Number of MST displays for Radeon RX 4xx / RX 5xx ?

[Alex Deucher]

On Mon, Mar 18, 2019 at 1:50 PM Klaus Kusche
<klaus.kusche at computerix.info> wrote:
>
>
> On 18/03/2019 15:17, Alex Deucher wrote:
> > On Mon, Mar 18, 2019 at 6:50 AM Klaus Kusche
> > <klaus.kusche at computerix.info> wrote:
> >
> >> Hello,
> >>
> >> I was unable to find any documentation (neither on the amd side
> >> nor on the linux side) giving the maximum number of independent displays
> >> supported by the amd polaris GPU's.
> >>
> >> The old cape verde GPU supports six displays,
> >> either with cards having 6 separate DP outputs or with MST hubs.
> >>
> >> But for polaris, the maximum number of outputs I've seen on graphics cards
> >> is 5 for 570 / 580 / 590, and 4 for 560 & WX 4100, and 3 for 550 & WX [23]100.
> >> There seems to be no card with 6 outputs.
> >>
> >> https://www.x.org/wiki/RadeonFeature/ says
> >> "Max Supported Displays (Eyefinity)" "4-6" for these cards,
> >> but it does not tell which ones are only 4 or 5 and which ones are 6.
> >>
> >> How many displays can be connected (in linux) to these cards using MST hubs,
> >> especially to the smaller ones WX [234]100 ? Also 6 for all cards?
> >>
> >> Many thanks for your help!
> >
> > Polaris 10 has 6 display controllers and Polaris 11 and 12 have 5
> > display controllers.  The numbers of displays supported on an MST link
> > (and in general) is limited by the bandwidth requirements of the
> > connected displays, bandwidth capabilities of the GPU, and the number
> > of type of physical connectors on the board itself.
> >
> > Alex
>
> I'm not sure how to interpret your answer.
>
> Does "and the number of type of physical connectors on the board itself"
> mean that if the board has only 3 DP connectors,
> it will be able to drive at most 3 DP displays, not 5 or 6,
> even with MST hubs, and even if the GPU has 5 or 6 display controllers?
>

If the OEM only puts 2 HDMI ports and 1 DVI port on the board, you are
limited to 3 displays because those display interfaces only support a
single monitor per connector.  If the OEM puts a DP connector on the
board, that opens up the possibility of DP MST.  The number of display
controllers is the hard limit.  This is true of all AMD GPUs.


> Or does this mean that with MST hubs, I can connect even more displays
> than display controllers on the GPU as long as I stay within bandwidth limits?
>

No.  The number of display controllers is the hard limit.

> As an example, as far as I know the WX4100 supports 4 simultaneous DP outputs
> with 4K @ 60 each. If I connect a triple MST hub to each of these outputs,
> and a HD display to each MST hub output, this would result in 12 HD displays
> on a single card, and this configuration should stay within all bandwidth
> limits (three HD @ 60 require less bandwidth than one 4K @ 60).
>
> So what is the maximum number of displays?
> Outputs of the card, display controllers of the GPU, or even more with MST hubs
> as long as the total bandwidth and bandwidth per output are not exceeded?

The maximum number of displays is the display controllers; 5 (polaris
11/12) or 6 (polaris 10).  You cannot go higher than that.  Assuming
you have enough bandwidth and enough connectors (either physical
connectors on the board or DP MST hubs), those are the limits.

Alex