Re: Very high data rate isochronous USB device ?

Le Monday 29 January 2007 19:50, Thierry Herbelot a écrit :
> Hello,
>
> I'm writing a USB device driver for a high-data rate device, for which I
> would like to use isochronous transfers (I already have working bulk
> transfers), but I can't get the expected (very high) data bandwidth.
>
> The situation today is following :
> - the device uses a High-speed USB 2.0 connection
> - the device advertises one isochronous Endpoint, with up to 3 packets of
> 1024 bytes per micro-frame, which corresponds to the designated target rate
> of around 200 Mbps (the endpoint descriptor has been checked and rechecked,
> and lsusb seems to acknowledge these parameters)
> - the PC I use as host has three EHCI USB bridges : one is on the
> motherboard, and comes from Intel (it is a recent ICH6), and two
> additionnal PCI boards use chipsets from VIA and Ali
> - I am using a quite recent linux kernel (2.6.18, coming with with OpenSuse
> 10.2)
> - I have copied the urb and iscochronous packet initialization and the
> completion routines from existing device drivers (one example I followed is
> drivers/media/dvb/b2c2/flexcop-usb.c)
> - like in flexcop-usb.c, I have initialized the URBs with many packets
> (urb->number_of_packets is now a parameter of my device driver, and current
> values are 8 or 16)
> - like in flexcop-usb.c, I have a big buffer cut in 8 or 16 chunks of
> 1kbyte, with offsets of 1 kbyte, one chunk for each packet of the URB
> - like in flexcop-usb.c, the URBs are scheduled as fast as possible
> (urb->interval = 1;)
> - I can read some data in isochronous mode, but with a lower data rate than
> the target : I only get one IN token for each micro-frame, instead of the 3
> expected
> - I get the same behavior with all thre EHCI controllers (Intel, VIA and
> Ali) - I have had a quick look at drivers/usb/host/ehci-sched.c, where the
> scheduling occurs, but I could not determine from iso_stream_init(), for
> example, which parameters I should set to have a higher data rate
>
> Thus the question is : is there any driver in the linux source tree which
> can read more than one 1kbyte packet par micro-frame ? (or, which parameter
> set in flexcop_usb_transfer_init() should I change to attain a higher data
> rate ?)

(for the archive ?)

after some more homework, I have found one strange but working answer in the 
gspcav1-20070110 driver :
urb->iso_frame_desc[j].length for each "packet" should in fact be set to 
(number of packets expected per micro-frame)*(size of each packet), that is a 
hard-coded 3*1024 for my case.

the example computes in gspca_core.c::gspca_init_transfert() : 
	psize = le16_to_cpu(ep->desc.wMaxPacketSize);
	psize = (psize & 0x07ff) * (1 + ((psize >> 11) & 3));
then :
		urb->iso_frame_desc[fx].length = psize;

	TfH	

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