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Subject: Re: [PATCH v6 1/3] dt-bindings: display: bridge: Document
 THC63LVD1024 LVDS decoder
To:     jacopo mondi <jacopo@jmondi.org>
References: <4060923.7DxT9ae38L@avalon>
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CC:     Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>,
        Andrzej Hajda <a.hajda@samsung.com>,
        Jacopo Mondi <jacopo+renesas@jmondi.org>,
        Rob Herring <robh@kernel.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        <architt@codeaurora.org>, <airlied@linux.ie>, <horms@verge.net.au>,
        <magnus.damm@gmail.com>, <geert@linux-m68k.org>,
        <niklas.soderlund@ragnatech.se>, <mark.rutland@arm.com>,
        <dri-devel@lists.freedesktop.org>,
        <linux-renesas-soc@vger.kernel.org>, <devicetree@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
From:   Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Message-ID: <6353eb46-5a2d-7b75-a60b-c31c59c8636a@mentor.com>
Date:   Tue, 27 Mar 2018 14:03:25 +0300
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Hi Jacopo,

On 03/27/2018 01:10 PM, jacopo mondi wrote:
> Hi Vladimir,
> 
> On Tue, Mar 27, 2018 at 12:37:31PM +0300, Vladimir Zapolskiy wrote:
>> Hi Jacopo,
>>
>> On 03/27/2018 11:57 AM, jacopo mondi wrote:
>>> Hi Vladimir,
>>>
>>> On Tue, Mar 27, 2018 at 11:30:29AM +0300, Vladimir Zapolskiy wrote:
>>>> Hi Sergei,
>>>>
>>>> On 03/27/2018 11:27 AM, Sergei Shtylyov wrote:
>>>>> Hello!
>>>>>
>>>>> On 3/27/2018 10:33 AM, jacopo mondi wrote:
>>>>> [...]
>>>>>>>>>>> Document Thine THC63LVD1024 LVDS decoder device tree bindings.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
>>>>>>>>>>> Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
>>>>>>>>>>> Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
>>>>>>>>>>> ---
>>>>>>>>>>>   .../bindings/display/bridge/thine,thc63lvd1024.txt | 66 +++++++++++++++++++
>>>>>>>>>>>   1 file changed, 66 insertions(+)
>>>>>>>>>>>   create mode 100644
>>>>>>>>>>> Documentation/devicetree/bindings/display/bridge/thine,thc63lvd1024.txt
>>>>>>>>>>>
>>>>>>>>>>> diff --git
>>>>>>>>>>> a/Documentation/devicetree/bindings/display/bridge/thine,thc63lvd1024.txt
>>>>>>>>>>> b/Documentation/devicetree/bindings/display/bridge/thine,thc63lvd1024.txt
>>>>>>>>>>> new file mode 100644
>>>>>>>>>>> index 0000000..8225c6a
>>>>>>>>>>> --- /dev/null
>>>>>>>>>>> +++
>>>>>>>>>>> b/Documentation/devicetree/bindings/display/bridge/thine,thc63lvd1024.txt
>>>>>>>>>>> @@ -0,0 +1,66 @@
>>>>>>>>>>> +Thine Electronics THC63LVD1024 LVDS decoder
>>>>>>>>>>> +-------------------------------------------
>>>>>>>>>>> +
>>>>>>>>>>> +The THC63LVD1024 is a dual link LVDS receiver designed to convert LVDS
>>>>>>>>>>> streams
>>>>>>>>>>> +to parallel data outputs. The chip supports single/dual input/output modes,
>>>>>>>>>>> +handling up to two two input LVDS stream and up to two digital CMOS/TTL
>>>>>>>>>>> outputs.
>>>>>>>>>>> +
>>>>>>>>>>> +Single or dual operation modes, output data mapping and DDR output modes
>>>>>>>>>>> are
>>>>>>>>>>> +configured through input signals and the chip does not expose any control
>>>>>>>>>>> bus.
>>>>>>>>>>> +
>>>>>>>>>>> +Required properties:
>>>>>>>>>>> +- compatible: Shall be "thine,thc63lvd1024"
>>>>>>>>>>> +
>>>>>>>>>>> +Optional properties:
>>>>>>>>>>> +- vcc-supply: Power supply for TTL output and digital circuitry
>>>>>>>>>>> +- cvcc-supply: Power supply for TTL CLOCKOUT signal
>>>>>>>>>>> +- lvcc-supply: Power supply for LVDS inputs
>>>>>>>>>>> +- pvcc-supply: Power supply for PLL circuitry
>>>>>>>>>> As explained in a comment to one of the previous versions of this series, I'm
>>>>>>>>>> tempted to make vcc-supply mandatory and drop the three other power supplies
>>>>>>>>>> for now, as I believe there's very little chance they will be connected to
>>>>>>>>>> separately controllable regulators (all supplies use the same voltage). In the
>>>>>>>>>> very unlikely event that this occurs in design we need to support in the
>>>>>>>>>> future, the cvcc, lvcc and pvcc supplies can be added later as optional
>>>>>>>>>> without breaking backward compatibility.
>>>>>>>>> I'm okay with that.
>>>>>>>>>
>>>>>>>>>> Apart from that,
>>>>>>>>>>
>>>>>>>>>> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>>>>>>>>>>
>>>>>>>>>>> +- pdwn-gpios: Power down GPIO signal. Active low
>>>>>>>>> powerdown-gpios is the semi-standard name.
>>>>>>>>>
>>>>>>>> right, I've also noticed it. If possible please avoid shortenings in
>>>>>>>> property names.
>>>>>>>
>>>>>>> It is not shortening, it just follow pin name from decoder's datasheet.
>>>>>>>
>>>>>>>>
>>>>>>>>>>> +- oe-gpios: Output enable GPIO signal. Active high
>>>>>>>>>>> +
>>>>>>>> And this one is also a not ever met property name, please consider to
>>>>>>>> rename it to 'enable-gpios', for instance display panels define it.
>>>>>>>
>>>>>>>
>>>>>>> Again, it follows datasheet naming scheme. Has something changed in DT
>>>>>>> conventions?
>>>>>>
>>>>>> Seconded. My understanding is that the property name should reflect
>>>>>> what reported in the the chip manual. For THC63LVD1024 the enable and
>>>>>> power down pins are named 'OE' and 'PDWN' respectively.
>>>>>
>>>>>     But don't we need the vendor prefix in the prop names then, like
>>>>> "renesas,oe-gpios" then?
>>>>>
>>>>
>>>> Seconded, with a correction to "thine,oe-gpios".
>>>>
>>>
>>> mmm, okay then...
>>>
>>> A grep for that semi-standard properties names in Documentation/
>>> returns only usage examples and no actual definitions, so I assume this
>>> is why they are semi-standard.
>>
>> Here we have to be specific about a particular property, let it be 'oe-gpios'
>> vs. 'enable-gpios' and let's collect some statistics:
>>
>> % grep -Hr oe-gpios Documentation/devicetree/bindings/* | wc -l
>> 0
>>
>> $ grep -Hr enable-gpios Documentation/devicetree/bindings/* | wc -l
>> 86
>>
>> While 'thine,oe-gpios' would be correct, I see no reason to introduce a vendor
>> specific property to define a pin with a common and well understood purpose.
>>
>> If you go forward with the vendor specific prefix, apparently you can set the name
>> to 'thine,oe-gpio' (single) or even to 'thine,oe', or does the datasheet names
>> the pin as "OE GPIO" or "OE connected to a GPIO"? I guess no.
>>
> 
> Let me clarify I don't want to push for a vendor specific name or
> similar, I'm fine with using 'semi-standard' names, I'm just confused
> by the 'semi-standard' definition. I guess from your examples, the
> usage count makes a difference here.

yes, in gneneral you can read "semi-standard" as "widely used", thus collecting
statistics is a good enough method to make a reasoning.

Hopefully the next evolutionary step of "widely used" is "described in standard".

>> Standards do not define '-gpios' suffix, but partially the description is found
>> in Documentation/bindings/gpio/gpio.txt, still it is not a section in any
>> standard as far as I know.
> 
>>
>>> Seems like there is some tribal knowledge involved in defining what
>>> is semi-standard and what's not, or are those properties documented somewhere?
>>>
>>
>> The point is that there is no formal standard which describes every IP,
>> every IC and every single their property, some device node names and property
>> names are recommended in ePAPR and Devicetree Specification though.
>>
>> Think of a confusion if 'rst-gpios' (have you seen any ICs with an RST pin?) and
>> 'reset-gpios' are different. Same applies to 'pdwn-gpios' vs. 'powerdown-gpios'.
> 
> I see all your points and I agree with most of them. Anyway, if the
> chip manual describes a pin as 'RST' I would not find it confusing to
> have a 'rst-gpio' defined in bindings :)
> 
> Let me be a bit pesky here: what if a chip defines a reset GPIO, which
> is definitely a reset, but names it, say "XYZ" ? Would you prefer to
> see it defined as "reset-gpios" for consistency with other bindings,
> or "xyz-gpios" for consistency with documentation?

If a pin is definitely an IC reset as you said, then my preference is to see
it described under 'reset-gpios' property name, plus a comment in the IC
device tree documentation document about it. I can provide two reasons to
advocate my position:

1) developers spend significantly more time reading and editing the actual
   DTSI/DTS board files rather than reading and editing documentation,
   it makes sense to use common property names to save time and reduce
   amount of "what does 'oe' stand for?" type of questions; I suppose
   that the recommendation to avoid not "widely used" abbreviations in
   device node and property names arises from the same reasoning,

2) "widely used" and "standard" properties are excellent candidates for
   developing (or re-using) generalization wrappers, it happened so many
   times in the past, and this process shall be supported in my opinion;
   due to compatibility restrictions it might be problematic to change
   property names, and every new exception to "widely used" properties
   makes problematic to develop and maintain these kinds of wrappers, and
   of course it postpones a desired "described in standard" recognition.

If my point of view is accepted, I do admit that a developer who
translates a board schematics to board DTS file may experience a minor
discomfort, which is mitigated if relevant pin names are found in device
tree binding documentation in comments to properties, still the overall
gain is noticeably higher in my personal opinion.

--
With best wishes,
Vladimir