Made the optional DPM functions actually optional

Now simple Sinks are easier to implement, as not all DPM functions have
to be provided.

lib/include/pdb_dpm.h

  * PD Buddy firmware library device policy manager callback structure
  *
  * Optional functions may be set to NULL if the associated functionality is not
- * required. (TODO)
+ * required.
  */
 struct pdb_dpm_callbacks {
     pdb_dpm_eval_cap_func evaluate_capability;
     pdb_dpm_get_sink_cap_func get_sink_capability;
-    pdb_dpm_giveback_func giveback_enabled;
-    pdb_dpm_tcc_func evaluate_typec_current; /* Optional */
+    pdb_dpm_giveback_func giveback_enabled; /* Optional (missing means no GiveBack) */
+    pdb_dpm_tcc_func evaluate_typec_current; /* Optional (missing means no Type-C Current) */
     pdb_dpm_func pd_start; /* Optional */
     pdb_dpm_func transition_default;
     pdb_dpm_func transition_min; /* Optional if no GiveBack */
     pdb_dpm_func transition_standby;
     pdb_dpm_func transition_requested;
-    pdb_dpm_func transition_typec; /* Optional */
+    pdb_dpm_func transition_typec; /* Optional if no Type-C Current */
 };
 
 

lib/src/policy_engine.c

 {
     /* We don't have an explicit contract currently */
     cfg->pe._explicit_contract = false;
-    /* Tell the DPM that we've started negotiations */
-    cfg->dpm.pd_start(cfg);
+    /* Tell the DPM that we've started negotiations, if it cares */
+    if (cfg->dpm.pd_start != NULL) {
+        cfg->dpm.pd_start(cfg);
+    }
 
     /* No need to reset the protocol layer here.  There are two ways into this
      * state: startup and exiting hard reset.  On startup, the protocol layer
             /* Handle GotoMin messages */
             } else if (PD_MSGTYPE_GET(cfg->pe._message) == PD_MSGTYPE_GOTOMIN
                     && PD_NUMOBJ_GET(cfg->pe._message) == 0) {
-                if (cfg->dpm.giveback_enabled(cfg)) {
+                if (cfg->dpm.giveback_enabled != NULL
+                        && cfg->dpm.giveback_enabled(cfg)) {
                     /* Transition to the minimum current level */
                     cfg->dpm.transition_min(cfg);
                     cfg->pe._min_power = true;
  */
 static enum policy_engine_state pe_sink_source_unresponsive(struct pdb_config *cfg)
 {
-    int tcc_match = cfg->dpm.evaluate_typec_current(cfg, fusb_get_typec_current(&cfg->fusb));
+    /* If the DPM can evaluate the Type-C Current advertisement */
+    if (cfg->dpm.evaluate_typec_current != NULL) {
+        /* Make the DPM evaluate the Type-C Current advertisement */
+        int tcc_match = cfg->dpm.evaluate_typec_current(cfg,
+                fusb_get_typec_current(&cfg->fusb));
+
+        /* If the last two readings are the same, set the output */
+        if (cfg->pe._old_tcc_match == tcc_match) {
+            cfg->dpm.transition_typec(cfg);
+        }
 
-    /* If the last two readings are the same, set the output */
-    if (cfg->pe._old_tcc_match == tcc_match) {
-        cfg->dpm.transition_typec(cfg);
+        /* Remember whether or not the last measurement succeeded */
+        cfg->pe._old_tcc_match = tcc_match;
     }
 
-    /* Remember whether or not the last measurement succeeded */
-    cfg->pe._old_tcc_match = tcc_match;
-
     /* Wait tPDDebounce between measurements */
     chThdSleep(PD_T_PD_DEBOUNCE);