CVT reduced blanking.

[Luc Verhaegen]

On Tue, Dec 27, 2005 at 04:21:03PM +1100, Benjamin Herrenschmidt wrote:
> 
> All the IBM ones I have seen at work have a DVI connector that only
> carries the analog signal. I yet have to see a single CRT that uses the
> digital signals (I didn't say there is none, I'm just saying that it's
> easy to be confused as it's a DVI connector but it only really carries
> analog signaling in most cases).
You're right about the p260/p275. That's pretty nasty. I would've 
expected your employer to not be pulling that kind of tricks.

Same thing for the nec one. bleh.

This iiyama one is the real thing though. Limited to 1280x1024 at 85Hz too 
because of it (analog is limited by the bandwidth of the monitor itself).
Sadly, since i'm using nothing but unichromes currently, the dvi link is 
not in use and i need a secondary monitor (that 17"er) to do any 
testing.

When i bought that iiyama, i was convinced that dvi was the way to go, 
even for CRTs. But if other manufacturers tricked customers by just 
providing a DVI-A (or a deprecated I) connection, then i see why this 
didn't happen even on high end CRTs. 5y on, i'm amazed that this device 
now turns out to be some sort of unicorn.

> Well, I would suggest for now that you provide a way (flag or whatever)
> for the driver to inform the mode calculation stuff that it knows for
> sure it's a digitally connected display (some have ways to detect it).
> In absence of that, don't allow CVT unless it's explicitely enabled in
> the config file.
> 
> Ben.

There is no flag needed. If we scrap modes on HBlank < 25% then we can 
easily pick out those with 160clocks hblank, which are the CVT reduced 
ones.

The code that currently sits in the bugzilla does all that and 
implements Option "ReducedBlanking" <Bool> for the "Monitor" Section.

I'm going to repeat some pros/cons for limiting it:
- There are worries about possible damage on CRTs, but here those don't 
  seem to care much.
- Blanking was never really checked before.
- I'm not at all happy with a .conf option that requires a server 
  restart and user intervention. And i'm sure many will agree.
- A blacklist on the digital bit should be easy to implement, as 
  there seems to be only one entry currently.
- What about analogly connected panels? Do we really want to deny 
  reduced modes there? Or is the native mode at 60Hz CVT always 
  considered good enough? Can anyone verify that those tend to accept 
  reduced modes too, because i'd like to know.

My guess is that the conclusion will be:
- MonPtr->reducedblanking should be added (in patch already).
- Monitor Section Option "ReducedBlanking" should be added (in patch 
already).
- Use edids digital bit to set the same, with a blacklist containing at 
least: Manufacturer: IVM  Model: 1930. There will likely CRTs which have 
bad edid blocks too.

Luc Verhaegen.