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abis_rsl: fix frame number calculation 

The formula that is used to recover the (relative) frame number from the
T1, T2, T3 parameters matches the definition in the spec, but since the
partial term t3-t2 can be negative special precaution is required when
performing the MOD 26 operation.  This is due to the truncated modulo
implementation in C/C++, which has a very specific understanding on how
to deal with negative input parameters.

The libosmocore gsm_gsmtime2fn(() offers a correct implementation, so
lets use it.

Change-Id: I5fb2b0ada8d409730ac22963741fb4ab0026abdd
Related: OS#5198
This commit is contained in:
Philipp Maier 2023-01-10 18:02:58 +01:00
parent 5cd6d86e94
commit 9b45d86e5e
1 changed files with 7 additions and 7 deletions
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14
src/osmo-bsc/abis_rsl.c
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@ -1800,10 +1800,10 @@ struct chan_rqd {
/* Handle packet channel rach requests */
static int rsl_rx_pchan_rqd(struct chan_rqd *rqd)
{
uint8_t t1, t2, t3;
uint32_t fn;
uint8_t rqd_ta;
uint8_t is_11bit;
struct gsm_time gsm_time;
/* Process rach request and forward contained information to PCU */
if (rqd->ref.ra == 0x7F) {
@ -1816,13 +1816,13 @@ static int rsl_rx_pchan_rqd(struct chan_rqd *rqd)
is_11bit = 0;
rqd_ta = rqd->ta;
/* See also 3GPP TS 04.08, section 10.5.2.38 and 3GPP TS 08.58, section 9.3.8 */
t1 = rqd->ref.t1;
t2 = rqd->ref.t2;
t3 = rqd->ref.t3_low | (rqd->ref.t3_high << 3);
fn = (51 * ((t3-t2) % 26) + t3 + 51 * 26 * t1);
gsm_time.t1 = rqd->ref.t1;
gsm_time.t2 = rqd->ref.t2;
gsm_time.t3 = rqd->ref.t3_low | (rqd->ref.t3_high << 3);
fn = gsm_gsmtime2fn(&gsm_time);
LOG_BTS(rqd->bts, DRSL, LOGL_INFO, "CHAN RQD: fn(t1=%u,t3=%u,t2=%u) = %u\n", t1, t3, t2, fn);
LOG_BTS(rqd->bts, DRSL, LOGL_INFO, "CHAN RQD: fn(t1=%u,t3=%u,t2=%u) = %u\n",
gsm_time.t1, gsm_time.t3, gsm_time.t2, fn);
}
return pcu_tx_rach_ind(rqd->bts, rqd_ta, rqd->ref.ra, fn, is_11bit,