drivers: ieee802154: cc13/26xx_subg: fix/document non-standard CSMA/CA

The CC13xx/CC26xx Sub-GHz driver announces a hardware CSMA/CA capability
which it provides only partially. This change documents the gap.

The change also fixes two related issues with the current CCA
implementation:

- The given default ED threshold was above the allowed threshold
  defined in the specification.

- The CCA timeout was not calculated according to the requirements
  defined in the standard.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>

drivers/ieee802154/Kconfig.cc13xx_cc26xx

@@ -71,11 +71,17 @@ config IEEE802154_CC13XX_CC26XX_SUB_GHZ_NUM_RX_BUF
 	  receive buffers.
 
 config IEEE802154_CC13XX_CC26XX_SUB_GHZ_CS_THRESHOLD
-	int "TI CC13xx / CC26xx IEEE 802.15.4g Carrier Sense Threshold in dBm"
+	int "TI CC13xx / CC26xx IEEE 802.15.4g Carrier Sense ED Threshold in dBm"
-	default -70
+	default -80 # Based on SUN FSK, 200 kHz bit rate, no FEC, see IEEE 802.15.4-2020, section 19.6.7
 	help
-	  This option sets RSSI threshold for carrier sense in the CSMA/CA
+	  This option sets the energy detection (ED) threshold for
-	  algorithm.
+	  clear channel assessment (CCA) modes 1 or 3 in the CSMA/CA
+	  algorithm. Except for the SUN O-QPSK PHY, the ED threshold
+	  shall correspond to a received signal power of at most 10 dB
+	  greater than the specified receiver sensitivity for that PHY,
+	  or in accordance with local regulations (see IEEE 802.15.4-2020,
+	  section 10.2.8). For the SUN O-QPSK PHY, the ED threshold shall
+	  comply with IEEE 802.15.4-2020, section 21.5.13.
 
 config IEEE802154_CC13XX_CC26XX_SUB_GHZ_INIT_PRIO
 	int "TI CC13xx / CC26xx IEEE 802.15.4g initialization priority"

drivers/ieee802154/ieee802154_cc13xx_cc26xx_subg.c

@@ -335,6 +335,7 @@ static enum ieee802154_hw_caps
 ieee802154_cc13xx_cc26xx_subg_get_capabilities(const struct device *dev)
 {
 	/* TODO: enable IEEE802154_HW_FILTER */
+	/* TODO: remove or actually implement IEEE802154_HW_CSMA */
 	return IEEE802154_HW_FCS | IEEE802154_HW_CSMA
 	       | IEEE802154_HW_SUB_GHZ;
 }
@@ -526,11 +527,14 @@ static int ieee802154_cc13xx_cc26xx_subg_tx(const struct device *dev,
 		if (drv_data->cmd_prop_cs.status != PROP_DONE_IDLE) {
 			LOG_DBG("Channel access failure (0x%x)",
 				drv_data->cmd_prop_cs.status);
-			/* Collision Avoidance is a WIP
+			/* TODO: This is not a compliant CSMA/CA algorithm, use soft CSMA/CA
-			 * Currently, we just wait a random amount of us in the
+			 *       instead as the SubGHz radio of this SoC has no HW CSMA/CA backoff
-			 * range [0,256) but k_busy_wait() is fairly inaccurate in
+			 *       algorithm support as required by IEEE 802.15.4, section 6.2.5.1.
-			 * practice. Future revisions may attempt to use the RAdio
+			 *       Alternatively construct compliant CSMA/CA with a combination
-			 * Timer (RAT) to measure this somewhat more precisely.
+			 *       of CMD_NOP, CMD_PROP_CS and CMD_COUNT_BRANCH commands, see
+			 *       SimpleLink SDK (rfListenBeforeTalk.c) or calculate proper backoff
+			 *       period as in the SimpleLink WiSUN stack's mac_tx.c.
+			 * Currently, we just wait a random amount of us in the range [0,256).
 			 */
 			k_busy_wait(sys_rand32_get() & 0xff);
 			continue;
@@ -863,8 +867,10 @@ static struct ieee802154_cc13xx_cc26xx_subg_data ieee802154_cc13xx_cc26xx_subg_d
 		},
 		.rssiThr = CONFIG_IEEE802154_CC13XX_CC26XX_SUB_GHZ_CS_THRESHOLD,
 		.csEndTrigger.triggerType = TRIG_REL_START,
-		/* 8 symbol periods. 802.15.4-2015 Table 11.1 */
+		/* see IEEE 802.15.4, section 11.3, table 11-1 and section 10.2.8 */
-		.csEndTime = 5000,
+		.csEndTime = RF_convertUsToRatTicks(
+			IEEE802154_PHY_A_CCA_TIME *
+				IEEE802154_PHY_SUN_FSK_863MHZ_915MHZ_SYMBOL_PERIOD_US),
 	},
 
 	.cmd_prop_tx_adv = {