[Bug 81476] three monitors on two radeon cards works with some layouts not others

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=81476

--- Comment #18 from Alex Deucher <agd5f at yahoo.com> ---
(In reply to comment #17)
> 
> Phoronix suggests they don't all work well with open source drivers, those
> that do work often don't do well multi-monitor, and most don't do triple DVI
> output unless one buys one or more active adapters.

Displays should generally work fine.  Note that Evergreen, NI, and SI parts
only have two non-DP PLLs so they are limited to two independent non-DP clocks.
 So what this means is that you can only use two non-DP monitors with
independent timings.  If you want to use more than two non-DP monitors, you
have to use the exact same pixel clocks on several of the monitors so you don't
use more then two different clocks.  E.g., if all three of your monitors are
identical use the same modeline and pixel clock, you'd only need one PLL so you
should be fine.

> 
> I don't want to pick a card that becomes obsolete and where the driver
> writers have moved on and aren't interested in fixing the old bugs (e.g. the
> above bug in the radeon driver).

TBH, I think it's a hw limitation on r5xx hardware that's not properly handled.
 Unfortunately, X didn't support multi-card xrandr support when r5xx was still
actively being developed.  At that time, the only way to support multi-card
multi-head was zaphod mode which uses separate buffers for each display so you
never hit the hw limits.

> 
> Are the open source drivers for the evergreen cards (and newer) under active
> development?

Yes.

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