linux-kernel - Re: [PATCH RFC v2] media: v4l2-ctrl: Add gain controls

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Message-Id: <201212110956.43081.hverkuil@xs4all.nl>
Date:	Tue, 11 Dec 2012 09:56:42 +0100
From:	Hans Verkuil <hverkuil@...all.nl>
To:	Sakari Ailus <sakari.ailus@....fi>
Cc:	Prabhakar Lad <prabhakar.csengg@...il.com>,
	LMML <linux-media@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	LDOC <linux-doc@...r.kernel.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Hans de Goede <hdegoede@...hat.com>,
	Chris MacGregor <chris@...ermato.com>,
	Rob Landley <rob@...dley.net>,
	Jeongtae Park <jtp.park@...sung.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>
Subject: Re: [PATCH RFC v2] media: v4l2-ctrl: Add gain controls

On Thu 6 December 2012 10:54:32 Sakari Ailus wrote:
> Hi Prabhakar and Hans,
> 
> On Thu, Dec 06, 2012 at 10:24:18AM +0530, Prabhakar Lad wrote:
> > Hi Hans,
> > 
> > On Wed, Dec 5, 2012 at 5:38 PM, Hans Verkuil <hverkuil@...all.nl> wrote:
> > > (resend without HTML formatting)
> > >
> > > On Wed 5 December 2012 12:49:29 Prabhakar Lad wrote:
> > >> From: Lad, Prabhakar <prabhakar.csengg@...il.com>
> > >>
> > >> add support for per color component digital/analog gain controls
> > >> and also their corresponding offset.
> > >
> > > Some obvious questions below...
> > >
> > >>
> > >> Signed-off-by: Lad, Prabhakar <prabhakar.csengg@...il.com>
> > >> Cc: Sakari Ailus <sakari.ailus@....fi>
> > >> Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> > >> Cc: Kyungmin Park <kyungmin.park@...sung.com>
> > >> Cc: Guennadi Liakhovetski <g.liakhovetski@....de>
> > >> Cc: Sylwester Nawrocki <s.nawrocki@...sung.com>
> > >> Cc: Hans Verkuil <hans.verkuil@...co.com>
> > >> Cc: Hans de Goede <hdegoede@...hat.com>
> > >> Cc: Chris MacGregor <chris@...ermato.com>
> > >> Cc: Rob Landley <rob@...dley.net>
> > >> Cc: Jeongtae Park <jtp.park@...sung.com>
> > >> Cc: Mauro Carvalho Chehab <mchehab@...radead.org>
> > >> ---
> > >>  Changes for v2:
> > >>  1: Fixed review comments pointed by Laurent.
> > >>  2: Rebased on latest tree.
> > >>
> > >>  Documentation/DocBook/media/v4l/controls.xml |   54 ++++++++++++++++++++++++++
> > >>  drivers/media/v4l2-core/v4l2-ctrls.c         |   11 +++++
> > >>  include/uapi/linux/v4l2-controls.h           |   11 +++++
> > >>  3 files changed, 76 insertions(+), 0 deletions(-)
> > >>
> > >> diff --git a/Documentation/DocBook/media/v4l/controls.xml b/Documentation/DocBook/media/v4l/controls.xml
> > >> index 7fe5be1..847a9bb 100644
> > >> --- a/Documentation/DocBook/media/v4l/controls.xml
> > >> +++ b/Documentation/DocBook/media/v4l/controls.xml
> > >> @@ -4543,6 +4543,60 @@ interface and may change in the future.</para>
> > >>           specific test patterns can be used to test if a device is working
> > >>           properly.</entry>
> > >>         </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GAIN_RED</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GAIN_GREEN_RED</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GAIN_GREEN_BLUE</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GAIN_BLUE</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GAIN_GREEN</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="descr"> Some capture/sensor devices have
> > >> +         the capability to set per color component digital/analog gain values.</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GAIN_OFFSET</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_BLUE_OFFSET</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_RED_OFFSET</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GREEN_OFFSET</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GREEN_RED_OFFSET</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="id"><constant>V4L2_CID_GREEN_BLUE_OFFSET</constant></entry>
> > >> +         <entry>integer</entry>
> > >> +       </row>
> > >> +       <row>
> > >> +         <entry spanname="descr"> Some capture/sensor devices have the
> > >> +         capability to set per color component digital/analog gain offset values.
> > >> +         V4L2_CID_GAIN_OFFSET is the global gain offset and the rest are per
> > >> +         color component gain offsets.</entry>
> > >
> > > If I set both V4L2_CID_GAIN_RED and V4L2_CID_RED_OFFSET, how are they supposed
> > > to interact? Or are they mutually exclusive?
> > >
> > > And if I set both V4L2_CID_GAIN_OFFSET and V4L2_CID_RED_OFFSET, how are they supposed
> > > to interact?
> > >
> > > This questions should be answered in the documentation...
> > >
> > I haven’t worked on the hardware which supports both, What is the general
> > behaviour when the hardware supports both per color component and global
> > and both of them are set ? That could be helpful for me to document.
> 
> I'd guess most of the time only either one is supported,

Are you talking about GAIN_RED vs GAIN_RED_OFFSET or GAIN_OFFSET vs RED_OFFSET?
Or both?

> and when someone
> thinks of supporting both on the same device, we can start thinking of the
> interaction of per-component and global ones. That may be hardware specific
> as well, so standardising it might not be possible.
> 
> I think it'd be far more important to know which unit is it. Many such
> controls are indeed fixed point values but the location of the point varies.
> For unstance, u16,u16 and u8,u8 aren't uncommon. We currently have no way to
> tell this to the user space. This isn't in any way specific to gain or
> offset controls, though.

There are no standardized units for gain at the moment, and I don't really see
that happening any time soon. Fixed point isn't supported at all as a control
type, so that will have to be converted to an integer anyway.

Prabhakar, which of these controls are actually supported by your hardware?

Regards,

	Hans
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