Linux那些事儿 之 戏说USB(34)驱动的生命线(二)
关系是跑出来的，感情是喝出来的，朋友是处出来的，事业是干出来的，但设备是配置出来的，绝非吹出来的。core配置设备使用的是message.c里的usb_set_configuration函数
1388 /*
1389* usb_set_configuration - Makes a particular device setting be current
1390* @dev: the device whose configuration is being updated
1391* @configuration: the configuration being chosen.
1392* Context: !in_interrupt(), caller owns the device lock
1393*
1394* This is used to enable non-default device modes.Not all devices
1395* use this kind of configurability; many devices only have one
1396* configuration.
1397*
1398* @configuration is the value of the configuration to be installed.
1399* According to the USB spec (e.g. section 9.1.1.5), configuration values
1400* must be non-zero; a value of zero indicates that the device in
1401* unconfigured.However some devices erroneously use 0 as one of their
1402* configuration values.To help manage such devices, this routine will
1403* accept @configuration = -1 as indicating the device should be put in
1404* an unconfigured state.
1405*
1406* USB device configurations may affect Linux interoperability,
1407* power consumption and the functionality available.For example,
1408* the default configuration is limited to using 100mA of bus power,
1409* so that when certain device functionality requires more power,
1410* and the device is bus powered, that functionality should be in some
1411* non-default device configuration.Other device modes may also be
1412* reflected as configuration options, such as whether two ISDN
1413* channels are available independently; and choosing between open
1414* standard device protocols (like CDC) or proprietary ones.
1415*
1416* Note that USB has an additional level of device configurability,
1417* associated with interfaces.That configurability is accessed using
1418* usb_set_interface().
1419*
1420* This call is synchronous. The calling context must be able to sleep,
1421* must own the device lock, and must not hold the driver model's USB
1422* bus mutex; usb device driver probe() methods cannot use this routine.
1423
*
1424
* Returns zero on success, or else the status code returned by the
1425
* underlying call that failed.On successful completion, each interface
1426
* in the original device configuration has been destroyed, and each one
1427
* in the new configuration has been probed by all relevant usb device
1428
* drivers currently known to the kernel.
1429
*/
1430
int usb_set_configuration(struct usb_device *dev, int configuration)
1431
{
1433 struct usb_host_config *cp = NULL;
1434 struct usb_interface **new_interfaces = NULL;
1435 int n, nintf;
1437 if (configuration == -1)
1438 configuration = 0;
1439 else {
1442 configuration) {
1444 break;
1445 }
1446 }
1447 }
1451 /* The USB spec says configuration 0 means unconfigured.
1452 * But if a device includes a configuration numbered 0,
1453 * we will accept it as a correctly configured state.
1454 * Use -1 if you really want to unconfigure the device.
1455
*/
1459 /* Allocate memory for new interfaces before doing anything else,
1460 * so that if we run out then nothing will have changed. */
1461 n = nintf = 0;
1466 if (!new_interfaces) {
1469 }
1471 for (; n < nintf; ++n) {
1473 sizeof(struct usb_interface),
1475 if (!new_interfaces[n]) {
1478
free_interfaces:
1479
while (--n >= 0)
1483
}
1484
}
1489
"limit by %dmA/n",
1491
}
1493
/* Wake up the device so we can send it the Set-Config request */
1496
goto free_interfaces;
1498
/* if it's already configured, clear out old state first.
1499
* getting rid of old interfaces means unbinding their drivers.
1500
*/
1502
usb_disable_device (dev, 1); // Skip ep0
1504
if ((ret = usb_control_msg(dev, usb_sndctrlpipe(dev, 0),
1505
USB_REQ_SET_CONFIGURATION, 0, configuration, 0,
1506
NULL, 0, USB_CTRL_SET_TIMEOUT)) < 0) {
1508
/* All the old state is gone, so what else can we do?
1509
* The device is probably useless now anyway.
1510
*/
1512
}
1516
usb_set_device_state(dev, USB_STATE_ADDRESS);
1518
goto free_interfaces;
1519
}
1520
usb_set_device_state(dev, USB_STATE_CONFIGURED);
1522
/* Initialize the new interface structures and the
1523 * hc/hcd/usbcore interface/endpoint state.
1524 */
1526 struct usb_interface_cache *intfc;
1527 struct usb_interface *intf;
1528 struct usb_host_interface *alt;
1532 intf->altsetting = intfc->altsetting;
1533 intf->num_altsetting = intfc->num_altsetting;
1536 alt = usb_altnum_to_altsetting(intf, 0);
1538 /* No altsetting 0?We'll assume the first altsetting.
1539 * We could use a GetInterface call, but if a device is
1540 * so non-compliant that it doesn't have altsetting 0
1541 * then I wouldn't trust its reply anyway.
1542 */
1544 alt = &intf->altsetting[0];
1547
usb_enable_interface(dev, intf);
1550
intf->dev.bus = &usb_bus_type;
1551
intf->dev.type = &usb_if_device_type;
1553
device_initialize (&intf->dev);
1554
mark_quiesced(intf);
1558
}
1564
/* Now that all the interfaces are set up, register them
1565
* to trigger binding of drivers to interfaces.probe()
1566
* routines may install different altsettings and may
1567
* claim() any interfaces not yet bound.Many class drivers
1568
* need that: CDC, audio, video, etc.
1569
*/
1571
struct usb_interface *intf = cp->interface[i];
1574
"adding %s (config #%d, interface %d)/n",
1577
ret = device_add (&intf->dev);
1581 continue;
1582 }
1583 usb_create_sysfs_intf_files (intf);
1584 }
1587 return 0;
1588
}
这个函数很迷信，从1388行开始，到1588行结束，怎么着它都要发发，希望咱们看的时候也能沾上点财气运气。那就先对着它许个愿吧，并不是真的相信它，但是反正也是免费的，而且也没有证据证明它不灵。
说代码前咱们再聊点这个函数背后的人生哲学，你设备不是和usb_generic_driver这个大美女配对成功了么，可是这并不是就说你可以高枕无忧了，要想保持和她的这种亲密关系，你得想办法让她得到满足，你就得抱着一颗勇敢的心，准备着让她去配置，准备着她想让你什么样你就什么样。你要明白，吸引住男人的办法就是让他一直得不到，吸引住女人的办法正好相反，就是让她一直满足。从这个角度看，这个函数就可以泾渭分明的分成三个部分三个阶段，从1432到1491的这几十行是准备阶段，做做常规检查啊，申请申请内存啊，搞点前戏什么的。1498到1520这部分可是重头戏，就是在这里设备从Address发展到了Configured，可算是高潮阶段，别看它短，这是事物发展的规律，也是每个男人女人的规律。1522到1584这阶段也挺重要的，主要就是充实充实设备的每个接口并提交给设备模型，为它们寻找命中注定的接口驱动，最后温存温存，过了这个后戏阶段，usb_generic_driver也就彻底从你设备那儿得到满足了，generic_probe的历史使命也就完成了。事物的发展大体上就脱离不了这三个阶段，再比如股票，买、卖、回味。
先看第一阶段，要做一个好男人，按顺序一步一步来。1437行，configuration是前边儿choose_configuration()那里返回回来的，找到合意的配置的话，就返回那个配置的bConfigurationValue值，没有找到称心的配置的话，就返回-1，所以这里的configuration值就可能有两种情况，或者为-1，或者为配置的bConfigurationValue值。当configuration为-1时这里为啥又要把它改为0捏？要知道configuration这个值是要在后面的高潮阶段里发送SET_CONFIGURATION请求时用的，关于SET_CONFIGURATION请求，spec里说，这个值必须为0或者与配置描述符的bConfigurationValue一致，如果为0，则设备收到SET_CONFIGURATION请求后，仍然会待在Address状态。这里当configuration为-1也就是没有发现满意的配置时，设备不能进入Configured，所以要把configuration的值改为0，以便满足SET_CONFIGURATION请求的要求。
那接下来的问题就出来了，在没有找到合适配置的时候直接给configuration这个参数传个0，也就是让choose_configuration()返回个0不就得了，干吗还这么麻烦先返回个-1再把它改成0，不是脱裤子放屁多此一举么？你别愤怒，这归根结底还是那句话，林子大了什么鸟都有，有些设备就是有拿0当配置bConfigurationValue值的毛病，你又不能不让它用，不是有句俗话么：既然说人生是本书，那出现几个错别字就没什么大不了的。人生都能出现错字，那usb世界里出现几个有毛病的设备也没啥大不了的，这里妥协一下就是了，想让设备回到Address状态时，usb_set_configuration()就别传递0了，传递个-1，里边儿去处理一下。如果configuration值为0或大于0的值，就从设备struct usb_device结构体的config数组里将相应配置的描述信息，也就是struct usb_host_config结构体给取出来。
1448行，如果没有拿到配置的内容，configuration值就必须为0了，让设备待在Address那儿别动。这也很好理解，配置的内容都找不到了，还配置个什么劲儿。当然，如果拿到了配置的内容，但同时configuration为0，这就是对应了上面说的那种有毛病的设备的情况，就提出一下警告，告诉你不正常现象出现了。
1461行，过了这个if，第一阶段就告结束了。如果配置是实实在在存在的，就为它使用的那些接口都准备一个struct usb_interface结构体。new_interfaces是开头儿就定义好的一个struct usb_interface结构体指针数组，数组的每一项都指向了一个struct usb_interface结构体，所以这里申请内存也要分两步走，先申请指针数组的，再申请每一项的。