[Openchrome-devel] [Bug 91966] No signal to monitor with X and openchrome using VX855 chipset graphics

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91966

--- Comment #300 from Kevin Brace <kevinbrace at gmx.com> ---
(In reply to Christopher from comment #296)

Hi Christopher,

> Just a thought -- a monitor shows up on a VGA connector as a set of 68-ohm
> resistors on the Red Green and Blue lines to their respective grounds.
> 
> Is the stuff you're working with low-level enough that you can detect
> whether the thing is there based on that, at least for analog displays...?

Christopher, I am not an analog guy, and in fact I struggle in anything analog
(^_^;
The typical method used for detecting a VGA monitor is to use I2C bus or what
is also called VESA DDC (Display Data Channel) to obtain EDID (Extended Display
Identification Data).
EDID tells the graphics device driver about screen resolution support.
In the case of OpenChrome and other Linux graphics device drivers, the EDID
identifying the interface type as analog type is used to determine whether or
not the device driver is trying to detect a VGA interface.
OpenChrome apparently used to do this, and then one day Xavier removed it for
some reason.
However, when I tried to fix your DVI to VGA adapter problem, not looking into
the analog or digital type for the interface type became a problem (for
hardware that does not have DVI like my laptop), so after looking at other
graphics device driver source code, I put the interface type check back in.
    However, at least for some VIA hardware, they do have a hardware level VGA
interface presence detection circuit (this is what people call mixed signal IC
design) that can actual probe the presence of a VGA monitor.
The problem of relying on this is that it is not clear if every VIA past
hardware supports it (OpenChrome tries to support all the way back to CLE266
chipset which was released around 2003).
I will imagine that the existence of this feature is to support VGA hot
plugging.

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