Messages in this thread | | | Date | Sat, 25 Nov 2006 21:42:18 +0100 | From | "Ilyes Gouta" <> | Subject | [USB] urb->number_of_packets = 256 ! |
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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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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