gsm: Introduce functions to convert between FN and RFN (Reduced FN)

Implementation ported from osmo-pcu.git
e98b315d12fb009359410809f4169f9380f3d933, function bts_rfn_to_fn().

This functionality can be used by osmo-pcu, libosmo-gprs or any other
related code which needs to handle RFNs.

Change-Id: Ib71e8da976f6cc84c3a4ab17b0a8c2101492e243

include/osmocom/gsm/gsm_utils.h

@@ -188,6 +188,14 @@ char *osmo_dump_gsmtime(const struct gsm_time *tm);
 char *osmo_dump_gsmtime_buf(char *buf, size_t buf_len, const struct gsm_time *tm);
 char *osmo_dump_gsmtime_c(const void *ctx, const struct gsm_time *tm);
 
+/* Reduced Frame Number (3GPP TS 44.018 §10.5.2.38) */
+#define GSM_RFN_MODULUS 42432
+uint32_t gsm_rfn2fn(uint16_t rfn, uint32_t curr_fn);
+static inline uint16_t gsm_fn2rfn(uint32_t fn)
+{
+	return fn % GSM_RFN_MODULUS;
+}
+
 /* GSM TS 03.03 Chapter 2.6 */
 enum gprs_tlli_type {
 	TLLI_LOCAL,

src/gsm/gsm_utils.c

@@ -80,6 +80,7 @@
 #include <osmocom/gsm/gsm_utils.h>
 #include <osmocom/gsm/meas_rep.h>
 #include <osmocom/gsm/protocol/gsm_04_08.h>
+#include <osmocom/gsm/gsm0502.h>
 
 #include <stdlib.h>
 #include <stdint.h>
@@ -924,6 +925,45 @@ char *osmo_dump_gsmtime_c(const void *ctx, const struct gsm_time *tm)
 	return osmo_dump_gsmtime_buf(buf, 64, tm);
 }
 
+#define GSM_RFN_THRESHOLD (GSM_RFN_MODULUS / 2)
+uint32_t gsm_rfn2fn(uint16_t rfn, uint32_t curr_fn)
+{
+	uint32_t curr_rfn;
+	uint32_t fn_rounded;
+	const uint32_t rfn32 = rfn; /* used as 32bit for calculations */
+
+	/* Ensure that all following calculations are performed with the
+	 * relative frame number */
+	OSMO_ASSERT(rfn32 < GSM_RFN_MODULUS);
+
+	/* Compute an internal relative frame number from the full internal
+	   frame number */
+	curr_rfn = gsm_fn2rfn(curr_fn);
+
+	/* Compute a "rounded" version of the internal frame number, which
+	 * exactly fits in the RFN_MODULUS raster */
+	fn_rounded = GSM_TDMA_FN_SUB(curr_fn, curr_rfn);
+
+	/* If the delta between the internal and the external relative frame
+	 * number exceeds a certain limit, we need to assume that the incoming
+	 * request belongs to a the previous rfn period. To correct this,
+	 * we roll back the rounded frame number by one RFN_MODULUS */
+	if (GSM_TDMA_FN_DIFF(rfn32, curr_rfn) > GSM_RFN_THRESHOLD) {
+		/* Race condition between rfn and curr_fn detected:  rfn belongs
+		to the previous RFN_MODULUS cycle, wrapping... */
+		if (fn_rounded < GSM_RFN_MODULUS) {
+			/* Corner case detected: wrapping crosses GSM_MAX_FN border */
+			fn_rounded = GSM_TDMA_FN_SUB(GSM_MAX_FN, (GSM_TDMA_FN_SUB(GSM_RFN_MODULUS, fn_rounded)));
+		} else {
+			fn_rounded = GSM_TDMA_FN_SUB(fn_rounded, GSM_RFN_MODULUS);
+		}
+	}
+
+	/* The real frame number is the sum of the rounded frame number and the
+	 * relative framenumber computed via RACH */
+	return GSM_TDMA_FN_SUM(fn_rounded, rfn32);
+}
+
 /*! append range1024 encoded data to bit vector
  *  \param[out] bv Caller-provided output bit-vector
  *  \param[in] r Input Range1024 sructure */

src/gsm/libosmogsm.map

@@ -544,6 +544,7 @@ gsm_gsmtime2fn;
 osmo_dump_gsmtime;
 osmo_dump_gsmtime_buf;
 osmo_dump_gsmtime_c;
+gsm_rfn2fn;
 
 gsm_milenage;
 gsm_septet_encode;

tests/gsm0408/gsm0408_test.c

@@ -1946,6 +1946,31 @@ static void test_gsm_gsmtime2fn(void)
 	}
 }
 
+static void test_gsm_rfn2fn(void)
+{
+	unsigned int i;
+	struct {
+		uint32_t curr_fn;
+		uint16_t rfn;
+		uint32_t exp_fn;
+	} input[] = {
+		{ .curr_fn = 0,		.rfn = 0,	.exp_fn = 0 },
+		{ .curr_fn = 0,		.rfn = 4,	.exp_fn = 4 },
+		{ .curr_fn = 2229729,	.rfn = 23322,	.exp_fn = 2229786 },
+		{ .curr_fn = 2229777,	.rfn = 23322,	.exp_fn = 2229786 },
+		{ .curr_fn = 1320458,	.rfn = 5070,	.exp_fn = 1320462 },
+	};
+
+	for (i = 0; i < ARRAY_SIZE(input); i++) {
+		uint32_t fn = gsm_rfn2fn(input[i].rfn, input[i].curr_fn);
+		if (fn != input[i].exp_fn) {
+			printf("Wrong frame number computed! curr_fn=%u, rfn=%u ==> fn=%u, expected fn=%u\n",
+			       input[i].curr_fn, input[i].rfn, fn, input[i].exp_fn);
+			OSMO_ASSERT(false);
+		}
+	}
+}
+
 int main(int argc, char **argv)
 {
 	test_bearer_cap();
@@ -1967,6 +1992,7 @@ int main(int argc, char **argv)
 	test_power_ctrl();
 	test_rach_tx_integer_raw2val();
 	test_gsm_gsmtime2fn();
+	test_gsm_rfn2fn();
 
 	return EXIT_SUCCESS;
 }