1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * Copyright 2012-2013, Marco Porsch <marco.porsch@s2005.tu-chemnitz.de>
4 * Copyright 2012-2013, cozybit Inc.
5 * Copyright (C) 2021 Intel Corporation
6 * Copyright (C) 2023 Intel Corporation
7 */
8
9#include "mesh.h"
10#include "wme.h"
11
12
13/* mesh PS management */
14
15/**
16 * mps_qos_null_get - create pre-addressed QoS Null frame for mesh powersave
17 * @sta: the station to get the frame for
18 *
19 * Returns: A newly allocated SKB
20 */
21static struct sk_buff *mps_qos_null_get(struct sta_info *sta)
22{
23 struct ieee80211_sub_if_data *sdata = sta->sdata;
24 struct ieee80211_local *local = sdata->local;
25 struct ieee80211_hdr *nullfunc; /* use 4addr header */
26 struct sk_buff *skb;
27 int size = sizeof(*nullfunc);
28 __le16 fc;
29
30 skb = dev_alloc_skb(length: local->hw.extra_tx_headroom + size + 2);
31 if (!skb)
32 return NULL;
33 skb_reserve(skb, len: local->hw.extra_tx_headroom);
34
35 nullfunc = skb_put(skb, len: size);
36 fc = cpu_to_le16(IEEE80211_FTYPE_DATA | IEEE80211_STYPE_QOS_NULLFUNC);
37 ieee80211_fill_mesh_addresses(hdr: nullfunc, fc: &fc, da: sta->sta.addr,
38 sa: sdata->vif.addr);
39 nullfunc->frame_control = fc;
40 nullfunc->duration_id = 0;
41 nullfunc->seq_ctrl = 0;
42 /* no address resolution for this frame -> set addr 1 immediately */
43 memcpy(nullfunc->addr1, sta->sta.addr, ETH_ALEN);
44 skb_put_zero(skb, len: 2); /* append QoS control field */
45 ieee80211_mps_set_frame_flags(sdata, sta, hdr: nullfunc);
46
47 return skb;
48}
49
50/**
51 * mps_qos_null_tx - send a QoS Null to indicate link-specific power mode
52 * @sta: the station to send to
53 */
54static void mps_qos_null_tx(struct sta_info *sta)
55{
56 struct sk_buff *skb;
57
58 skb = mps_qos_null_get(sta);
59 if (!skb)
60 return;
61
62 mps_dbg(sta->sdata, "announcing peer-specific power mode to %pM\n",
63 sta->sta.addr);
64
65 /* don't unintentionally start a MPSP */
66 if (!test_sta_flag(sta, flag: WLAN_STA_PS_STA)) {
67 u8 *qc = ieee80211_get_qos_ctl(hdr: (void *) skb->data);
68
69 qc[0] |= IEEE80211_QOS_CTL_EOSP;
70 }
71
72 ieee80211_tx_skb(sdata: sta->sdata, skb);
73}
74
75/**
76 * ieee80211_mps_local_status_update - track status of local link-specific PMs
77 *
78 * @sdata: local mesh subif
79 *
80 * sets the non-peer power mode and triggers the driver PS (re-)configuration
81 * Return BSS_CHANGED_BEACON if a beacon update is necessary.
82 *
83 * Returns: BSS_CHANGED_BEACON if a beacon update is in order.
84 */
85u64 ieee80211_mps_local_status_update(struct ieee80211_sub_if_data *sdata)
86{
87 struct ieee80211_if_mesh *ifmsh = &sdata->u.mesh;
88 struct sta_info *sta;
89 bool peering = false;
90 int light_sleep_cnt = 0;
91 int deep_sleep_cnt = 0;
92 u64 changed = 0;
93 enum nl80211_mesh_power_mode nonpeer_pm;
94
95 rcu_read_lock();
96 list_for_each_entry_rcu(sta, &sdata->local->sta_list, list) {
97 if (sdata != sta->sdata)
98 continue;
99
100 switch (sta->mesh->plink_state) {
101 case NL80211_PLINK_OPN_SNT:
102 case NL80211_PLINK_OPN_RCVD:
103 case NL80211_PLINK_CNF_RCVD:
104 peering = true;
105 break;
106 case NL80211_PLINK_ESTAB:
107 if (sta->mesh->local_pm == NL80211_MESH_POWER_LIGHT_SLEEP)
108 light_sleep_cnt++;
109 else if (sta->mesh->local_pm == NL80211_MESH_POWER_DEEP_SLEEP)
110 deep_sleep_cnt++;
111 break;
112 default:
113 break;
114 }
115 }
116 rcu_read_unlock();
117
118 /*
119 * Set non-peer mode to active during peering/scanning/authentication
120 * (see IEEE802.11-2012 13.14.8.3). The non-peer mesh power mode is
121 * deep sleep if the local STA is in light or deep sleep towards at
122 * least one mesh peer (see 13.14.3.1). Otherwise, set it to the
123 * user-configured default value.
124 */
125 if (peering) {
126 mps_dbg(sdata, "setting non-peer PM to active for peering\n");
127 nonpeer_pm = NL80211_MESH_POWER_ACTIVE;
128 } else if (light_sleep_cnt || deep_sleep_cnt) {
129 mps_dbg(sdata, "setting non-peer PM to deep sleep\n");
130 nonpeer_pm = NL80211_MESH_POWER_DEEP_SLEEP;
131 } else {
132 mps_dbg(sdata, "setting non-peer PM to user value\n");
133 nonpeer_pm = ifmsh->mshcfg.power_mode;
134 }
135
136 /* need update if sleep counts move between 0 and non-zero */
137 if (ifmsh->nonpeer_pm != nonpeer_pm ||
138 !ifmsh->ps_peers_light_sleep != !light_sleep_cnt ||
139 !ifmsh->ps_peers_deep_sleep != !deep_sleep_cnt)
140 changed = BSS_CHANGED_BEACON;
141
142 ifmsh->nonpeer_pm = nonpeer_pm;
143 ifmsh->ps_peers_light_sleep = light_sleep_cnt;
144 ifmsh->ps_peers_deep_sleep = deep_sleep_cnt;
145
146 return changed;
147}
148
149/**
150 * ieee80211_mps_set_sta_local_pm - set local PM towards a mesh STA
151 *
152 * @sta: mesh STA
153 * @pm: the power mode to set
154 * Returns: BSS_CHANGED_BEACON if a beacon update is in order.
155 */
156u64 ieee80211_mps_set_sta_local_pm(struct sta_info *sta,
157 enum nl80211_mesh_power_mode pm)
158{
159 struct ieee80211_sub_if_data *sdata = sta->sdata;
160
161 if (sta->mesh->local_pm == pm)
162 return 0;
163
164 mps_dbg(sdata, "local STA operates in mode %d with %pM\n",
165 pm, sta->sta.addr);
166
167 sta->mesh->local_pm = pm;
168
169 /*
170 * announce peer-specific power mode transition
171 * (see IEEE802.11-2012 13.14.3.2 and 13.14.3.3)
172 */
173 if (sta->mesh->plink_state == NL80211_PLINK_ESTAB)
174 mps_qos_null_tx(sta);
175
176 return ieee80211_mps_local_status_update(sdata);
177}
178
179/**
180 * ieee80211_mps_set_frame_flags - set mesh PS flags in FC (and QoS Control)
181 *
182 * @sdata: local mesh subif
183 * @sta: mesh STA
184 * @hdr: 802.11 frame header
185 *
186 * see IEEE802.11-2012 8.2.4.1.7 and 8.2.4.5.11
187 *
188 * NOTE: sta must be given when an individually-addressed QoS frame header
189 * is handled, for group-addressed and management frames it is not used
190 */
191void ieee80211_mps_set_frame_flags(struct ieee80211_sub_if_data *sdata,
192 struct sta_info *sta,
193 struct ieee80211_hdr *hdr)
194{
195 enum nl80211_mesh_power_mode pm;
196 u8 *qc;
197
198 if (WARN_ON(is_unicast_ether_addr(hdr->addr1) &&
199 ieee80211_is_data_qos(hdr->frame_control) &&
200 !sta))
201 return;
202
203 if (is_unicast_ether_addr(addr: hdr->addr1) &&
204 ieee80211_is_data_qos(fc: hdr->frame_control) &&
205 sta->mesh->plink_state == NL80211_PLINK_ESTAB)
206 pm = sta->mesh->local_pm;
207 else
208 pm = sdata->u.mesh.nonpeer_pm;
209
210 if (pm == NL80211_MESH_POWER_ACTIVE)
211 hdr->frame_control &= cpu_to_le16(~IEEE80211_FCTL_PM);
212 else
213 hdr->frame_control |= cpu_to_le16(IEEE80211_FCTL_PM);
214
215 if (!ieee80211_is_data_qos(fc: hdr->frame_control))
216 return;
217
218 qc = ieee80211_get_qos_ctl(hdr);
219
220 if ((is_unicast_ether_addr(addr: hdr->addr1) &&
221 pm == NL80211_MESH_POWER_DEEP_SLEEP) ||
222 (is_multicast_ether_addr(addr: hdr->addr1) &&
223 sdata->u.mesh.ps_peers_deep_sleep > 0))
224 qc[1] |= (IEEE80211_QOS_CTL_MESH_PS_LEVEL >> 8);
225 else
226 qc[1] &= ~(IEEE80211_QOS_CTL_MESH_PS_LEVEL >> 8);
227}
228
229/**
230 * ieee80211_mps_sta_status_update - update buffering status of neighbor STA
231 *
232 * @sta: mesh STA
233 *
234 * called after change of peering status or non-peer/peer-specific power mode
235 */
236void ieee80211_mps_sta_status_update(struct sta_info *sta)
237{
238 enum nl80211_mesh_power_mode pm;
239 bool do_buffer;
240
241 /* For non-assoc STA, prevent buffering or frame transmission */
242 if (sta->sta_state < IEEE80211_STA_ASSOC)
243 return;
244
245 /*
246 * use peer-specific power mode if peering is established and the
247 * peer's power mode is known
248 */
249 if (sta->mesh->plink_state == NL80211_PLINK_ESTAB &&
250 sta->mesh->peer_pm != NL80211_MESH_POWER_UNKNOWN)
251 pm = sta->mesh->peer_pm;
252 else
253 pm = sta->mesh->nonpeer_pm;
254
255 do_buffer = (pm != NL80211_MESH_POWER_ACTIVE);
256
257 /* clear the MPSP flags for non-peers or active STA */
258 if (sta->mesh->plink_state != NL80211_PLINK_ESTAB) {
259 clear_sta_flag(sta, flag: WLAN_STA_MPSP_OWNER);
260 clear_sta_flag(sta, flag: WLAN_STA_MPSP_RECIPIENT);
261 } else if (!do_buffer) {
262 clear_sta_flag(sta, flag: WLAN_STA_MPSP_OWNER);
263 }
264
265 /* Don't let the same PS state be set twice */
266 if (test_sta_flag(sta, flag: WLAN_STA_PS_STA) == do_buffer)
267 return;
268
269 if (do_buffer) {
270 set_sta_flag(sta, flag: WLAN_STA_PS_STA);
271 atomic_inc(v: &sta->sdata->u.mesh.ps.num_sta_ps);
272 mps_dbg(sta->sdata, "start PS buffering frames towards %pM\n",
273 sta->sta.addr);
274 } else {
275 ieee80211_sta_ps_deliver_wakeup(sta);
276 }
277}
278
279static void mps_set_sta_peer_pm(struct sta_info *sta,
280 struct ieee80211_hdr *hdr)
281{
282 enum nl80211_mesh_power_mode pm;
283 u8 *qc = ieee80211_get_qos_ctl(hdr);
284
285 /*
286 * Test Power Management field of frame control (PW) and
287 * mesh power save level subfield of QoS control field (PSL)
288 *
289 * | PM | PSL| Mesh PM |
290 * +----+----+---------+
291 * | 0 |Rsrv| Active |
292 * | 1 | 0 | Light |
293 * | 1 | 1 | Deep |
294 */
295 if (ieee80211_has_pm(fc: hdr->frame_control)) {
296 if (qc[1] & (IEEE80211_QOS_CTL_MESH_PS_LEVEL >> 8))
297 pm = NL80211_MESH_POWER_DEEP_SLEEP;
298 else
299 pm = NL80211_MESH_POWER_LIGHT_SLEEP;
300 } else {
301 pm = NL80211_MESH_POWER_ACTIVE;
302 }
303
304 if (sta->mesh->peer_pm == pm)
305 return;
306
307 mps_dbg(sta->sdata, "STA %pM enters mode %d\n",
308 sta->sta.addr, pm);
309
310 sta->mesh->peer_pm = pm;
311
312 ieee80211_mps_sta_status_update(sta);
313}
314
315static void mps_set_sta_nonpeer_pm(struct sta_info *sta,
316 struct ieee80211_hdr *hdr)
317{
318 enum nl80211_mesh_power_mode pm;
319
320 if (ieee80211_has_pm(fc: hdr->frame_control))
321 pm = NL80211_MESH_POWER_DEEP_SLEEP;
322 else
323 pm = NL80211_MESH_POWER_ACTIVE;
324
325 if (sta->mesh->nonpeer_pm == pm)
326 return;
327
328 mps_dbg(sta->sdata, "STA %pM sets non-peer mode to %d\n",
329 sta->sta.addr, pm);
330
331 sta->mesh->nonpeer_pm = pm;
332
333 ieee80211_mps_sta_status_update(sta);
334}
335
336/**
337 * ieee80211_mps_rx_h_sta_process - frame receive handler for mesh powersave
338 *
339 * @sta: STA info that transmitted the frame
340 * @hdr: IEEE 802.11 (QoS) Header
341 */
342void ieee80211_mps_rx_h_sta_process(struct sta_info *sta,
343 struct ieee80211_hdr *hdr)
344{
345 if (is_unicast_ether_addr(addr: hdr->addr1) &&
346 ieee80211_is_data_qos(fc: hdr->frame_control)) {
347 /*
348 * individually addressed QoS Data/Null frames contain
349 * peer link-specific PS mode towards the local STA
350 */
351 mps_set_sta_peer_pm(sta, hdr);
352
353 /* check for mesh Peer Service Period trigger frames */
354 ieee80211_mpsp_trigger_process(qc: ieee80211_get_qos_ctl(hdr),
355 sta, tx: false, acked: false);
356 } else {
357 /*
358 * can only determine non-peer PS mode
359 * (see IEEE802.11-2012 8.2.4.1.7)
360 */
361 mps_set_sta_nonpeer_pm(sta, hdr);
362 }
363}
364
365
366/* mesh PS frame release */
367
368static void mpsp_trigger_send(struct sta_info *sta, bool rspi, bool eosp)
369{
370 struct ieee80211_sub_if_data *sdata = sta->sdata;
371 struct sk_buff *skb;
372 struct ieee80211_hdr *nullfunc;
373 struct ieee80211_tx_info *info;
374 u8 *qc;
375
376 skb = mps_qos_null_get(sta);
377 if (!skb)
378 return;
379
380 nullfunc = (struct ieee80211_hdr *) skb->data;
381 if (!eosp)
382 nullfunc->frame_control |=
383 cpu_to_le16(IEEE80211_FCTL_MOREDATA);
384 /*
385 * | RSPI | EOSP | MPSP triggering |
386 * +------+------+--------------------+
387 * | 0 | 0 | local STA is owner |
388 * | 0 | 1 | no MPSP (MPSP end) |
389 * | 1 | 0 | both STA are owner |
390 * | 1 | 1 | peer STA is owner | see IEEE802.11-2012 13.14.9.2
391 */
392 qc = ieee80211_get_qos_ctl(hdr: nullfunc);
393 if (rspi)
394 qc[1] |= (IEEE80211_QOS_CTL_RSPI >> 8);
395 if (eosp)
396 qc[0] |= IEEE80211_QOS_CTL_EOSP;
397
398 info = IEEE80211_SKB_CB(skb);
399
400 info->flags |= IEEE80211_TX_CTL_NO_PS_BUFFER |
401 IEEE80211_TX_CTL_REQ_TX_STATUS;
402
403 mps_dbg(sdata, "sending MPSP trigger%s%s to %pM\n",
404 rspi ? " RSPI" : "", eosp ? " EOSP" : "", sta->sta.addr);
405
406 ieee80211_tx_skb(sdata, skb);
407}
408
409/**
410 * mpsp_qos_null_append - append QoS Null frame to MPSP skb queue if needed
411 * @sta: the station to handle
412 * @frames: the frame list to append to
413 *
414 * To properly end a mesh MPSP the last transmitted frame has to set the EOSP
415 * flag in the QoS Control field. In case the current tailing frame is not a
416 * QoS Data frame, append a QoS Null to carry the flag.
417 */
418static void mpsp_qos_null_append(struct sta_info *sta,
419 struct sk_buff_head *frames)
420{
421 struct ieee80211_sub_if_data *sdata = sta->sdata;
422 struct sk_buff *new_skb, *skb = skb_peek_tail(list_: frames);
423 struct ieee80211_hdr *hdr = (struct ieee80211_hdr *) skb->data;
424 struct ieee80211_tx_info *info;
425
426 if (ieee80211_is_data_qos(fc: hdr->frame_control))
427 return;
428
429 new_skb = mps_qos_null_get(sta);
430 if (!new_skb)
431 return;
432
433 mps_dbg(sdata, "appending QoS Null in MPSP towards %pM\n",
434 sta->sta.addr);
435 /*
436 * This frame has to be transmitted last. Assign lowest priority to
437 * make sure it cannot pass other frames when releasing multiple ACs.
438 */
439 new_skb->priority = 1;
440 skb_set_queue_mapping(skb: new_skb, queue_mapping: IEEE80211_AC_BK);
441 ieee80211_set_qos_hdr(sdata, skb: new_skb);
442
443 info = IEEE80211_SKB_CB(skb: new_skb);
444 info->control.vif = &sdata->vif;
445 info->control.flags |= IEEE80211_TX_INTCFL_NEED_TXPROCESSING;
446
447 __skb_queue_tail(list: frames, newsk: new_skb);
448}
449
450/**
451 * mps_frame_deliver - transmit frames during mesh powersave
452 *
453 * @sta: STA info to transmit to
454 * @n_frames: number of frames to transmit. -1 for all
455 */
456static void mps_frame_deliver(struct sta_info *sta, int n_frames)
457{
458 struct ieee80211_local *local = sta->sdata->local;
459 int ac;
460 struct sk_buff_head frames;
461 struct sk_buff *skb;
462 bool more_data = false;
463
464 skb_queue_head_init(list: &frames);
465
466 /* collect frame(s) from buffers */
467 for (ac = 0; ac < IEEE80211_NUM_ACS; ac++) {
468 while (n_frames != 0) {
469 skb = skb_dequeue(list: &sta->tx_filtered[ac]);
470 if (!skb) {
471 skb = skb_dequeue(
472 list: &sta->ps_tx_buf[ac]);
473 if (skb)
474 local->total_ps_buffered--;
475 }
476 if (!skb)
477 break;
478 n_frames--;
479 __skb_queue_tail(list: &frames, newsk: skb);
480 }
481
482 if (!skb_queue_empty(list: &sta->tx_filtered[ac]) ||
483 !skb_queue_empty(list: &sta->ps_tx_buf[ac]))
484 more_data = true;
485 }
486
487 /* nothing to send? -> EOSP */
488 if (skb_queue_empty(list: &frames)) {
489 mpsp_trigger_send(sta, rspi: false, eosp: true);
490 return;
491 }
492
493 /* in a MPSP make sure the last skb is a QoS Data frame */
494 if (test_sta_flag(sta, flag: WLAN_STA_MPSP_OWNER))
495 mpsp_qos_null_append(sta, frames: &frames);
496
497 mps_dbg(sta->sdata, "sending %d frames to PS STA %pM\n",
498 skb_queue_len(&frames), sta->sta.addr);
499
500 /* prepare collected frames for transmission */
501 skb_queue_walk(&frames, skb) {
502 struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
503 struct ieee80211_hdr *hdr = (void *) skb->data;
504
505 /*
506 * Tell TX path to send this frame even though the
507 * STA may still remain is PS mode after this frame
508 * exchange.
509 */
510 info->flags |= IEEE80211_TX_CTL_NO_PS_BUFFER;
511
512 if (more_data || !skb_queue_is_last(list: &frames, skb))
513 hdr->frame_control |=
514 cpu_to_le16(IEEE80211_FCTL_MOREDATA);
515 else
516 hdr->frame_control &=
517 cpu_to_le16(~IEEE80211_FCTL_MOREDATA);
518
519 if (skb_queue_is_last(list: &frames, skb) &&
520 ieee80211_is_data_qos(fc: hdr->frame_control)) {
521 u8 *qoshdr = ieee80211_get_qos_ctl(hdr);
522
523 /* MPSP trigger frame ends service period */
524 *qoshdr |= IEEE80211_QOS_CTL_EOSP;
525 info->flags |= IEEE80211_TX_CTL_REQ_TX_STATUS;
526 }
527 }
528
529 ieee80211_add_pending_skbs(local, skbs: &frames);
530 sta_info_recalc_tim(sta);
531}
532
533/**
534 * ieee80211_mpsp_trigger_process - track status of mesh Peer Service Periods
535 *
536 * @qc: QoS Control field
537 * @sta: peer to start a MPSP with
538 * @tx: frame was transmitted by the local STA
539 * @acked: frame has been transmitted successfully
540 *
541 * NOTE: active mode STA may only serve as MPSP owner
542 */
543void ieee80211_mpsp_trigger_process(u8 *qc, struct sta_info *sta,
544 bool tx, bool acked)
545{
546 u8 rspi = qc[1] & (IEEE80211_QOS_CTL_RSPI >> 8);
547 u8 eosp = qc[0] & IEEE80211_QOS_CTL_EOSP;
548
549 if (tx) {
550 if (rspi && acked)
551 set_sta_flag(sta, flag: WLAN_STA_MPSP_RECIPIENT);
552
553 if (eosp)
554 clear_sta_flag(sta, flag: WLAN_STA_MPSP_OWNER);
555 else if (acked &&
556 test_sta_flag(sta, flag: WLAN_STA_PS_STA) &&
557 !test_and_set_sta_flag(sta, flag: WLAN_STA_MPSP_OWNER))
558 mps_frame_deliver(sta, n_frames: -1);
559 } else {
560 if (eosp)
561 clear_sta_flag(sta, flag: WLAN_STA_MPSP_RECIPIENT);
562 else if (sta->mesh->local_pm != NL80211_MESH_POWER_ACTIVE)
563 set_sta_flag(sta, flag: WLAN_STA_MPSP_RECIPIENT);
564
565 if (rspi && !test_and_set_sta_flag(sta, flag: WLAN_STA_MPSP_OWNER))
566 mps_frame_deliver(sta, n_frames: -1);
567 }
568}
569
570/**
571 * ieee80211_mps_frame_release - release frames buffered due to mesh power save
572 *
573 * @sta: mesh STA
574 * @elems: IEs of beacon or probe response
575 *
576 * For peers if we have individually-addressed frames buffered or the peer
577 * indicates buffered frames, send a corresponding MPSP trigger frame. Since
578 * we do not evaluate the awake window duration, QoS Nulls are used as MPSP
579 * trigger frames. If the neighbour STA is not a peer, only send single frames.
580 */
581void ieee80211_mps_frame_release(struct sta_info *sta,
582 struct ieee802_11_elems *elems)
583{
584 int ac, buffer_local = 0;
585 bool has_buffered = false;
586
587 if (sta->mesh->plink_state == NL80211_PLINK_ESTAB)
588 has_buffered = ieee80211_check_tim(tim: elems->tim, tim_len: elems->tim_len,
589 aid: sta->mesh->aid);
590
591 if (has_buffered)
592 mps_dbg(sta->sdata, "%pM indicates buffered frames\n",
593 sta->sta.addr);
594
595 /* only transmit to PS STA with announced, non-zero awake window */
596 if (test_sta_flag(sta, flag: WLAN_STA_PS_STA) &&
597 (!elems->awake_window || !get_unaligned_le16(p: elems->awake_window)))
598 return;
599
600 if (!test_sta_flag(sta, flag: WLAN_STA_MPSP_OWNER))
601 for (ac = 0; ac < IEEE80211_NUM_ACS; ac++)
602 buffer_local += skb_queue_len(list_: &sta->ps_tx_buf[ac]) +
603 skb_queue_len(list_: &sta->tx_filtered[ac]);
604
605 if (!has_buffered && !buffer_local)
606 return;
607
608 if (sta->mesh->plink_state == NL80211_PLINK_ESTAB)
609 mpsp_trigger_send(sta, rspi: has_buffered, eosp: !buffer_local);
610 else
611 mps_frame_deliver(sta, n_frames: 1);
612}
613

source code of linux/net/mac80211/mesh_ps.c