Re: [PATCH 1/2] OMAPDSS: DISPC: Enable predecimation

On Thu, Mar 15, 2012 at 05:18:28PM +0530, Chandrabhanu Mahapatra wrote:

In OMAP3 and OMAP4, the DISPC Scaler can downscale an image up to 4 times, and
up to 2 times in OMAP2. However, with predecimation, the image can be reduced
to 16 times by fetching only the necessary pixels in memory. Then this
predecimated image can be downscaled further by the DISPC scaler.

Now, where does that number 16 come from? IIRC the hardware can skip
basically any number of pixels/rows. I certainly didn't add any such
limit to the code in the harmattan kernel, and distinctly remember
being able to downscale the N9/N950 UI even down to 1 pixel size :)

Based on the downscaling required, a prior calculation of predecimation values
for width and height of an image is done. Since, Predecimation reduces quality
of an image higher priorty is given to DISPC Scaler for downscaling.

This code was successfully tested on OMAP2, OMAP3 and OMAP4. Horizontal and
vertical predecimation worked fine except for some synclost errors due to
undocumented errata in OMAP3 which are fixed later and skewed images were seen
on OMAP2 and OMAP3 during horizontal predecimation which will be addressed in
the future patches.

All the rotation offset calculations still look suspiciously different
to what is in the harmattan kernel. I remember that the original code
was quite broken, and I fixed a lot of things when I was implementing
pre-decimation and some rotation stuff for the N9/N950. Too bad I never
managed to push that stuff upstream...

-- 
Ville Syrjälä
syrjala@sci.fi
http://www.sci.fi/~syrjala/