Re: [Libusb-win32-devel] Limit on Maximum Asynchronous URBs

[Stephan M. <ste...@we...>]

Libusb's kernel driver limits the maximum data size to 64k for
each URB although Windows' host controller drivers support
larger sizes.

See: http://support.microsoft.com/default.aspx=3Fscid=3Dkb;%5BLN%5D;832430



> Thank you Stephan.  That information is useful.
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> One question though.  I wasn't aware that the maximum URB size was 64 kB=
 in=20
> Windows.  Is this true=3F  In Linux I believe the maximum is 16384 (and on=
ly=20
> with Kernel versions 2.6 and up).
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> Gopal
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> On Tuesday 13 June 2006 10:18, Stephan Meyer wrote:
> > Libusb's kernel driver doesn't have any built-in limitations because
> > it doesn't internally queue any data. It just allocates an URB for eac=
h
> > userspace request and passes that URB down the stack.
> >
> > But it should be obvious that the host controller driver has to queue
> > these request and that the size of this queue is limited since kernel
> > memory is a limited resource.
> >
> > I wrote a small test program (see attachment) to see how many requests=

> > Windows' USB stack can handle at once. And the limit on WinXP-SP2
> > (with 1GB of RAM) seems to be exactly 2500 URBs regardless of the URB'=
s
> > data size.
> >
> > The maximum amount of data I was able to request at once with the test=

> > program was 2500 URB each with 64kB of data. That's more than 150MB!
> >
> > This limit may depend on the host controller driver and the Windows
> > version.
> >
> >
> > Stephan
> >
> > > Hi all,
> > >
> > > I did some poking around on the web but have had a hard time finding=
 an
> > > answer to this question.  Maybe someone here knows.
> > >
> > > Is there a maximum number of URBs that can be submitted to an endpoi=
nt /
> > > interface / device at any given time=3F  For example, can I submit 102=
4
> > > URBs each with 16 KB data to a read endpoint or will I be restricted=
 by
> > > the driver stack, host controller, or something else=3F  If there is s=
uch a
> > > restriction, does it apply to the device as a whole, just the interf=
ace,
> > > or just the endpoint=3F
> > >
> > > Any insights would be much appreciated.
> > >
> > > Best regards,
> > > Gopal
> > >
> > >
> > > =5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F
> > > Libusb-win32-devel mailing list
> > > Lib...@li...
> > > https://lists.sourceforge.net/lists/listinfo/libusb-win32-devel
> >
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