ba72329f6de1b576f64efabb77653c3528f8430b
[openwrt.git] / package / madwifi / patches / 300-napi_polling.patch
1 --- a/ath/if_ath.c
2 +++ b/ath/if_ath.c
3 @@ -184,7 +184,11 @@
4         struct sk_buff *, int, int, u_int64_t);
5  static void ath_setdefantenna(struct ath_softc *, u_int);
6  static struct ath_txq *ath_txq_setup(struct ath_softc *, int, int);
7 -static void ath_rx_tasklet(TQUEUE_ARG);
8 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
9 +static int ath_rx_poll(struct napi_struct *napi, int budget);
10 +#else
11 +static int ath_rx_poll(struct net_device *dev, int *budget);
12 +#endif
13  static int ath_hardstart(struct sk_buff *, struct net_device *);
14  static int ath_mgtstart(struct ieee80211com *, struct sk_buff *);
15  #ifdef ATH_SUPERG_COMP
16 @@ -376,6 +380,9 @@
17                 u_int32_t new_clamped_maxtxpower);
18  static u_int32_t ath_get_real_maxtxpower(struct ath_softc *sc);
19  
20 +static void ath_poll_disable(struct net_device *dev);
21 +static void ath_poll_enable(struct net_device *dev);
22 +
23  /* calibrate every 30 secs in steady state but check every second at first. */
24  static int ath_calinterval = ATH_SHORT_CALINTERVAL;
25  static int ath_countrycode = CTRY_DEFAULT;     /* country code */
26 @@ -547,7 +554,6 @@
27  
28         atomic_set(&sc->sc_txbuf_counter, 0);
29  
30 -       ATH_INIT_TQUEUE(&sc->sc_rxtq,     ath_rx_tasklet,       dev);
31         ATH_INIT_TQUEUE(&sc->sc_txtq,     ath_tx_tasklet,       dev);
32         ATH_INIT_TQUEUE(&sc->sc_bmisstq,  ath_bmiss_tasklet,    dev);
33         ATH_INIT_TQUEUE(&sc->sc_bstucktq, ath_bstuck_tasklet,   dev);
34 @@ -821,6 +827,12 @@
35         dev->set_mac_address = ath_set_mac_address;
36         dev->change_mtu = ath_change_mtu;
37         dev->tx_queue_len = ATH_TXBUF - ATH_TXBUF_MGT_RESERVED;
38 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
39 +       netif_napi_add(dev, &sc->sc_napi, ath_rx_poll, 64);
40 +#else
41 +       dev->poll = ath_rx_poll;
42 +       dev->weight = 64;
43 +#endif
44  #ifdef USE_HEADERLEN_RESV
45         dev->hard_header_len += sizeof(struct ieee80211_qosframe) +
46                                 sizeof(struct llc) +
47 @@ -2216,6 +2228,7 @@
48                 (status & HAL_INT_GLOBAL)       ? " HAL_INT_GLOBAL"     : ""
49                 );
50  
51 +       sc->sc_isr = status;
52         status &= sc->sc_imask;                 /* discard unasked for bits */
53         /* As soon as we know we have a real interrupt we intend to service, 
54          * we will check to see if we need an initial hardware TSF reading. 
55 @@ -2273,7 +2286,23 @@
56                 }
57                 if (status & (HAL_INT_RX | HAL_INT_RXPHY)) {
58                         ath_uapsd_processtriggers(sc, hw_tsf);
59 -                       ATH_SCHEDULE_TQUEUE(&sc->sc_rxtq, &needmark);
60 +                       sc->sc_isr &= ~HAL_INT_RX;
61 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
62 +                       if (netif_rx_schedule_prep(dev, &sc->sc_napi))
63 +#else
64 +                       if (netif_rx_schedule_prep(dev))
65 +#endif
66 +                       {
67 +#ifndef ATH_PRECISE_TSF
68 +                               sc->sc_imask &= ~HAL_INT_RX;
69 +                               ath_hal_intrset(ah, sc->sc_imask);
70 +#endif
71 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
72 +                               __netif_rx_schedule(dev, &sc->sc_napi);
73 +#else
74 +                               __netif_rx_schedule(dev);
75 +#endif
76 +                       }
77                 }
78                 if (status & HAL_INT_TX) {
79  #ifdef ATH_SUPERG_DYNTURBO
80 @@ -2299,6 +2328,11 @@
81                                 }
82                         }
83  #endif
84 +                       /* disable transmit interrupt */
85 +                       sc->sc_isr &= ~HAL_INT_TX;
86 +                       ath_hal_intrset(ah, sc->sc_imask & ~HAL_INT_TX);
87 +                       sc->sc_imask &= ~HAL_INT_TX;
88 +
89                         ATH_SCHEDULE_TQUEUE(&sc->sc_txtq, &needmark);
90                 }
91                 if (status & HAL_INT_BMISS) {
92 @@ -2511,6 +2545,7 @@
93         if (sc->sc_tx99 != NULL)
94                 sc->sc_tx99->start(sc->sc_tx99);
95  #endif
96 +       ath_poll_enable(dev);
97  
98  done:
99         ATH_UNLOCK(sc);
100 @@ -2551,6 +2586,9 @@
101                 if (sc->sc_tx99 != NULL)
102                         sc->sc_tx99->stop(sc->sc_tx99);
103  #endif
104 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
105 +               ath_poll_disable(dev);
106 +#endif
107                 netif_stop_queue(dev);  /* XXX re-enabled by ath_newstate */
108                 dev->flags &= ~IFF_RUNNING;     /* NB: avoid recursion */
109                 ieee80211_stop_running(ic);     /* stop all VAPs */
110 @@ -4009,6 +4047,39 @@
111         return ath_keyset(sc, k, mac, vap->iv_bss);
112  }
113  
114 +static void ath_poll_disable(struct net_device *dev)
115 +{
116 +       struct ath_softc *sc = dev->priv;
117 +
118 +       /*
119 +        * XXX Using in_softirq is not right since we might
120 +        * be called from other soft irq contexts than
121 +        * ath_rx_poll
122 +        */
123 +       if (!in_softirq()) {
124 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
125 +               napi_disable(&sc->sc_napi);
126 +#else
127 +               netif_poll_disable(dev);
128 +#endif
129 +       }
130 +}
131 +
132 +static void ath_poll_enable(struct net_device *dev)
133 +{
134 +       struct ath_softc *sc = dev->priv;
135 +
136 +       /* NB: see above */
137 +       if (!in_softirq()) {
138 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
139 +               napi_enable(&sc->sc_napi);
140 +#else
141 +               netif_poll_enable(dev);
142 +#endif
143 +       }
144 +}
145 +
146 +
147  /*
148   * Block/unblock tx+rx processing while a key change is done.
149   * We assume the caller serializes key management operations
150 @@ -4026,13 +4097,7 @@
151          * When called from the rx tasklet we cannot use
152          * tasklet_disable because it will block waiting
153          * for us to complete execution.
154 -        *
155 -        * XXX Using in_softirq is not right since we might
156 -        * be called from other soft irq contexts than
157 -        * ath_rx_tasklet.
158          */
159 -       if (!in_softirq())
160 -               tasklet_disable(&sc->sc_rxtq);
161         netif_stop_queue(dev);
162  }
163  
164 @@ -4043,9 +4108,9 @@
165         struct ath_softc *sc = dev->priv;
166  
167         DPRINTF(sc, ATH_DEBUG_KEYCACHE, "End\n");
168 -       netif_wake_queue(dev);
169 -       if (!in_softirq())              /* NB: see above */
170 -               tasklet_enable(&sc->sc_rxtq);
171 +
172 +       if (dev->flags&IFF_RUNNING)
173 +               netif_wake_queue(dev);
174  }
175  
176  /*
177 @@ -6350,15 +6415,25 @@
178         sc->sc_rxotherant = 0;
179  }
180  
181 -static void
182 -ath_rx_tasklet(TQUEUE_ARG data)
183 +static int
184 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
185 +ath_rx_poll(struct napi_struct *napi, int budget)
186 +#else
187 +ath_rx_poll(struct net_device *dev, int *budget)
188 +#endif
189  {
190  #define        PA2DESC(_sc, _pa) \
191         ((struct ath_desc *)((caddr_t)(_sc)->sc_rxdma.dd_desc + \
192                 ((_pa) - (_sc)->sc_rxdma.dd_desc_paddr)))
193 -       struct net_device *dev = (struct net_device *)data;
194 -       struct ath_buf *bf;
195 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
196 +       struct ath_softc *sc = container_of(napi, struct ath_softc, sc_napi);
197 +       struct net_device *dev = sc->sc_dev;
198 +       u_int rx_limit = budget;
199 +#else
200         struct ath_softc *sc = dev->priv;
201 +       u_int rx_limit = min(dev->quota, *budget);
202 +#endif
203 +       struct ath_buf *bf;
204         struct ieee80211com *ic = &sc->sc_ic;
205         struct ath_hal *ah = sc ? sc->sc_ah : NULL;
206         struct ath_desc *ds;
207 @@ -6368,8 +6443,10 @@
208         unsigned int len;
209         int type;
210         u_int phyerr;
211 +       u_int processed = 0, early_stop = 0;
212  
213         DPRINTF(sc, ATH_DEBUG_RX_PROC, "invoked\n");
214 +process_rx_again:
215         do {
216                 bf = STAILQ_FIRST(&sc->sc_rxbuf);
217                 if (bf == NULL) {               /* XXX ??? can this happen */
218 @@ -6393,6 +6470,15 @@
219                         /* NB: never process the self-linked entry at the end */
220                         break;
221                 }
222 +
223 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
224 +               processed++;
225 +#endif
226 +               if (rx_limit-- < 0) {
227 +                       early_stop = 1;
228 +                       break;
229 +               }
230 +
231                 skb = bf->bf_skb;
232                 if (skb == NULL) {
233                         EPRINTF(sc, "Dropping; buffer contains NULL skbuff.\n");
234 @@ -6440,6 +6526,7 @@
235                                 sc->sc_stats.ast_rx_phyerr++;
236                                 phyerr = rs->rs_phyerr & 0x1f;
237                                 sc->sc_stats.ast_rx_phy[phyerr]++;
238 +                               goto rx_next;
239                         }
240                         if (rs->rs_status & HAL_RXERR_DECRYPT) {
241                                 /*
242 @@ -6635,9 +6722,43 @@
243                 STAILQ_INSERT_TAIL(&sc->sc_rxbuf, bf, bf_list);
244                 ATH_RXBUF_UNLOCK_IRQ(sc);
245         } while (ath_rxbuf_init(sc, bf) == 0);
246 +       if (!early_stop) {
247 +               unsigned long flags;
248 +               /* Check if more data is received while we were
249 +                * processing the descriptor chain.
250 +                */
251 +#ifndef ATH_PRECISE_TSF
252 +               local_irq_save(flags);
253 +               if (sc->sc_isr & HAL_INT_RX) {
254 +                       u_int64_t hw_tsf = ath_hal_gettsf64(ah);
255 +                       sc->sc_isr &= ~HAL_INT_RX;
256 +                       local_irq_restore(flags);
257 +                       ath_uapsd_processtriggers(sc, hw_tsf);
258 +                       goto process_rx_again;
259 +               }
260 +#endif
261 +#ifndef ATH_PRECISE_TSF
262 +               sc->sc_imask |= HAL_INT_RX;
263 +               ath_hal_intrset(ah, sc->sc_imask);
264 +               local_irq_restore(flags);
265 +#endif
266 +       }
267 +
268 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
269 +       netif_rx_complete(dev, napi);
270 +#else
271 +       netif_rx_complete(dev);
272 +       *budget -= processed;
273 +       dev->quota -= processed;
274 +#endif
275  
276         /* rx signal state monitoring */
277         ath_hal_rxmonitor(ah, &sc->sc_halstats, &sc->sc_curchan);
278 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
279 +       return processed;
280 +#else
281 +       return early_stop;
282 +#endif
283  #undef PA2DESC
284  }
285  
286 @@ -8288,12 +8409,24 @@
287  {
288         struct net_device *dev = (struct net_device *)data;
289         struct ath_softc *sc = dev->priv;
290 +       unsigned long flags;
291  
292 +process_tx_again:
293         if (txqactive(sc->sc_ah, 0))
294                 ath_tx_processq(sc, &sc->sc_txq[0]);
295         if (txqactive(sc->sc_ah, sc->sc_cabq->axq_qnum))
296                 ath_tx_processq(sc, sc->sc_cabq);
297  
298 +       local_irq_save(flags);
299 +       if (sc->sc_isr & HAL_INT_TX) {
300 +               sc->sc_isr &= ~HAL_INT_TX;
301 +               local_irq_restore(flags);
302 +               goto process_tx_again;
303 +       }
304 +       sc->sc_imask |= HAL_INT_TX;
305 +       ath_hal_intrset(sc->sc_ah, sc->sc_imask);
306 +       local_irq_restore(flags);
307 +
308         netif_wake_queue(dev);
309  
310         if (sc->sc_softled)
311 @@ -8309,7 +8442,9 @@
312  {
313         struct net_device *dev = (struct net_device *)data;
314         struct ath_softc *sc = dev->priv;
315 +       unsigned long flags;
316  
317 +process_tx_again:
318         /*
319          * Process each active queue.
320          */
321 @@ -8330,6 +8465,16 @@
322         if (sc->sc_uapsdq && txqactive(sc->sc_ah, sc->sc_uapsdq->axq_qnum))
323                 ath_tx_processq(sc, sc->sc_uapsdq);
324  
325 +       local_irq_save(flags);
326 +       if (sc->sc_isr & HAL_INT_TX) {
327 +               sc->sc_isr &= ~HAL_INT_TX;
328 +               local_irq_restore(flags);
329 +               goto process_tx_again;
330 +       }
331 +       sc->sc_imask |= HAL_INT_TX;
332 +       ath_hal_intrset(sc->sc_ah, sc->sc_imask);
333 +       local_irq_restore(flags);
334 +
335         netif_wake_queue(dev);
336  
337         if (sc->sc_softled)
338 @@ -8345,13 +8490,25 @@
339         struct net_device *dev = (struct net_device *)data;
340         struct ath_softc *sc = dev->priv;
341         unsigned int i;
342 +       unsigned long flags;
343  
344         /* Process each active queue. This includes sc_cabq, sc_xrtq and
345          * sc_uapsdq */
346 +process_tx_again:
347         for (i = 0; i < HAL_NUM_TX_QUEUES; i++)
348                 if (ATH_TXQ_SETUP(sc, i) && txqactive(sc->sc_ah, i))
349                         ath_tx_processq(sc, &sc->sc_txq[i]);
350  
351 +       local_irq_save(flags);
352 +       if (sc->sc_isr & HAL_INT_TX) {
353 +               sc->sc_isr &= ~HAL_INT_TX;
354 +               local_irq_restore(flags);
355 +               goto process_tx_again;
356 +       }
357 +       sc->sc_imask |= HAL_INT_TX;
358 +       ath_hal_intrset(sc->sc_ah, sc->sc_imask);
359 +       local_irq_restore(flags);
360 +
361         netif_wake_queue(dev);
362  
363         if (sc->sc_softled)
364 @@ -10284,9 +10441,9 @@
365         dev->mtu = mtu;
366         if ((dev->flags & IFF_RUNNING) && !sc->sc_invalid) {
367                 /* NB: the rx buffers may need to be reallocated */
368 -               tasklet_disable(&sc->sc_rxtq);
369 +               ath_poll_disable(dev);
370                 error = ath_reset(dev);
371 -               tasklet_enable(&sc->sc_rxtq);
372 +               ath_poll_enable(dev);
373         }
374         ATH_UNLOCK(sc);
375  
376 --- a/ath/if_athvar.h
377 +++ b/ath/if_athvar.h
378 @@ -53,6 +53,10 @@
379  # include      <asm/bitops.h>
380  #endif
381  
382 +#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,0)
383 +#define irqs_disabled()                        0
384 +#endif
385 +
386  /*
387   * Deduce if tasklets are available.  If not then
388   * fall back to using the immediate work queue.
389 @@ -616,6 +620,9 @@
390  struct ath_softc {
391         struct ieee80211com sc_ic;              /* NB: must be first */
392         struct net_device *sc_dev;
393 +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,24)
394 +       struct napi_struct sc_napi;
395 +#endif
396         void __iomem *sc_iobase;                /* address of the device */
397         struct semaphore sc_lock;               /* dev-level lock */
398         struct net_device_stats sc_devstats;    /* device statistics */
399 @@ -730,7 +737,6 @@
400         struct ath_buf *sc_rxbufcur;            /* current rx buffer */
401         u_int32_t *sc_rxlink;                   /* link ptr in last RX desc */
402         spinlock_t sc_rxbuflock;
403 -       struct ATH_TQ_STRUCT sc_rxtq;           /* rx intr tasklet */
404         struct ATH_TQ_STRUCT sc_rxorntq;        /* rxorn intr tasklet */
405         u_int8_t sc_defant;                     /* current default antenna */
406         u_int8_t sc_rxotherant;                 /* RXs on non-default antenna */
407 @@ -745,6 +751,7 @@
408         u_int sc_txintrperiod;                  /* tx interrupt batching */
409         struct ath_txq sc_txq[HAL_NUM_TX_QUEUES];
410         struct ath_txq *sc_ac2q[WME_NUM_AC];    /* WME AC -> h/w qnum */
411 +       HAL_INT sc_isr;                         /* unmasked ISR state */
412         struct ATH_TQ_STRUCT sc_txtq;           /* tx intr tasklet */
413         u_int8_t sc_grppoll_str[GRPPOLL_RATE_STR_LEN];
414         struct ath_descdma sc_bdma;             /* beacon descriptors */
415 @@ -858,6 +865,8 @@
416  #define        ATH_TXBUF_LOCK_CHECK(_sc)
417  #endif
418  
419 +#define ATH_DISABLE_INTR               local_irq_disable
420 +#define ATH_ENABLE_INTR                local_irq_enable
421  
422  #define        ATH_RXBUF_LOCK_INIT(_sc)        spin_lock_init(&(_sc)->sc_rxbuflock)
423  #define        ATH_RXBUF_LOCK_DESTROY(_sc)
424 --- a/net80211/ieee80211_input.c
425 +++ b/net80211/ieee80211_input.c
426 @@ -1198,7 +1198,7 @@
427                         /* attach vlan tag */
428                         struct ieee80211_node *ni_tmp = SKB_CB(skb)->ni;
429                         if (vlan_hwaccel_receive_skb(skb, vap->iv_vlgrp, ni->ni_vlan) == NET_RX_DROP) {
430 -                               /* If netif_rx dropped the packet because 
431 +                               /* If netif_receive_skb dropped the packet because
432                                  * device was too busy */
433                                 if (ni_tmp != NULL) {
434                                         /* node reference was leaked */
435 @@ -1209,8 +1209,8 @@
436                         skb = NULL; /* SKB is no longer ours */
437                 } else {
438                         struct ieee80211_node *ni_tmp = SKB_CB(skb)->ni;
439 -                       if (netif_rx(skb) == NET_RX_DROP) {
440 -                               /* If netif_rx dropped the packet because 
441 +                       if (netif_receive_skb(skb) == NET_RX_DROP) {
442 +                               /* If netif_receive_skb dropped the packet because
443                                  * device was too busy */
444                                 if (ni_tmp != NULL) {
445                                         /* node reference was leaked */
446 @@ -2322,8 +2322,8 @@
447                 skb1->protocol = __constant_htons(0x0019);  /* ETH_P_80211_RAW */
448  
449                 ni_tmp = SKB_CB(skb1)->ni;
450 -               if (netif_rx(skb1) == NET_RX_DROP) {
451 -                       /* If netif_rx dropped the packet because 
452 +               if (netif_receive_skb(skb1) == NET_RX_DROP) {
453 +                       /* If netif_receive_skb dropped the packet because
454                          * device was too busy */
455                         if (ni_tmp != NULL) {
456                                 /* node reference was leaked */
457 --- a/net80211/ieee80211_monitor.c
458 +++ b/net80211/ieee80211_monitor.c
459 @@ -584,8 +584,8 @@
460                         skb1->protocol = 
461                                 __constant_htons(0x0019); /* ETH_P_80211_RAW */
462  
463 -                       if (netif_rx(skb1) == NET_RX_DROP) {
464 -                               /* If netif_rx dropped the packet because 
465 +                       if (netif_receive_skb(skb1) == NET_RX_DROP) {
466 +                               /* If netif_receive_skb dropped the packet because
467                                  * device was too busy, reclaim the ref. in 
468                                  * the skb. */
469                                 if (SKB_CB(skb1)->ni != NULL)
470 --- a/net80211/ieee80211_skb.c
471 +++ b/net80211/ieee80211_skb.c
472 @@ -73,7 +73,7 @@
473  #undef dev_queue_xmit
474  #undef kfree_skb
475  #undef kfree_skb_fast
476 -#undef netif_rx
477 +#undef netif_receive_skb
478  #undef pskb_copy
479  #undef skb_clone
480  #undef skb_copy
481 @@ -638,8 +638,8 @@
482                 grp, vlan_tag);
483  }
484  
485 -int netif_rx_debug(struct sk_buff *skb, const char* func, int line) {
486 -       return netif_rx(untrack_skb(skb, 0, func, line, __func__, __LINE__));
487 +int netif_receive_skb_debug(struct sk_buff *skb, const char* func, int line) {
488 +       return netif_receive_skb(untrack_skb(skb, 0, func, line, __func__, __LINE__));
489  }
490  
491  struct sk_buff * alloc_skb_debug(unsigned int length, gfp_t gfp_mask,
492 @@ -760,7 +760,7 @@
493  }
494  
495  EXPORT_SYMBOL(vlan_hwaccel_receive_skb_debug);
496 -EXPORT_SYMBOL(netif_rx_debug);
497 +EXPORT_SYMBOL(netif_receive_skb_debug);
498  EXPORT_SYMBOL(alloc_skb_debug);
499  EXPORT_SYMBOL(dev_alloc_skb_debug);
500  EXPORT_SYMBOL(skb_clone_debug);
501 --- a/net80211/ieee80211_skb.h
502 +++ b/net80211/ieee80211_skb.h
503 @@ -116,7 +116,7 @@
504  int  vlan_hwaccel_receive_skb_debug(struct sk_buff *skb, 
505                                     struct vlan_group *grp, unsigned short vlan_tag, 
506                                     const char* func, int line);
507 -int netif_rx_debug(struct sk_buff *skb, const char* func, int line);
508 +int netif_receive_skb_debug(struct sk_buff *skb, const char* func, int line);
509  struct sk_buff * alloc_skb_debug(unsigned int length, gfp_t gfp_mask,
510                                  const char *func, int line);
511  struct sk_buff * dev_alloc_skb_debug(unsigned int length,
512 @@ -151,7 +151,7 @@
513  #undef dev_queue_xmit
514  #undef kfree_skb
515  #undef kfree_skb_fast
516 -#undef netif_rx
517 +#undef netif_receive_skb
518  #undef pskb_copy
519  #undef skb_clone
520  #undef skb_copy
521 @@ -168,8 +168,8 @@
522         skb_copy_expand_debug(_skb, _newheadroom, _newtailroom, _gfp_mask, __func__, __LINE__)
523  #define vlan_hwaccel_receive_skb(_skb, _grp, _tag) \
524         vlan_hwaccel_receive_skb_debug(_skb, _grp, _tag, __func__, __LINE__)
525 -#define netif_rx(_skb) \
526 -       netif_rx_debug(_skb, __func__, __LINE__)
527 +#define netif_receive_skb(_skb) \
528 +       netif_receive_skb_debug(_skb, __func__, __LINE__)
529  #define        alloc_skb(_length, _gfp_mask) \
530         alloc_skb_debug(_length, _gfp_mask, __func__, __LINE__)
531  #define        dev_alloc_skb(_length) \