linux-kernel - Re: [PATCH v4 2/2] media: dt-bindings: media: st,stm32-dcmi: Add support of BT656

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Message-ID: <657634eb-690a-53a6-2ac1-de3c06a1cec4@st.com>
Date:   Wed, 21 Oct 2020 14:24:08 +0000
From:   Hugues FRUCHET <hugues.fruchet@...com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
CC:     Alexandre TORGUE <alexandre.torgue@...com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Rob Herring <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        Alain VOLMAT <alain.volmat@...com>,
        Yannick FERTRE <yannick.fertre@...com>,
        Philippe CORNU <philippe.cornu@...com>
Subject: Re: [PATCH v4 2/2] media: dt-bindings: media: st,stm32-dcmi: Add
 support of BT656

Hi Sakari,

On 10/21/20 3:00 PM, Sakari Ailus wrote:
> Hi Hugues,
> 
> On Tue, Oct 20, 2020 at 12:14:49PM +0200, Hugues Fruchet wrote:
>> Add support of BT656 parallel bus mode in DCMI.
>> This mode is enabled when hsync-active & vsync-active
>> fields are not specified.
>>
>> Signed-off-by: Hugues Fruchet <hugues.fruchet@...com>
>> ---
>>   .../devicetree/bindings/media/st,stm32-dcmi.yaml   | 30 ++++++++++++++++++++++
>>   1 file changed, 30 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml b/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml
>> index 3fe778c..1ee521a 100644
>> --- a/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml
>> +++ b/Documentation/devicetree/bindings/media/st,stm32-dcmi.yaml
>> @@ -44,6 +44,36 @@ properties:
>>         bindings defined in
>>         Documentation/devicetree/bindings/media/video-interfaces.txt.
>>   
>> +    properties:
>> +      endpoint:
>> +        type: object
>> +
>> +        properties:
>> +          bus-width: true
>> +
>> +          hsync-active:
>> +            description:
>> +              If both HSYNC and VSYNC polarities are not specified, BT656
>> +              embedded synchronization is selected.
>> +            default: 0
>> +
>> +          vsync-active:
>> +            description:
>> +              If both HSYNC and VSYNC polarities are not specified, BT656
>> +              embedded synchronization is selected.
>> +            default: 0
> 
> Should I understand this as if the polarities were not specified, BT.656
> will be used?

Yes, this is what is documented in video-interfaces.txt:
"
   Note, that if HSYNC and VSYNC polarities are not specified, embedded
   synchronization may be required, where supported.
"
and
"
				/* If hsync-active/vsync-active are missing,
				   embedded BT.656 sync is used */
				hsync-active = <0>;	/* Active low */
				vsync-active = <0>;	/* Active low */
"
and I found also this in 
Documentation/devicetree/bindings/media/renesas,vin.yaml
"
           hsync-active:
             description:
               If both HSYNC and VSYNC polarities are not specified, 
embedded
               synchronization is selected.
             default: 1

           vsync-active:
             description:
               If both HSYNC and VSYNC polarities are not specified, 
embedded
               synchronization is selected.
             default: 1
"

In the other hand I've found few occurences of "bus-type" 
(marvell,mmp2-ccic.yaml), it is why I asked you if "bus-type" is the new 
way to go versus previous way to signal BT656 (without hsync/vsync) ?
As explained previously, I prefer this last way for backward compatibility.


The bindings previously documented BT.601 (parallel) only, so
> it was somewhat ambigious to begin with. Is there a risk of interpreting
> old BT.601 bindings as BT.656?
I don't think so.

With bus-type property, I believe you could
> avoid at least that risk.
yes but as explained, I'll prefer not to amend current boards device 
tree files.

> 
> Also not specifying at least one of the default values leads to BT.656
> without bus-type. That could be addressed by removing the defaults.
> 
I'm new to yaml, I've taken that from renesas,vin.yaml. Should I just 
drop the "default: 1" lines ?

>> +
>> +          pclk-sample: true
>> +
>> +          remote-endpoint: true
>> +
>> +        required:
>> +          - remote-endpoint
>> +
>> +        additionalProperties: false
>> +
>> +    additionalProperties: false
>> +
>>   required:
>>     - compatible
>>     - reg
> 

BR,
Hugues.