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Date: Wed, 26 Sep 2012 22:23:02 +0300
From: Sakari Ailus <sakari.ailus@iki.fi>
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To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
CC: Chris MacGregor <chris@cybermato.com>,
	Prabhakar Lad <prabhakar.csengg@gmail.com>,
	Hans Verkuil <hverkuil@xs4all.nl>,
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	Prabhakar Lad <prabhakar.lad@ti.com>,
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Subject: Re: Gain controls in v4l2-ctrl framework
References: <CA+V-a8vYDFhJzKVKsv7Q_JOQzDDYRyev15jDKio0tG2CP8iCCw@mail.gmail.com> <20120926074240.GM12025@valkosipuli.retiisi.org.uk> <50631461.7080903@cybermato.com> <1703218.fEdZSF7M3x@avalon>
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Hi Laurent and Chris,

Laurent Pinchart wrote:
> On Wednesday 26 September 2012 07:42:41 Chris MacGregor wrote:
...
>> Sorry to make this more complicated, but the Aptina MT9P031, for
>> instance (datasheet at
>> http://www.aptina.com/assets/downloadDocument.do?id=865 - see page 35),
>> has Digital Gain, an Analog Multiplier, and Analog Gain (for each of R,
>> Gr, Gb, and B). For each color channel, there is one register, with the
>> bits divided up into the three gain types. Furthermore, the different
>> gain types have different units (increments).
>>
>> Currently (at least in the last version I've used), the driver hides all
>> this and provides a single gain control, and prioritizes which gain
>> types are adjusted at different user-level gain settings in accordance
>> with the datasheet recommendations (e.g. keep the analog gain between 1
>> and 4 for best noise performance, and use the multiplier for gains
>> between 4 and 8). This seems very sensible.
>
> I think it should be fine for now. If we later find out that a user space
> application really needs to control the analog and digital gains individually
> and precisely we can always split the controls then. For now I think a single
> gain control (per channel) that groups analog and digital gains should be
> enough.

Agreed. I think there's much less reason to keep them separate for 
per-colour controls (compared to global gain), so little it's probably 
not worth it.

Cheers,

-- 
Sakari Ailus
sakari.ailus@iki.fi