More questions and patches for 835GM/ns2501 DVO

[Thomas Richter <thor@math.tu-berlin.de>]

Hi Daniel, dear intel experts,

again trying to get the old Fujitsu laptop to work. The problem with the 
latest drm-nightly built is that
the system again locks up - if the bios is configured to show an image 
only on the internal display and
nothing on the external VGA. If the bios is configured to "shared 
video", booting works fine.

This seems to be related to the problem that, apparently, the bios seems 
to prefer to connect the internal
display to pipe B and not pipe A, and hence during bootstrap just 
configuring the dpll for pipe A is not enough.

That being said, the following modifications for ns2501 will fix this:

struct ns2501_priv need to get a new field int pll_b. In enable_dvo(), 
the setting for the dpll_b needs to be
saved, too, and installed, too:
     I915_WRITE(_DPLL_A, 0xd0820000);
     I915_WRITE(_DPLL_B, 0xd0820000);

It is absolutely unnecessary to overwrite the DVOC register, this is 
configured fine. In "restore_dvo()", the dpll_b
configuration needs to be restored as well. The DVOC register need not 
to be touched. In fact, the current
enable_dvo() has a bug in so far as it saves the wrong register.

However, what is more stunning is how this bug is triggered. Actually, 
intel_display.c computes the dpll register
value correctly (as it seems), but __intel_set_mode() (around line 9356) 
is a bit strange:

First, it disables the crtcs, then sets the mode, and the enables the 
crtcs. Unfortunately, this cannot work with
with the ns2501 since a disabled PLL will block any communication with 
with the DVO. I tried to move the "enable"
call above the intel_crtc_mode_set(), but this did not work either. I do 
not know enough about the inner workings
of intel_display.c to fix this properly, but the problem seems to be 
exactly that: An incorrectly configured DPLL
disables the communication with the DVO, hence the need for the workaround.

Last, a question: All I can get with the current intel driver is a 
"shared display" between the internal and external
display. Is there any way (through xrandr) to get two different 
configurations such that the external monitor is
using one configuration feed through pipe A, and the internal display is 
feed through pipe B with another configuration?

And finally: As the internal display is only a 6 bit display, is there a 
way to enable dithering on the 835GM to avoid the banding artifacts?

Greetings,
     Thomas