topology2: document HDA backend ID assignments

Document the fixed HDA backend IDs assigned by the kernel machine driver
alongside the corresponding topology IDs and variables.

HDA topologies hard-code the analog BE ID, while iDisp, DMIC and UAOL
IDs are derived or supplied through topology variables. Recording both
sides makes mismatches visible when either the machine driver or
topology definitions change.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>

Author: ujfalusi

tools/topology/topology2/README.md

@@ -276,6 +276,29 @@ default ID ranges for the same endpoint types.
 | Deep Buffer | 31 | `DEEP_BUFFER_PCM_ID` |
 | Compress HDA Analog | 50 | `COMPR_PCM_ID` |
 
+**Intel HDA Backend DAI Link IDs:**
+
+These IDs match the fixed backend DAI-link IDs defined by the kernel machine
+driver. They are independent of PCM IDs and pipeline indexes. The kernel defines
+base IDs for iDisp HDMI, HDA, and UAOL. The HDA helper increments the iDisp and
+HDA base IDs when it creates additional links; UAOL stream backends also use
+consecutive IDs.
+
+| Backend | DAI Link ID | Kernel Define | Topology Variable |
+|---|---:|---|---|
+| iDisp HDMI 1 | 1 | `IDISP_HDMI_BE_ID` | `HDMI1_ID` |
+| iDisp HDMI 2 | 2 | `IDISP_HDMI_BE_ID + 1` | `HDMI2_ID` |
+| iDisp HDMI 3 | 3 | `IDISP_HDMI_BE_ID + 2` | `HDMI3_ID` |
+| HDA Analog | 4 | `HDA_BE_ID` | Hard-coded as `id 4` |
+| HDA Digital | 5 | `HDA_BE_ID + 1` | — |
+| DMIC01 | 6 | `DMIC01_BE_ID` | `DMIC0_ID` |
+| DMIC 16 kHz | 7 | `DMIC16K_BE_ID` | `DMIC1_ID` |
+| Bluetooth Offload | 8 | `BT_OFFLOAD_BE_ID` | Not used by HDA generic |
+| UAOL Play0 | 9 | `UAOL_BE_ID` | `UAOL_PB_BE_ID` |
+| UAOL Cap0 | 10 | `UAOL_BE_ID + 1` | `UAOL_CP_BE_ID` |
+| UAOL Play1 | 11 | `UAOL_BE_ID + 2` | `UAOL_PB1_BE_ID` |
+| UAOL Cap1 | 12 | `UAOL_BE_ID + 3` | `UAOL_CP1_BE_ID` |
+
 Key rules:
 
 * PCM ID 0 is always the primary playback endpoint