Messages in this thread | | | Date | Thu, 30 Aug 2012 20:54:52 +0300 | From | Sakari Ailus <> | Subject | Re: [PATCH v2] media: v4l2-ctrls: add control for dpcm predictor |
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Hi Hans and Prabhakar,
Hans Verkuil wrote: > Hi Prabhakar! > > I've got some documentation review comments below... > > On Thu August 30 2012 00:58:16 Prabhakar Lad wrote: >> From: Lad, Prabhakar <prabhakar.lad@ti.com> >> >> add V4L2_CID_DPCM_PREDICTOR control of type menu, which >> determines the dpcm predictor. The predictor can be either >> simple or advanced. >> >> Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com> >> Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com> >> Cc: Sakari Ailus <sakari.ailus@iki.fi> >> Cc: Hans Verkuil <hans.verkuil@cisco.com> >> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com> >> Cc: Mauro Carvalho Chehab <mchehab@infradead.org> >> Cc: Sylwester Nawrocki <s.nawrocki@samsung.com> >> Cc: Hans de Goede <hdegoede@redhat.com> >> Cc: Kyungmin Park <kyungmin.park@samsung.com> >> --- >> This patches has one checkpatch warning for line over >> 80 characters altough it can be avoided I have kept it >> for consistency. >> >> Changes for v2: >> 1: Added documentaion in controls.xml pointed by Sylwester. >> 2: Chnaged V4L2_DPCM_PREDICTOR_ADVANCE to V4L2_DPCM_PREDICTOR_ADVANCED >> pointed by Sakari. >> >> Documentation/DocBook/media/v4l/controls.xml | 25 ++++++++++++++++++++++++- >> drivers/media/v4l2-core/v4l2-ctrls.c | 9 +++++++++ >> include/linux/videodev2.h | 5 +++++ >> 3 files changed, 38 insertions(+), 1 deletions(-) >> >> diff --git a/Documentation/DocBook/media/v4l/controls.xml b/Documentation/DocBook/media/v4l/controls.xml >> index 93b9c68..84746d0 100644 >> --- a/Documentation/DocBook/media/v4l/controls.xml >> +++ b/Documentation/DocBook/media/v4l/controls.xml >> @@ -4267,7 +4267,30 @@ interface and may change in the future.</para> >> pixels / second. >> </entry> >> </row> >> - <row><entry></entry></row> >> + <row> >> + <entry spanname="id"><constant>V4L2_CID_DPCM_PREDICTOR</constant></entry> >> + <entry>menu</entry> >> + </row> >> + <row id="v4l2-dpcm-predictor"> >> + <entry spanname="descr"> DPCM Predictor: depicts what type of prediction >> + is used simple or advanced. > > This is not useful information. It basically just rephrases the name of the > define without actually telling me anything. > > I would expect to see here at least the following: > > - what the DPCM abbreviation stands for > - a link or bibliography reference to the relevant standard (if there is any)
There's a Wikipedia article:
<URL:http://en.wikipedia.org/wiki/Differential_pulse-code_modulation>
It's the same DPCM encoding as in many of the compressed raw bayer formats.
> - a high-level explanation of what this do and what the difference is between > simple and advanced.
That's somewhat subjective and hardware dependent. If I understand this correctly, the "advanced" should differ only quality-wise from "simple" option. Why one would use "simple" instead of "advanced" then? Perhaps mostly for testing purposes; it might be that the advanced predictor could have issues in certain cases where the simple would not. However this isn't the only piece of hardware where I see that this is configurable, and the simple one was even the default.
> If this is part of a video compression standard, then this control would probably > belong to the MPEG control class as well.
It's not --- DPCM compression is typically used on the bus between the sensor and the receiver to compress the data as the bus is often a limiting factor in the transfer rate. DPCM compression can be used to squeeze the data from 10 to 8 bits without much loss in quality or dynamic range.
Cheers,
-- Sakari Ailus sakari.ailus@iki.fi
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