Re: Advice needed concerning ISOC transactions for a webcam

On Sun, 7 Oct 2007, Ilyes Gouta wrote:

> Hi Alan,
> 
> Thanks for the reply.
> 
> Well, the webcam (0402:5602) is a USB 2.0 Camera that comes embedded into laptops (ex: Acer).
> Here is the output of lsusb -v:
> 
> Bus 001 Device 003: ID 0402:5602 ALi Corp.
> Device Descriptor:
>    bLength                18
>    bDescriptorType         1
>    bcdUSB               2.00
>    bDeviceClass            0 (Defined at Interface level)
>    bDeviceSubClass         0
>    bDeviceProtocol         0
>    bMaxPacketSize0        64
>    idVendor           0x0402 ALi Corp.
>    idProduct          0x5602
>    bcdDevice            1.00
>    iManufacturer           0
>    iProduct                1 USB2.0 Camera
>    iSerial                 0
>    bNumConfigurations      1
>    Configuration Descriptor:
>      bLength                 9
>      bDescriptorType         2
>      wTotalLength          101
>      bNumInterfaces          1
>      bConfigurationValue     1
>      iConfiguration          0
>      bmAttributes         0xa0
>        Remote Wakeup
>      MaxPower              500mA
>      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

A maxpacket size of 0 is unusual but it's legal.  And for Isochronous 
endpoints in altsetting 0, it is the _only_ legal size.  This means 
that Iso transfers require the altsetting to be different from 0.

>        Endpoint Descriptor:
>          bLength                 7
>          bDescriptorType         5
>          bEndpointAddress     0x82  EP 2 IN
>          bmAttributes            3
>            Transfer Type            Interrupt
>            Synch Type               None
>            Usage Type               Data
>          wMaxPacketSize     0x0000  1x 0 bytes
>          bInterval               4

An interrupt endpoint with a maxpacket size of 0 is rather unusual.  I 
guess it just shows the camera doesn't expect to be used for anything 
while it is in altsetting 0.

The rest of the descriptors looked more or less normal.

> The camera is high-speed capable

But are you _running_ it at high speed?  Judging by the descriptors, it 
looks like you are.

>  and I'm not really managing any polling interval (and why I need this? ISOCs should be 
> self-sustainable).

Yes you are.  The polling interval is one of the fields you have to 
fill in before submitting URBs (urb->interval).  The descriptors show 
the interval to be 1 microframe.

>  I start by allocating ISOCs URBs, fill the fields accordingly and submit them. Once an ISOC URB comes 
> back the system calls my completion handler, I process it and resubmit it. Standard procedure. The problem is that 
> sometimes, URBs don't come back (or seems so). Is it possible that I'm using the wrong alternate setting? I have to 
> check this...

Certainly it's possible that you're using the wrong altsetting, but 
that shouldn't prevent your URBs from coming back.  Have you tried 
using usbmon to see what's happening?  Instructions are in the kernel 
source file Documentation/usb/usbmon.txt.

Alan Stern


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