Display routing can be confusing so here are the basics. The simplified route from framebuffer to monitor looks like this:
framebuffer -> crtc -> encoder -> transmitter -> connector -> monitor
The framebuffer is just a buffer vram that has an image encoded in it as an array of pixels.
The crtc reads the data out of the framebuffer and generates the video mode timing in conjunction with a PLL. The crtc also determines what part of the framebuffer is read; e.g., when multi-head is enabled, each crtc scans out of a different part of vram; in clone mode, each crtc scans out of the same part of vram.
The encoder takes the digital bitstream from the crtc and converts it to the appropriate format for the requested output.
The transmitter takes the digital representation and converts that to the appropriate analog levels for transmission across the connector to the monitor.
The connector provides the appropriate plug for the monitor to connect to.
On older asics (radeon (r1xx-r4xx), DCE 1.x (r5xx) and DCE 2.x (early R6xx/RS600/RS690/RS740)) the encoders and transmitters
tended to be one combined block that supported a single output type; e.g., a TMDS/HDMI block or an LVDS block. The DCE1.x/2.x LVTMA block was kind of an exception in that it could support both TMDS and LVDS signaling, but the encoder part and the transmitter part were not routeable. The LVTMA encoder was hardwired to the LVTMA transmitter; the TMDSA encoder was hardwired to the TMDSA transmitter, etc. Analog outputs (DACs) are also generally one block rather than being split into a separate routeable encoder and transmitter since there are fewer types of analog outputs (pretty much just TV and VGA). The digital outputs were split into separate encoders and transmitters because there are lots of different types of digital outputs required for systems (HDMI, DP, DVI, LVDS, eDP, etc.). The links (A,B,A+B) are required for things like dual link DVI or LVDS, where one link doesn’t provide enough for a particular mode. In that case, two links are used rather than one to transmit the data to the monitor. The LVTMA block on DCE3.0 was basically a UNIPHY block that also supported LVDS; DCE3.0 UNIPHY only supported TMDS/HDMI/DP. On DCE3.2, all the UNIPHY blocks supported all output types (LVDS/TMDS/HDMI/DP/etc.), so there was no need to make a distinction.
One or more transmitters are wired to a connector. Having support for six transmitters means we can support up to six digital connectors (three transmitters (UNIPHY0/1/2, two links each (A,B)). So physically your system might look like:
1. DACB + UNIPHY0 links A,B -> dual link DVI-I port
2. UNIPHY1 link B -> single link DVI-D port
3. UNIPHY1 link A -> HDMI type A port
4. UNIPHY2 link A -> single link LVDS port
5. DACB -> TV port
6. DACA -> VGA port
You need to drive those connectors with timing (crtc), and the proper digital or analog data stream encoding (DIG encoder or DAC), and
appropriate transmission levels (UNIPHY/LVTMA/DAC). So the logical path would look like:
timing -> encoder -> transmitter -> connector
So for example 2 above the path might look like:
crtc 0 -> DIG1 -> UNIPHY1 link B -> single link DVI-D port
And example 4 might look like:
crtc 0 -> DIG2 -> UNIPHY2 link A -> LVDS port
And example 6 might looks like:
crtc 1 -> DACA -> DACA -> VGA port
Until evergreen, radeon chips only supported two crtcs, so there were only two encoder blocks. With evergreen, there are six crtcs, so there are also have six digital encoder blocks since you might want to run six independent displays. In a way evergreen has gone back to being more like the earlier DCE 1.x/2.x designs in that the encoders are not individually routeable anymore; they are hardwired to a particular transmitter. E.g., on evergreen DIG0/1 are hardcoded to UNIPHY0. There are two encoders since UNIPHY links A and B can be used independently or combined (for dual link). In the combined case, you’d only use one encoder, but it would drive both links. On DCE3.x, the encoders and transmitters are separately routeable since you have more transmitters (three transmitters, six possible links) than encoders (two) and you need to be able to drive different combinations.
The number of active heads supported depends what connectors the OEMs put on their boards. Generally, most seem to be one DP and several non-DP outputs for a total of three possible independent screens, but you could in theory design a board with more two or more DP outputs and some combination of non-DP ports for between two and six independent screens. However, as you add more possible simultaneous screens, you need more memory bandwidth, so that needs to be taken into account when designing the board. A lot of current boards have two dual-link DVI ports, an HDMI port, and a DP port. That combination uses all the possible encoders/transmitters, so you’d have to give up one of those to add another DP port.
The evergreen hardware has two PLLs, six crtcs, two DACs, and six digital encoders/transmitters (which can be used for LVDS/TMDS/DP/eDP/HDMI). DP runs at a fixed clock, so you don’t need a separate programmable PLL for it. That gives you some combination of up to two non-DP outputs, and up to six DP outputs for a maximum of six possible independent screens. Dual-link DVI ports require two digital transmitters (one for each link), so a dual-link DVI port would use two of the six possible transmitters, leaving four for other digital outputs. Two dual-link DVI ports would use four digital transmitters which would leave two for other digital outputs. DP only requires 1 transmitter. So in order to use a dual-link DVI monitor on a DP port, you need an active converter since native dual-link is not possible due to the lack of a second transmitter when running in DVI pass-through mode. You can use a passive DP->DVI converter on any DP port; all of the DP ports support pass-through and can be configured for HDMI or DVI. However, you are limited to two active non-DP monitors (due to there only being two PLLs) at a time. Also the monitors being used for pass-through (passive converter) have to be single link DVI since DP only has one digital transmitter connected to it. For more than 2 non-DP monitors, or dual-link DVI, you will need an active converter.