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Subject: Re: devicetree: media: Documentation of Bt.656 Bus DT bindings
To:     Ken Sloat <ken.sloat@ohmlinxelectronics.com>
Cc:     linux-media@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, yong.deng@magewell.com,
        mchehab@kernel.org, robh+dt@kernel.org, mark.rutland@arm.com,
        maxime.ripard@bootlin.com, wens@csie.org,
        kieran.bingham@ideasonboard.com, laurent.pinchart@ideasonboard.com,
        jean-michel.hautbois@vodalys.com,
        Nate Drude <nate.drude@ohmlinxelectronics.com>,
        Sakari Ailus <sakari.ailus@linux.intel.com>
References: <CAPo_4QDW0r22ZTqtS_NDFWB3NFLBx9YEGgWKb-P9A3t_TBAFMQ@mail.gmail.com>
From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <c4d68627-b26a-6402-daf4-5cd103ec9fd0@xs4all.nl>
Date:   Mon, 28 Jan 2019 14:11:54 +0100
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+Sakari

On 1/24/19 3:53 AM, Ken Sloat wrote:
> There are a number of v4l2 subdevices in the kernel that support a
> Bt.656 bus also known as "embedded sync." Previously in older versions
> of the kernel (and in the current 4.14 LTS kernel), the standard way
> to enable this in device tree on a parallel bus was to simply omit all
> hysync and vsync flags.
> 
> During some other kernel development I was doing, it was brought to my
> attention that there is now a standard defined binding in
> "video-interfaces.txt" called "bus-type" that should be used in order
> to enable Bt.656 mode. While omitting the flags still appears to work
> because of other assumptions made in v4l2-fwnode driver, this method
> is now outdated and improper.
> 
> However, I have noticed that several dt binding docs have not been
> updated to reflect this change and still reference the old method:
> 
> Documentation/devicetree/bindings/media/sun6i-csi.txt
> /* If hsync-active/vsync-active are missing,
>    embedded BT.656 sync is used */
> 
> Documentation/devicetree/bindings/media/i2c/tvp5150.txt
> "If none of hsync-active, vsync-active and field-even-active is specified,
> the endpoint is assumed to use embedded BT.656 synchronization."
> 
> Documentation/devicetree/bindings/media/i2c/adv7604.txt
> "If none of hsync-active, vsync-active and pclk-sample is specified the
>   endpoint will use embedded BT.656 synchronization."
> 
> and amazingly even
> Documentation/devicetree/bindings/media/video-interfaces.txt in one of
> the code snippets
> /* If hsync-active/vsync-active are missing,
>    embedded BT.656 sync is used */
> 
> In order to avoid future confusion in the matter and ensure that the
> proper bindings are used, I am proposing submitting patches to update
> these docs to at minimum remove these statements and maybe even adding
> additional comments specifying the optional property and value for
> Bt.656 where missing. I wanted to open a discussion here first before
> doing this though. Thoughts?
> 
> Thanks,
> Ken Sloat
> 

I certainly agree that this should be updated to make it all consistent.

Regards,

	Hans