LCM flicker
Cesar Eduardo Barros
cesarb at cesarb.net
Tue Mar 11 00:41:07 CET 2008
Werner Almesberger escreveu:
> Andy Green wrote:
>> Maybe the GTA01 batch of LCMs are
>> themselves different from the ones we currently use.
>
> I connected my GTA01Bv4 LCM to a GTA02v4, and it showed all the same
> flicker.
>
>> Maybe this is because the GTA01 issued the video at a higher
>> frame rate and the flicker is less visible.
>
> That is the case, yes. I just measured that HSYNC on GTA02 is 40.9kHz,
> while on GTA01 it's 55.4kHz (!)
With GTA01, it does flicker when using cpufreq on some frequencies.
The problem on GTA01 is that the base frequency can be either HCLK/4 or
HCLK/2, and neither gets near the ideal set of values. The kernel will
always try to stay below the specified "pixclock" parameter; but, while
that is set on mach-gta01.c to 40000, the value actually used after the
rouding (on the default core frequency) corresponds to a pixclock of 30075.
To reduce the issue, I changed the parameter on mach-gta01.c to 35000
instead of 40000 on my cpufreq tree (the frequencies which flicker the
most are the ones where the effective value is near 40000). I also made
cpufreq stop using the slower frequencies (ones which would want to use
HCLK instead of HCLK/2, which is not possible) when the LCD controller
is enabled (and, unfortunately, it currently never gets disabled; the
screen only gets blanked). Both changes combined make the effective
pixclock range 17500-35000 instead of 20000-40000. However, that's only
a workaround (the flicker is still there, only reduced, and you can
still notice when "ondemand" is giving you a slower frequency).
--
Cesar Eduardo Barros
cesarb at cesarb.net
cesar.barros at gmail.com
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