usb/110855: ugen: interrupt in msgs are truncated when buffer is full

[Markus Henschel]

The following reply was made to PR usb/110855; it has been noted by GNATS.

From: Markus Henschel <m.henschel at bally-wulff.de>
To: Hans Petter Selasky <hselasky at c2i.net>
Cc: freebsd-usb at freebsd.org,  freebsd-gnats-submit at freebsd.org
Subject: Re: usb/110855: ugen: interrupt in msgs are truncated when buffer
 is full
Date: Tue, 27 Mar 2007 11:30:25 +0200

 Hans Petter Selasky schrieb:
 > On Monday 26 March 2007 16:13, Markus Henschel wrote:
 >>> Number:         110855
 >>> Category:       usb
 >>> Synopsis:       ugen: interrupt in msgs are truncated when buffer is full
 >>> Confidential:   no
 >>> Severity:       serious
 >>> Priority:       medium
 >>> Responsible:    freebsd-usb
 >>> State:          open
 >>> Quarter:
 >>> Keywords:
 >>> Date-Required:
 >>> Class:          change-request
 >>> Submitter-Id:   current-users
 >>> Arrival-Date:   Mon Mar 26 14:20:04 GMT 2007
 >>> Closed-Date:
 >>> Last-Modified:
 >>> Originator:     Markus Henschel
 >>> Release:        6.2 custom kernel
 >>> Organization:
 >> Bally Wulff Automaten GmbH
 >>
 >>> Environment:
 >> FreeBSD freebsd-1.bally.de 6.2-RELEASE FreeBSD 6.2-RELEASE #11: Fri Mar 23
 >> 21:28:38 CET 2007    
 >> prog at freebsd-1.bally.de:/usr/obj/usr/src/sys/BALLYWULFF  i386
 >>
 >>> Description:
 >> We use ugen for some user space drivers. When an interrupt in endpoint is
 >> used ugen creates a queue that is filled by the kernel. The user space
 >> driver is responsible for reading data from the device file. If this
 >> happens too slow the queue is full and new msgs arriving from the usb
 >> device are lost. This behavior is OK.
 >>
 >> The problem is that the queue is not a multiple of the interrupt in
 >> endpoints msgs size. So it is possible that the last msg in the queue is
 >> truncated. This is very hard to detect for a user space driver. The data
 >> stream seen by the user space driver will contain an incomplete msgs
 >> directly followed by the next message without knowing truncation happened
 >> (except when using some data corruption detection mechanism).
 >>
 >> It would be much better if ugen would fill the queues of interrupt in
 >> endpoints until there is no more space for a complete msg. This way the
 >> user space driver will not loose sync with the incoming msgs.
 > 
 > The new USB stack has this fixed already. What I do is that the USB driver 
 > stops polling the interrupt endpoint when the user-land application does not 
 > read data. When the user-land application has read a packet, the interrupt 
 > endpoint is started again. The only problem is that some devices, like a 
 > Microsoft mouse I have, stops working immediately when its internal buffer 
 > overflows. Bad hardware design. But if your hardware is not like that, the 
 > new ugen, which is part of the new USB driver, will work great for you.
 > 
 > Also packet alignment is kept between reads: Only one packet per read.
 > 
 > See:
 > 
 > http://www.turbocat.net/~hselasky/usb4bsd
 > 
 > --HPS
 > 
 
 Thanks,
 
 I gave it a try and it seems to work fine :-). Could you please explain
 how reading an interrupt in endpoint works internally with the new ugen?
 Is there still a buffer that recieves data from the endpoint or is each
 read request from user land synchronously triggering a read data request
 on the interrupt endpoint? Why isn't O_NONBLOCK working anymore?
 
 
 
 -- 
 Regards,
 
 
 Markus Henschel
 Development
 
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