Hotplug of screens vs displays

[Jon Smirl]

I was confused about hotplugging screens vs displays. When you take a
card out of the system you are hotplugging a display, not a screen.
The number of screens on a card is fixed. So it is probably ok to go
back to screen numbers instead of opaque handles.

But this still leaves a problem we have on the radeon which has two
screens. How should these screens be numbered?

fbdev implements it this way:
two heads, DVI 0, Analog 1
one head DVI 0
one head Analog 0

The radeon supports hotplugging of monitors. fbdev doesn't currently
support monitor hotplug (I have a patch that turns it on) so it
doesn't deal with this. When monitors are hotplugged the screen
numbers will get assigned differently.

I think it would probably be best to always assigned both heads a
framebuffer device even if the monitor is not plugged in. You can tell
if there is a monitor there since the modes attribute in sysfs will be
empty if there is no monitor. Always assigning heads a device will
yield stable screen numbers.

Another variation on this is TV-OUT, the two DACs for a single screen
problem. This can be handled by adding a special mode to the mode
list. Setting the special mode will enable TV-OUT. No change to the
egl API is required.

-- 
Jon Smirl
jonsmirl at gmail.com