[Intel-gfx] Difference in dot clock between xserver 1.5.2 and 1.9.4 - intel driver

[Keith Packard]

On Mon, 28 Feb 2011 03:54:24 -0800 (PST), SD <sd.domrep at yahoo.com> wrote:

> (II) intel(0): using SSC reference clock of 96 MHz

That's the spread-spectrum clock reference value; it forms the basis
From which the actual dot clock is computed using a PLL. It is *not* the
actual dot clock.


> [   135.556] (II) intel(0): clock: 65.0 MHz   Image Size:  304 x 228
> mm

And there's the actual dot clock.

> And in MHz can be only dot clock, but if I use 1024x768x60Hz = ~ 47.2 MHz and we need to multiply on 3 (subpixels), it will be ~140 MHz
> I know that 96 in xserver 1.5.2 is not 140 but it is much closer to
> it, then 65.

You don't multiply by three; each channel has it's own wire in
LVDS. But, you have to add space for the retrace intervals. The standard
VESA mode for 1024x768x60 uses a 65MHz dot clock.

> Why in xserver 1.5.2 intel driver shows 96 MHz but in 1.9.4 it does
> only 65 MHz. And can it be the reason why my LCD shows picture like it
> use less then 50Hz vsync.

I'll bet both are using the same timings, but you can check for yourself
with the 'xrandr' utility:

$ xrandr --verbose
...
  1440x900 (0x45)   74.1MHz -HSync -VSync *current +preferred
        h: width  1440 start 1464 end 1480 total 1600 skew    0 clock   46.3KHz
        v: height  900 start  903 end  909 total  926           clock   50.0Hz
...

The mode marked '*current' is what the system is currently using.

You can help figure out what the problem is by running the
'intel_reg_dumper' utility on both distributions. That's available from:

git://anongit.freedesktop.org/git/xorg/app/intel-gpu-tools

Doing a 'diff' on the output of this under the two operating systems
will show every change in programming for your machine.

-- 
keith.packard at intel.com
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