static int ir_nec_decode(struct rc_dev *dev, struct ir_raw_event ev)
{
         struct nec_dec *data = &dev->raw->nec;
         u32 scancode;
         u8 address, not_address, command, not_command;
         bool send_32bits = false;

         if (!(dev->enabled_protocols & RC_BIT_NEC))              //
 
判斷協議是否支援
                  return 0;
         //… …
         switch (data->state) {

         case STATE_INACTIVE:
                  if (!ev.pulse)
                          break;

                  if (eq_margin(ev.duration, NEC_HEADER_PULSE, NEC_UNIT * 2)) {           //判斷ev.duration 是否等於9ms頭引導碼
 

                          data->is_nec_x = false;           //標記當前格式不是NECX編碼格式
                          data->necx_repeat = false;
                  } else if (eq_margin(ev.duration, NECX_HEADER_PULSE, NEC_UNIT / 2)) //另一種不常見的NECX引導碼
                          data->is_nec_x = true;                     //
 
標記是NECX編碼格式
                  else
                          break;

                  data->count = 0;
                  data->state = STATE_HEADER_SPACE;  //進入判斷引導碼間隔值,是4.5ms還是2.25ms ?           
                  return 0;

         case STATE_HEADER_SPACE:
                  if (ev.pulse)
                          break;

                  if (eq_margin(ev.duration, NEC_HEADER_SPACE, NEC_UNIT)) {    //如果ev.duration=4.5ms  間隔引導碼
                          data->state = STATE_BIT_PULSE;                      //進入解析32bit模式
                          return 0; 
                  } else if (eq_margin(ev.duration, NEC_REPEAT_SPACE, NEC_UNIT / 2)) {  //如果ev.duration=2.5ms  ,表示重複引導碼
                          if (!dev->keypressed) {                 //dev->keypressed是鬆開的,則放棄(這裡有BUG,後面會分析到)
                                   IR_dprintk(1, "Discarding last key repeat: event after key up\n");
                          } else {
                                   rc_repeat(dev);               //dev->keypressed是未鬆開,則上報事件
                                   IR_dprintk(1, "Repeat last key\n");
                                   data->state = STATE_TRAILER_PULSE;
                          }
                          return 0;
                  }
                  break;

         case STATE_BIT_PULSE:                                    //接收資料位的脈衝數據
                  if (!ev.pulse)
                          break;
                  if (!eq_margin(ev.duration, NEC_BIT_PULSE, NEC_UNIT / 2))         //不等於0.56ms,則忽略掉
                          break;

                  data->state = STATE_BIT_SPACE;                            //等於0.56ms,接下來進入STATE_BIT_SPACE,開始解析資料bit
                  return 0;

         case STATE_BIT_SPACE:         
                  if (ev.pulse)
                          break;

                  if (data->necx_repeat && data->count == NECX_REPEAT_BITS &&
                          geq_margin(ev.duration,
                          NEC_TRAILER_SPACE, NEC_UNIT / 2)) {                //解析NECX編碼格式
                                   IR_dprintk(1, "Repeat last key\n");
                                   rc_repeat(dev);
                                   data->state = STATE_INACTIVE;
                                   return 0;

                  } else if (data->count > NECX_REPEAT_BITS)
                          data->necx_repeat = false;

                  data->bits <<= 1;
                  if (eq_margin(ev.duration, NEC_BIT_1_SPACE, NEC_UNIT / 2))          // 1.68ms  資料1
                          data->bits |= 1;
                  else if (!eq_margin(ev.duration, NEC_BIT_0_SPACE, NEC_UNIT / 2)) // 既不等於1.68ms,也不等於0.56ms,則是無效資料
                          break;
                  data->count++;

                  if (data->count == NEC_NBITS)                             //data->count == 32,則表示資料接收完成
                          data->state = STATE_TRAILER_PULSE;
                  else
                          data->state = STATE_BIT_PULSE;
                  return 0;

         case STATE_TRAILER_PULSE:
                  if (!ev.pulse)
                          break;
                  if (!eq_margin(ev.duration, NEC_TRAILER_PULSE, NEC_UNIT / 2))
                          break;
                  data->state = STATE_TRAILER_SPACE;
                  return 0;

         case STATE_TRAILER_SPACE:
                  if (ev.pulse)
                          break;

                  if (!geq_margin(ev.duration, NEC_TRAILER_SPACE, NEC_UNIT / 2))
                          break;

                  address     = bitrev8((data->bits >> 24) & 0xff);                        
                  not_address = bitrev8((data->bits >> 16) & 0xff);
                  command            = bitrev8((data->bits >>  8) & 0xff);
                  not_command = bitrev8((data->bits >>  0) & 0xff);

                  if ((command ^ not_command) != 0xff) {                    //解析資料
                          IR_dprintk(1, "NEC checksum error: received 0x%08x\n",
                                      data->bits);
                          send_32bits = true;
                  }

                  if (send_32bits) {
                          /* NEC transport, but modified protocol, used by at
                           * least Apple and TiVo remotes */
                          scancode = data->bits;
                          IR_dprintk(1, "NEC (modified) scancode 0x%08x\n", scancode);
                  } else if ((address ^ not_address) != 0xff) {
                          /* Extended NEC */
                          scancode = address     << 16 |
                                      not_address <<  8 |
                                      command;
                          IR_dprintk(1, "NEC (Ext) scancode 0x%06x\n", scancode);
                  } else {
                          /* Normal NEC */
                          scancode = address << 8 | command;
                          IR_dprintk(1, "NEC scancode 0x%04x\n", scancode);
                  }

                  if (data->is_nec_x)
                          data->necx_repeat = true;

                  rc_keydown(dev, scancode, 0);              //通過scancode編碼來上報按鍵事件
                  data->state = STATE_INACTIVE;
                  return 0;
         }
    //… …
}