Why /dev/grf3 text console pixel clock is limited to 29Mhz?

[Petri Koistinen <petri.koistinen%iki.fi@localhost>]

Hi!

I have been lately trying to find out why I can´t configure working
text console with settings imported from Picasso96Mode program.
I have finaly found the reason.

Those of you who are still interested about this problem, let me give
you some background information, if you have not already read it from
previous mails.

I have PicassoIV graphics card and would like to change the resolution
of text console because default is not suitable for my monitor.  It
works, but I would like to use resolution that fits perfectly.  I have
made those perfect settings with Picasso96Mode and PicassoModeTNG
(AmigaOS) programs.

Picasso96Mode is able to print screen mode definitions directly in
format that grfconfig tool accepts.  So it should be pretty simply use
those settings.  And so it has been, X works ok with those setting
(although it´s only 8 bit).  But console is messed up totaly.

Here is exsample of grfconfig resolution definition file contents:
1 74275558 1024 768 8 1024 1040 1168 1272 767 779 785 817 default
c 55738629 832 624 4 832 848 912 1000 623 632 635 696 default

And these setting are used with follow commandline:
$ grfconfig /dev/grf3 above_text_file

First line is for setting resolution for mode 1 that could used by X
for exsample and second line is for text console.  First letters tell
that.

Next thing is pixel clock in hertz.  And rest are some other
definitions for the screen mode.

-- IMPORTANT --
What I found was that the pixel clock could _not_ be over 29000000
hertz for text console entries with /dev/grf3 or the text console is
messed up.
-- IMPORTANT --

As you can see above exsample has pixel clock of 55738629 hertz and
using that mode with /dev/grf3 will mess up the text console.  And
after realizing this I tweaked some mode to prove this.

c 24999996 832 624 4 832 856 920 896 623 670 670 761 interlace
c 28863632 720 400 8 720 736 808 872 399 400 403 420 -hsync +vsync

These both modes work.  First one has resolution of 832x624 and second
one 720x400.  Both have pixel clock under 29Mhz.  Also noticed that
kernel default screenmode for /dev/grf3 has pixel clock of 25Mhz.

Ok, I have solved the problem, shouldn´t I be happy?  Maybe, but modes
with pixel clock under 29Mhz aren´t very nice.

So, now I would like to know why text console pixel clock is limited
to 29Mhz? And ofcource does this problem affect only on PicassoIV
or is this problem common for all /dev/grf3 supported graphicscards?
Magic numbers, here I come.  ;-)

Merry Christmas
Petri Koistinen

P.S.  I have tried to import checking from CyberVision graphicscard
drivers to Cirrus driver that user is not changing resolution with
grfconfig while X is running, but didn´t manage make it working, yet.