bssgp: Compute BVC bucket size and leak rate

Currently the PDCH assignment and coding scheme does not influence
the values transmitted by the FLOW-CONTROL-BVC messages.

This commit adds the computation of those values. If the leak rate is
not given explicitly, it is derived from the number of PDCH and the
allowed coding scheme. If the BVC bucket size is not given
explicitly, it is derived from the leak rate and the maximum buffer time.
The latter is taken from the 'queue lifetime' command (or 10s if this
has not been used). The MS default bucket size is set to 50% of the
BVC bucket size. The MS default rate computation assumes, that each MS
only supports up to 4 time slots for GPRS RX.

Sponsored-by: On-Waves ehf

src/gprs_bssgp_pcu.cpp

@@ -27,6 +27,16 @@
 #define BSSGP_TIMER_T1	30	/* Guards the (un)blocking procedures */
 #define BSSGP_TIMER_T2	30	/* Guards the reset procedure */
 
+/* Tuning parameters for BSSGP flow control */
+#define FC_DEFAULT_LIFE_TIME_SECS 10		/* experimental value, 10s */
+#define FC_MS_BUCKET_SIZE_BY_BMAX(bmax) ((bmax) / 2 + 500) /* experimental */
+#define FC_FALLBACK_BVC_BUCKET_SIZE 2000	/* e.g. on R = 0, value taken from PCAP files */
+#define FC_MS_MAX_RX_SLOTS 4			/* limit MS default R to 4 TS per MS */
+
+/* Constants for BSSGP flow control */
+#define FC_MAX_BUCKET_LEAK_RATE (6553500 / 8)	/* Byte/s */
+#define FC_MAX_BUCKET_SIZE 6553500		/* Octets */
+
 static struct gprs_bssgp_pcu the_pcu = { 0, };
 
 extern void *tall_pcu_ctx;
@@ -482,9 +492,71 @@ static int nsvc_signal_cb(unsigned int subsys, unsigned int signal,
 	return 0;
 }
 
+static unsigned count_pdch(const struct gprs_rlcmac_bts *bts)
+{
+	size_t trx_no, ts_no;
+	unsigned num_pdch = 0;
+
+	for (trx_no = 0; trx_no < ARRAY_SIZE(bts->trx); ++trx_no) {
+		const struct gprs_rlcmac_trx *trx = &bts->trx[trx_no];
+
+		for (ts_no = 0; ts_no < ARRAY_SIZE(trx->pdch); ++ts_no) {
+			const struct gprs_rlcmac_pdch *pdch = &trx->pdch[ts_no];
+
+			if (pdch->is_enabled())
+				num_pdch += 1;
+		}
+	}
+
+	return num_pdch;
+}
+
+static uint32_t gprs_bssgp_max_leak_rate(unsigned cs, int num_pdch)
+{
+	static const uint32_t max_lr_per_ts[4] = {
+		20 * (1000 / 20), /* CS-1: 20 byte payload per 20ms */
+		30 * (1000 / 20), /* CS-2: 30 byte payload per 20ms */
+		36 * (1000 / 20), /* CS-3: 36 byte payload per 20ms */
+		50 * (1000 / 20), /* CS-4: 50 byte payload per 20ms */
+	};
+
+	if (cs > ARRAY_SIZE(max_lr_per_ts))
+		cs = 1;
+
+	return max_lr_per_ts[cs-1] * num_pdch;
+}
+
+static uint32_t compute_bucket_size(struct gprs_rlcmac_bts *bts,
+	uint32_t leak_rate, uint32_t fallback)
+{
+	uint32_t bucket_size = 0;
+
+	if (bts->force_llc_lifetime == 0xffff)
+		bucket_size = FC_MAX_BUCKET_SIZE;
+
+	if (bucket_size == 0 && bts->force_llc_lifetime && leak_rate)
+		bucket_size = (uint64_t)leak_rate * bts->force_llc_lifetime / 100;
+
+	if (bucket_size == 0 && leak_rate)
+		bucket_size = leak_rate * FC_DEFAULT_LIFE_TIME_SECS;
+
+	if (bucket_size == 0)
+		bucket_size = fallback;
+
+	if (bucket_size > FC_MAX_BUCKET_SIZE)
+		bucket_size = FC_MAX_BUCKET_SIZE;
+
+	return bucket_size;
+}
+
 int gprs_bssgp_tx_fc_bvc(void)
 {
 	struct gprs_rlcmac_bts *bts;
+	uint32_t bucket_size; /* oct */
+	uint32_t ms_bucket_size; /* oct */
+	uint32_t leak_rate; /* oct/s */
+	uint32_t ms_leak_rate; /* oct/s */
+	int num_pdch = -1;
 
 	if (!the_pcu.bctx) {
 		LOGP(DBSSGP, LOGL_ERROR, "No bctx\n");
@@ -492,12 +564,68 @@ int gprs_bssgp_tx_fc_bvc(void)
 	}
 	bts = bts_main_data();
 
-	/* FIXME: use real values */
+	bucket_size = bts->fc_bvc_bucket_size;
+	leak_rate = bts->fc_bvc_leak_rate;
+	ms_bucket_size = bts->fc_ms_bucket_size;
+	ms_leak_rate = bts->fc_ms_leak_rate;
+
+	if (leak_rate == 0) {
+		if (num_pdch < 0)
+			num_pdch = count_pdch(bts);
+
+		leak_rate = gprs_bssgp_max_leak_rate(bts->initial_cs_dl, num_pdch);
+
+		LOGP(DBSSGP, LOGL_DEBUG,
+			"Computed BVC leak rate = %d, num_pdch = %d, cs = %d\n",
+			leak_rate, num_pdch, bts->initial_cs_dl);
+	};
+
+	if (ms_leak_rate == 0) {
+		int ms_num_pdch;
+
+		if (num_pdch < 0)
+			num_pdch = count_pdch(bts);
+
+		ms_num_pdch = num_pdch;
+		if (ms_num_pdch > FC_MS_MAX_RX_SLOTS)
+			ms_num_pdch = FC_MS_MAX_RX_SLOTS;
+
+		ms_leak_rate = gprs_bssgp_max_leak_rate(bts->initial_cs_dl,
+			ms_num_pdch);
+
+		/* TODO: To properly support multiple TRX, the per MS leak rate
+		 * should be derived from the max number of PDCH TS per TRX.
+		 */
+		LOGP(DBSSGP, LOGL_DEBUG,
+			"Computed MS default leak rate = %d, ms_num_pdch = %d, cs = %d\n",
+			ms_leak_rate, ms_num_pdch, bts->initial_cs_dl);
+	};
+
+	/* TODO: Force leak_rate to 0 on buffer bloat */
+
+	if (bucket_size == 0)
+		bucket_size = compute_bucket_size(bts, leak_rate,
+			FC_FALLBACK_BVC_BUCKET_SIZE);
+
+	if (ms_bucket_size == 0)
+		ms_bucket_size = compute_bucket_size(bts, ms_leak_rate,
+			FC_MS_BUCKET_SIZE_BY_BMAX(bucket_size));
+
+	if (leak_rate > FC_MAX_BUCKET_LEAK_RATE)
+		leak_rate = FC_MAX_BUCKET_LEAK_RATE;
+
+	if (ms_leak_rate > FC_MAX_BUCKET_LEAK_RATE)
+		ms_leak_rate = FC_MAX_BUCKET_LEAK_RATE;
+
+	/* TODO: Implement avg queue delay monitoring */
+
+	LOGP(DBSSGP, LOGL_DEBUG,
+		"Sending FLOW CONTROL BVC, Bmax = %d, R = %d, Bmax_MS = %d, R_MS = %d\n",
+		bucket_size, leak_rate, ms_bucket_size, ms_leak_rate);
+
 	return bssgp_tx_fc_bvc(the_pcu.bctx, 1,
-		bts->fc_bvc_bucket_size ? bts->fc_bvc_bucket_size : 6553500,
-		bts->fc_bvc_leak_rate   ? bts->fc_bvc_leak_rate   : 819100,
-		bts->fc_ms_bucket_size  ? bts->fc_ms_bucket_size  : 50000,
-		bts->fc_ms_leak_rate    ? bts->fc_ms_leak_rate    : 50000,
+		bucket_size, leak_rate,
+		ms_bucket_size, ms_leak_rate,
 		NULL, NULL);
 }