[USB] urb->number_of_packets = 256 !

[Ilyes Gouta]

Hi!

I'm working on a driver for my USB 2.0 high-speed webcam under Linux
and I'm using a tool
called usbsnoop, which was designed for Windows, to get an idea on the exchanged
information between the host and the webcam. By examining the produced
trace file, I found
that my PC is sending a bunch of isochronous URBs to the webcam where
each one contains
256 isochronous packets and each packet is 3072 bytes wide. This means
that every URB
points to a 256 * 3072 bytes sized buffer.

Here is an excerpt of the trace file:

[21303 ms]  >>>  URB 1640 going down  >>>
-- URB_FUNCTION_ISOCH_TRANSFER:
 PipeHandle           = ff27dd8c [endpoint 0x00000081]
 TransferFlags        = 00000005 (USBD_TRANSFER_DIRECTION_IN,
~USBD_SHORT_TRANSFER_OK,
USBD_START_ISO_TRANSFER_ASAP
 TransferBufferLength = 000c0000
 TransferBuffer       = fd319100
 TransferBufferMDL    = 00000000
 StartFrame           = 00000000
 NumberOfPackets      = 00000100 // in hex
 IsoPacket[0].Offset = 0
 IsoPacket[0].Length = 0
 IsoPacket[1].Offset = 3072
 IsoPacket[1].Length = 0
 IsoPacket[2].Offset = 6144
 IsoPacket[2].Length = 0

I tried to program this behavior in my custom, alpha stage, kernel
driver, however I was
disappointed since Linux fails to allocate, using kmalloc(), such a
huge buffer (which is
quite normal actually) to associated to the URB through the
transfer_buffer field. I also
tried __get_free_pages(GFP_KERNEL, 10) without any success whatsoever.

Splitting the transfer across multiple URB doesn't seem to work (I didn't really
investigate in depth this possibility).

Any ideas?

Thanks for your time!

P.S: Is it possible to CC me since I didn't subscribe to the mailing
list? Thanks!

Best regards,
Ilyes Gouta.
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