diff options
| author | Philipp Maier <pmaier@sysmocom.de> | 2023-01-12 12:58:11 +0100 |
|---|---|---|
| committer | Philipp Maier <pmaier@sysmocom.de> | 2023-01-12 15:30:24 +0100 |
| commit | a13d5662b7c37b9219e2dd11fda40f24c0046cbf (patch) | |
| tree | e9683e8535282b9da43e25598183206ee8e789df /src | |
| parent | b6a3836ad2b7c3a0b09577ce87cda02d333c22d5 (diff) | |
gsm_utils: improve gsm_gsmtime2fn()
The function gsm_gsmtime2fn() uses a hack to account for the truncated
modulo implementation of C/C++. libosmocore offers proven modulo
functions, so lets use OSMO_MOD_FLR() instead. Also arrange the formula
so that it looks more like the one in the spec.
Also add better spec references and a final modulo GSM_MAX_FN to
prevent frame number results that exceed the valid range.
Change-Id: Ibf94bca8223f1f7858a6dd67bf27de0ab6feab20
Diffstat (limited to 'src')
| -rw-r--r-- | src/gsm/gsm_utils.c | 15 |
1 files changed, 13 insertions, 2 deletions
diff --git a/src/gsm/gsm_utils.c b/src/gsm/gsm_utils.c index 3b0ec6a8..bbc7dd64 100644 --- a/src/gsm/gsm_utils.c +++ b/src/gsm/gsm_utils.c @@ -887,8 +887,19 @@ char *gsm_fn_as_gsmtime_str(uint32_t fn) * \returns GSM Frame Number */ uint32_t gsm_gsmtime2fn(struct gsm_time *time) { - /* TS 05.02 Chapter 4.3.3 TDMA frame number */ - return (51 * ((time->t3 - time->t2 + 26) % 26) + time->t3 + (26 * 51 * time->t1)); + uint32_t fn; + + /* See also: + * 3GPP TS 44.018, section 10.5.2.38, 3GPP TS 45.002 section 4.3.3, and + * 3GPP TS 48.058, section 9.3.8 */ + fn = 51 * OSMO_MOD_FLR((time->t3-time->t2), 26) + time->t3 + 51 * 26 * time->t1; + + /* Note: Corrupted input values may cause a resulting frame number + * larger then the maximum permitted value of GSM_MAX_FN. Even though + * the caller is expected to check the input values beforehand we must + * make sure that the result cannot exceed the value range of a valid + * GSM frame number. */ + return fn % GSM_MAX_FN; } char *osmo_dump_gsmtime_buf(char *buf, size_t buf_len, const struct gsm_time *tm) |