CVT reduced blanking.
Luc Verhaegen
libv at skynet.be
Tue Dec 27 08:38:32 PST 2005
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.
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