thanks HPS, yes, the buffer size works well.
It was strange, because for other endpoints I don't see this kind of behavior.
BTW now the problem is solved.
Thanks again


On 06/27/11 11:19, Hans Petter Selasky wrote:
On Monday 27 June 2011 10:43:03 Luca Pizzamiglio wrote:
Hi USB list,

I've a little problem, but I'm not able to find a solution...
I've a generic custom USB device with several end points.
One of them (a bulk one) is read periodically (polled, 64 byte) every
second. There was some time inconsistencies, so I checked the USB
traffic with a sniffer, and I discovered that transactions are compacted
at the start of the loop in a block of 64 read (64 byte * 64 msg = 4096
byte). In other words, on the bus the first read is followed immediately
by the next 63 read.

I guess that this is a pre-fetch optimization for BULK end points. How
can I disable this optimization for this end point? How can I come back
to a more "real-time" behavior? I tried to play with open() flags
(O_NONBLOCK, O_DIRECT) without luck.

Using FreeBSD 7 with HPS stack, this behavior didn't happen. Every read
consists in 1 read on the bus and everything is fine.

Thanks in advance!


There is an IOCTL to set the RX and TX buffer sizes. Please use that. See
dev/usb/usb_ioctl.h. The default is more than one full packet.


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