 <20181008211205.2900-5-vz@mleia.com> <20181012060314.GU4939@dell>
 <63733d2e-f95e-8894-f2b0-0b551b5cfeeb@mentor.com>
 <20181012083924.GW4939@dell>
 <eef99526-9232-8cd4-9a7c-c30114d58806@ideasonboard.com>
 <506c03d7-7986-44dd-3290-92d16a8106ad@mentor.com>
 <20181012113415.GZ4939@dell>
From:   Vladimir Zapolskiy <vz@mleia.com>
Message-ID: <d7bfd5d8-2c85-1f82-a673-4e8d159a15d8@mleia.com>
Date:   Fri, 12 Oct 2018 17:13:28 +0300
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Hi Lee,

On 10/12/2018 02:34 PM, Lee Jones wrote:
> On Fri, 12 Oct 2018, Vladimir Zapolskiy wrote:
>> 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@mentor.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@mentor.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.
> 
> You keep saying that "this is an MFD device" without any obvious
> comprehension of what an MFD is.  Just saying that it is one
> over-and-over does not make it so.
> An MFD should be little more than parent to other functional devices.
> Their role is to register children which in turn conduct operations
> on the hardware in a useful way.  Some MFDs also house common functions
> to save repetition of code in each of the child devices.  They do not
> tend to offer any useful functionality (stuff) in their own right. 

This describes the presented MFD driver quite closely, if I remove
a few OF controls from ds90ux9xx-core.c:
* ti,video-map-select-*,
* ti,pixel-clock-edge,
* ti,spread-spectrum-clock-generation

Then the MFD device driver will not have any useful functionality
apart of what you've listed above, please feel free to recheck.

Should I just go ahead and do the change with the assumption that
the modified MFD driver suits MFD framework?

> As I already mentioned, you need to figure out what this device is
> and move all of the functionality into the appropriate subsystem.

By definition as I comprehend it only MFD cell device drivers should
be relocated into the correspondent subsystems, but ds90ux9xx-core
remains in drivers/mfd, no?

Probably ds90ux9xx-i2c-bridge cell driver could enter drivers/misc.

> Since an MFD isn't a real thing/device (it's a Linuxy-shim which 
> only serves to register sub-devices and (sometimes) provide a space
> for common functionality to be located), drivers/mfd is not the
> subsystem which you seek. 

Oh, that's exactly the case with DS90Ux9xx driver 'ds90ux9xx-core.c',
it's just a common place to store the shared boilerplate code
snippets for all cell device drivers and various flavours of ICs
from the series.

>>> 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