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Date:   Fri, 12 Oct 2018 15:25:18 +0100
From:   Lee Jones <lee.jones@linaro.org>
To:     Vladimir Zapolskiy <vz@mleia.com>
Cc:     Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>,
        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
Subject: Re: [PATCH 4/7] mfd: ds90ux9xx: add TI DS90Ux9xx de-/serializer MFD
 driver
Message-ID: <20181012142518.GB4939@dell>
References: <20181008211205.2900-1-vz@mleia.com>
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 <63733d2e-f95e-8894-f2b0-0b551b5cfeeb@mentor.com>
 <20181012083924.GW4939@dell>
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On Fri, 12 Oct 2018, Vladimir Zapolskiy wrote:

> 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?

Since this device seems to be truly multi-function, I have no qualms
with it being represented by a parent MFD driver.  Providing any
useful functionality (code that actually does stuff) is removed and
placed in a more suitable location, it sounds like a reasonably good
fit for MFD.

> > 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?

Sounds about right.

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

Let's see what Wolfram has to say WRT Laurent's suggestion.

> > 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.

What you're using it for now is out-of-scope of an MFD driver.  Which
if the functions are called from more than 1 call-site?  Those have a
chance of residing - we'll discuss those on an ad-hoc basis.  Anything
else needs relocating to the relevant subsystem.

We should also speak to Mark Brown about the indirect Regmap stuff.
Perhaps this would better suit a header file.

> >>> 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?

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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