linux-kernel - Re: [PATCH RFC v2] media: OF: add sync-on-green endpoint property

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Message-ID: <44193648.yaA827Trlv@avalon>
Date:	Thu, 30 May 2013 05:21:57 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Sylwester Nawrocki <sylvester.nawrocki@...il.com>
Cc:	Prabhakar Lad <prabhakar.csengg@...il.com>,
	LMML <linux-media@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	DLOS <davinci-linux-open-source@...ux.davincidsp.com>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Sakari Ailus <sakari.ailus@....fi>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>
Subject: Re: [PATCH RFC v2] media: OF: add sync-on-green endpoint property

Hi Sylwester,

On Saturday 25 May 2013 16:11:52 Sylwester Nawrocki wrote:
> On 05/25/2013 11:17 AM, Prabhakar Lad wrote:
> >> > From looking at Figure 8 "TVP7002 Application Example" in the TVP7002's
> >> > datasheet ([2], p. 52) and your initial TVP7002 patches it looks like
> >> > what you want is to specify polarity of the SOGOUT signal, so the
> >> > processor that receives this signal can properly interpret it, is it
> >> > correct ?
> > 
> > Yes
> > 
> >> >  If so then wouldn't it be more appropriate to define e.g. 'sog-active'
> >> >  property and media bus flags:
> >> >           V4L2_MBUS_SYNC_ON_GREEN_ACTIVE_LOW
> >> >           V4L2_MBUS_SYNC_ON_GREEN_ACTIVE_HIGH
> >> >  
> >> >  ?
> > 
> > Agreed I'll add these flags.
> > 
> >> >  And for synchronisation method on the analog part we could perhaps
> >> >  define 'component-sync' or similar property that would enumerate all
> >> >  possible synchronisation methods. We might as well use separate
> >> >  boolean properties, but I'm a bit concerned about the increasing
> >> >  number of properties that need to be parsed for each parallel video
> >> >  bus "endpoint".
> > 
> > I am not clear on it can please elaborate more on this.
> 
> I thought about two possible options:
> 
> 1. single property 'component-sync' or 'video-sync' that would have values:
> 
> #define VIDEO_SEPARATE_SYNC	0x01
> #define VIDEO_COMPOSITE_SYNC	0x02
> #define VIDEO_SYNC_ON_COMPOSITE	0x04
> #define VIDEO_SYNC_ON_GREEN	0x08
> #define VIDEO_SYNC_ON_LUMINANCE	0x10
> 
> And we could put these definitions into a separate header, e.g.
> <dt-bindings/video-interfaces.h>
> 
> Then in a device tree source file one could have, e.g.
> 
> video-sync = <VIDEO_SYNC_ON_GREEN>;
> 
> 
> 2. Separate boolean property for each video sync type, e.g.
> 
> 	"video-composite-sync"
> 	"video-sync-on-composite"
> 	"video-sync-on-green"
> 	"video-sync-on-luminance"
> 
> Separate sync, with separate VSYNC, HSYNC lines, would be the default, when
> none of the above is specified and 'vsync-active', 'hsync-active' properties
> are present.

I prefer 1. over 2.

> However, I suppose the better would be to deduce the video synchronisation
> method from the sync signal polarity flags. Then, for instance, when an
> endpoint node contains "composite-sync-active" property the parser would
> determine the "composite sync" synchronisation type is used.
> 
> Thus it might make sense to have only following integer properties (added
> as needed):
> 
> composite-sync-active
> sync-on-green-active
> sync-on-comp-active
> sync-on-luma-active
> 
> This would allow to specify polarity of each signal and at the same time
> the parsing code could derive synchronisation type. A new field could be
> added to struct v4l2_of_parallel_bus, e.g. sync_type and it would be filled
> within v4l2_of_parse_endpoint().
> 
> What do you think ?

My gut feeling is that we should have separate properties for the video sync 
type and the synchronization signals polarities. We could have a chip that 
supports sync-on-green on the analog (input) side and outputs separate hsync 
and vsync signals only on the digital (output) side. There would be no sync-
on-green polarity in that case.

-- 
Regards,

Laurent Pinchart

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