Date:   Fri, 12 Oct 2018 10:20:53 +0100
From:   Kieran Bingham <kieran.bingham@...asonboard.com>
To:     Lee Jones <lee.jones@...aro.org>,
        Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
        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 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
certainly has different functions which are broken out in this
implementation.

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?

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.

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?

--
Regards

Kieran


>> Laurent, can you please share your opinion?

-- 
Regards
--
Kieran

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