Implement PD 3.0 collision avoidance

It's pretty easy to implement, it turns out!  A new event gets sent to
the protocol TX thread when the PE is starting an AMS.  Just before the
message is transmitted, the TX thread checks if that event was received,
and if it was, it waits for the Type-C Current to look like 3.0 A
(SinkTxOk) before proceeding.  Now, to the best of my knowledge, we're
actually doing Power Delivery 3.0 as correctly as we ever did Power
Delivery 2.0, which is to say, everything is right except we don't do
BIST.

lib/docs/firmware_structure.svg

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lib/include/pdb_fusb.h

     fusb_tcc_none = 0,
     fusb_tcc_default = 1,
     fusb_tcc_1_5 = 2,
-    fusb_tcc_3_0 = 3
+    fusb_sink_tx_ng = 2,
+    fusb_tcc_3_0 = 3,
+    fusb_sink_tx_ok = 3
 };
 
 

lib/src/policy_engine.c

 
     /* If the DPM wants us to, send a Get_Source_Cap message */
     if (evt & PDB_EVT_PE_GET_SOURCE_CAP) {
+        /* Tell the protocol layer we're starting an AMS */
+        chEvtSignal(cfg->prl.tx_thread, PDB_EVT_PRLTX_START_AMS);
         return PESinkGetSourceCap;
     }
 
             chPoolFree(&pdb_msg_pool, cfg->pe._message);
             cfg->pe._message = NULL;
         }
+        /* Tell the protocol layer we're starting an AMS */
+        chEvtSignal(cfg->prl.tx_thread, PDB_EVT_PRLTX_START_AMS);
         return PESinkEvalCap;
     }
 
     /* If SinkPPSPeriodicTimer ran out, send a new request */
     if (evt & PDB_EVT_PE_PPS_REQUEST) {
+        /* Tell the protocol layer we're starting an AMS */
+        chEvtSignal(cfg->prl.tx_thread, PDB_EVT_PRLTX_START_AMS);
         return PESinkSelectCap;
     }
 

lib/src/protocol_tx.c

     cfg->prl._tx_message->hdr &= ~PD_HDR_MESSAGEID;
     cfg->prl._tx_message->hdr |= (cfg->prl._tx_messageidcounter % 8) << PD_HDR_MESSAGEID_SHIFT;
 
+    /* PD 3.0 collision avoidance */
+    if ((cfg->pe.hdr_template & PD_HDR_SPECREV) == PD_SPECREV_3_0) {
+        /* If we're starting an AMS, wait for permission to transmit */
+        evt = chEvtGetAndClearEvents(PDB_EVT_PRLTX_START_AMS);
+        if (evt & PDB_EVT_PRLTX_START_AMS) {
+            while (fusb_get_typec_current(&cfg->fusb) != fusb_sink_tx_ok) {
+                chThdSleepMilliseconds(1);
+            }
+        }
+    }
+
     /* Send the message to the PHY */
     fusb_send_message(&cfg->fusb, cfg->prl._tx_message);
 

lib/src/protocol_tx.h

 #define PDB_EVT_PRLTX_I_RETRYFAIL EVENT_MASK(2)
 #define PDB_EVT_PRLTX_DISCARD EVENT_MASK(3)
 #define PDB_EVT_PRLTX_MSG_TX EVENT_MASK(4)
+#define PDB_EVT_PRLTX_START_AMS EVENT_MASK(5)
 
 
 /*