【原创】拒绝YY 从基础讲解USB/UAC运作原理 - 耳机网-耳机大家坛

Endpoint Descriptor:
------------------------------
0x07 bLength
0x05 bDescriptorType
0x05 bEndpointAddress (OUT Endpoint) #主机端->USB界面
0x05 bmAttributes (Transfer: Isochronous / Synch: Asynchronous / Usage: Data) #传输类型Isochronous 同步方式Asynchronous
0x0400 wMaxPacketSize (1024 Bytes) #最大包大小
0x01 bInterval #传输间隔 2^(1-1) x 125。也就是125us传输一次
Endpoint Descriptor:
------------------------------
0x07 bLength
0x05 bDescriptorType
0x81 bEndpointAddress (IN Endpoint) #USB界面->主机端
0x11 bmAttributes (Transfer: Isochronous / Synch: None / Usage: Feedback) #传输类型Isochronous 用于Feedback
0x0004 wMaxPacketSize (4 Bytes) #最大包大小4字节
0x06 bInterval #传输间隔 2^(6-1) x 125。也就是4000us(4ms)传输一次

复制代码

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1156       /*
1157       * process after playback sync complete
1158       *
1159       * Full speed devices report feedback values in 10.14 format as samples
1160       * per frame, high speed devices in 16.16 format as samples per
1161       * microframe.
1162       *
1163       * Because the Audio Class 1 spec was written before USB 2.0, many high
1164       * speed devices use a wrong interpretation, some others use an
1165       * entirely different format.
1166       *
1167       * Therefore, we cannot predict what format any particular device uses
1168       * and must detect it automatically.
1169       */
1170
1171       if (urb->iso_frame_desc[0].status != 0 ||
1172          urb->iso_frame_desc[0].actual_length < 3)
1173                return;
1174
1175       f = le32_to_cpup(urb->transfer_buffer);
1176       if (urb->iso_frame_desc[0].actual_length == 3)
1177                f &= 0x00ffffff;
1178       else
1179                f &= 0x0fffffff;
1180
1181       if (f == 0)
1182                return;
1183
1184       if (unlikely(sender->tenor_fb_quirk)) {
1185                /*
1186                * Devices based on Tenor 8802 chipsets (TEAC UD-H01
1187                * and others) sometimes change the feedback value
1188                * by +/- 0x1.0000.
1189                */
1190                if (f < ep->freqn - 0x8000)
1191                      f += 0xf000;
1192                else if (f > ep->freqn + 0x8000)
1193                      f -= 0xf000;
1194       } else if (unlikely(ep->freqshift == INT_MIN)) {
1195                /*
1196                * The first time we see a feedback value, determine its format
1197                * by shifting it left or right until it matches the nominal
1198                * frequency value.  This assumes that the feedback does not
1199                * differ from the nominal value more than +50% or -25%.
1200                */
1201                shift = 0;
1202                while (f < ep->freqn - ep->freqn / 4) {
1203                      f <<= 1;
1204                      shift++;
1205                }
1206                while (f > ep->freqn + ep->freqn / 2) {
1207                      f >>= 1;
1208                      shift--;
1209                }
1210                ep->freqshift = shift;
1211       } else if (ep->freqshift >= 0)
1212                f <<= ep->freqshift;
1213       else
1214                f >>= -ep->freqshift;
1215
1216       if (likely(f >= ep->freqn - ep->freqn / 8 && f <= ep->freqmax)) {
1217                /*
1218                * If the frequency looks valid, set it.
1219                * This value is referred to in prepare_playback_urb().
1220                */
1221                spin_lock_irqsave(&ep->lock, flags);
1222                ep->freqm = f;
1223                spin_unlock_irqrestore(&ep->lock, flags);
1224       } else {
1225                /*
1226                * Out of range; maybe the shift value is wrong.
1227                * Reset it so that we autodetect again the next time.
1228                */
1229                ep->freqshift = INT_MIN;
1230       }

145 int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep)
146 {
147       unsigned long flags;
148       int ret;
149
150       if (ep->fill_max)
151                return ep->maxframesize;
152
153       spin_lock_irqsave(&ep->lock, flags);
154       ep->phase = (ep->phase & 0xffff)
155                + (ep->freqm << ep->datainterval);
156       ret = min(ep->phase >> 16, ep->maxframesize);
157       spin_unlock_irqrestore(&ep->lock, flags);
158
159       return ret;
160 }

1451 static void prepare_playback_urb(struct snd_usb_substream *subs,
1452                               struct urb *urb)
1453 {
1454       struct snd_pcm_runtime *runtime = subs->pcm_substream->runtime;
1455       struct snd_usb_endpoint *ep = subs->data_endpoint;
1456       struct snd_urb_ctx *ctx = urb->context;
1457       unsigned int counts, frames, bytes;
1458       int i, stride, period_elapsed = 0;
1459       unsigned long flags;
1460
1461       stride = runtime->frame_bits >> 3;
1462
1463       frames = 0;
1464       urb->number_of_packets = 0;
1465       spin_lock_irqsave(&subs->lock, flags);
1466       subs->frame_limit += ep->max_urb_frames;
1467       for (i = 0; i < ctx->packets; i++) {
1468                if (ctx->packet_size)
1469                      counts = ctx->packet_size;
1470                else
1471                      counts = snd_usb_endpoint_next_packet_size(ep);
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