Isochronous transfers - Webcam

These past 2 weeks, I focused on getting a USB webcam to work, and particularly the 2 isochronous endpoints, used for audio and video coming from the webcam. This is a full speed device, and its descriptor is below (I trimmed it a bit):

Bus 002 Device 061: ID 046d:08da Logitech, Inc. QuickCam Messanger
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0         8
  idVendor           0x046d Logitech, Inc.
  idProduct          0x08da QuickCam Messanger
  bcdDevice            1.00
  iManufacturer           0 
  iProduct                0 
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength          336
    bNumInterfaces          3
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            1
          Transfer Type            Isochronous
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0000  1x 0 bytes
        bInterval               1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval              10
    Interface Descriptor: ...
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       6
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              0 
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            1
          Transfer Type            Isochronous
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0300  1x 768 bytes
        bInterval               1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval              10
    Interface Descriptor: ...
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       0
      ...
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        2
      bAlternateSetting       0
      bNumEndpoints           0
      bInterfaceClass         1 Audio
      bInterfaceSubClass      2 Streaming
      bInterfaceProtocol      0 
      iInterface              0 
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        2
      bAlternateSetting       1
      bNumEndpoints           1
      bInterfaceClass         1 Audio
      bInterfaceSubClass      2 Streaming
      bInterfaceProtocol      0 
      iInterface              0 
      AudioStreaming Interface Descriptor: ....
      AudioStreaming Interface Descriptor: ....
      Endpoint Descriptor:
        bLength                 9
        bDescriptorType         5
        bEndpointAddress     0x83  EP 3 IN
        bmAttributes            1
          Transfer Type            Isochronous
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0010  1x 16 bytes
        bInterval               1
        bRefresh                0
        bSynchAddress           0
        AudioControl Endpoint Descriptor: ....
Device Status:     0x0000
  (Bus Powered)

Audio comes through EP3, and video through EP1.

The first thing to note is that isochronous endpoints are not allowed to be part of the default configuration for a device, so, to support these transfers, the SET_INTERFACE command has to be supported. For example, when a video player is launched, the alternate setting 6 is selected for interface 0, allowing packets of up to 768 bytes on EP1 IN.

When I tried with audio playback, URBs could not be submitted to the device, with error -28, i.e. ENOSPC, which has something to do with a problem in USB bandwidth allocation. The kernel configuration CONFIG_USB_EHCI_TT_NEWSCHED is meant to help with that kind of problems, and it does, no more -28 errors, and audio worked. I realised much later that the same problem happens when I connect the webcam to my PC, through the same USB hub I normally use with the BeagleBoard, so this is probably a hub problem. The webcam works fine with another USB hub on my PC, but that USB hub refuses to work with the BeagleBoard, so I need to find a better supported USB hub.

The next problem was about the packet size used for EP1, 768 bytes is more than the FIFO size (512 bytes) in the default endpoint configurations (fifo_mode=5, see here, __initdata mode_5_cfg). I simply created a new configuration, with an EP1 FIFO size of 1024 bytes (fifo_mode=6).

And then, nothing. When I started a video player, some packets were received properly from the webcam. The proxy driver would then submit a request to the MUSB controller, and, then, the request completion handler was only called for the first request, and never for any subsequent packet. After a few hours of debugging, I found that the MUSB driver sends a zero-length packet after sending a packet smaller than the maximum size (see musb_gadget.c, around line 520). This may make sense for bulk transfers (and I have doubts...), but not for isochronous transfers. Furthermore, sending that packet seemed to stall the endpoint, and no further requests would be processed. I tried a quick hack (around line 503), and avoid sending a zero-length packet, but transfers were still not working.

After seeing the line testing #ifdef CONFIG_USB_INVENTRA_DMA around there, I started wondering about that DMA engine, and disabled it (use_dma=0 parameter to musb_hdrc), and the webcam video started to work.

Now, the question is, why did it work with audio, but not video? The answer is quite simple, audio packets are always filled: Each packet contains exactly 16 bytes, as can be seen in the following usbmon line:

ffff88006e576100 2368995185 C Zi:6:075:3 0:1:831804651:0 1 0:0:16 16 =
6aff81ff 65ff6fff 7fff6fff 9dff86ff

(The interesting bit is 0:0:16, the 3rd number indicates that the packet contains 16 bytes. Other packets are similar, just with different payload and timestamps.)

However, for the webcam, the packet size is variable:

ffff880080052000 2351992862 C Zi:6:075:1 0:1:831787618:0 32 0:0:250
0:768:292 0:1536:338 0:2304:362 0:3072:254 6807 = ...

Here, the first packet contains 250 bytes, the second one 292, then 338, etc., triggering the MUSB/DMA bug.

Finally, you can try the latest version by using the instructions found here, and the branch stable-20100702 (here).