Unfortunately this wasn't the case with me. I wrote the original patch
to do this (which is the correct thing to do according the USB spec).
The problem I was having was the device I was using didn't like it
sending 8 byte requests when the maximum packet size of the pipe was
larger. The device thought it was a short read and then terminated the
transfer, or more specifically, barf when it saw the next IN since it
was expecting a status packet.
Trying to read more data than there is descriptor data should be safe
since the last packet will be treated as a short read and the connection
will end per the spec.
Atleast, Windows tries to read too much and it works.
JE
On Tue, Apr 11, 2000, Nick Hibma <[EMAIL PROTECTED]> wrote:
>
> Um, not quite. The reason that one reads the first 8 bytes first is that
> some devices go pop if you read let's say 128 bytes while there is only
> 18 bytes in descriptors.
>
> Reading the first 8 bytes and then the the correct number of bytes
> avoids this problem.
>
> Normally one would read the whole set of descriptors in one go with a
> read for 256 bytes or so and chunk it into 8 byte packets.
>
> Nick
>
>
> On Tue, 11 Apr 2000, Dunlap, Randy wrote:
>
> > Hi,
> >
> > When usb_new_device() is called, the maxpacketsize for the
> > default control pipe is not known, so it's not safe to read more
> > than 8 bytes of the device descriptor. Since bMaxPacketSize0
> > (max packet size for the default control pipe) is in this first
> > 8 bytes, the code can read this byte and use it for future
> > reads of the control pipe. You can follow the code and see
> > that a few lines later it calls usb_get_device_descriptor().
> > This reads the entire descriptor.
> >
> > This shouldn't cause any problem(s) with the device you
> > are working with if it's built properly. If you see problems,
> > it could be a USB device silicon or firmware problem.
> >
> > For future linux-usb discussions, please use the
> > [EMAIL PROTECTED] mailing list.
> >
> > ~Randy
> >
> > > -----Original Message-----
> > > From: Karel Srsen [mailto:[EMAIL PROTECTED]]
> > >
> > > Hello,
> > > is there somebody who understands linux USB driver ?
> > > I have one question:
> > > there is following piece of code in drivers/usb/usb.c (2.3.50 kernel):
> > >
> > > int usb_new_device(struct usb_device *dev)
> > > {
> > > ...
> > > ...
> > > err = usb_get_descriptor(dev, USB_DT_DEVICE, 0,
> > > &dev->descriptor, 8);
> > > if (err < 8) {
> > > ...
> > > ...
> > > }
> > >
> > > that's nice but in fact USB device descriptor's length is not 8 bytes
> > > but 18 (0x12). Does it means that driver reads only leading 8 bytes
> > > instead of whole descriptor ? I thing it's wrong, isn't ? Can it cause
> > > any error ?
> > >
> > > I haven't subscribed this mailing list. If you decide to answer, send
> > > reply to [EMAIL PROTECTED] please.
> > >
> > > Karel
> >
> >
> > ---------------------------------------------------------------------
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> > For additional commands, e-mail: [EMAIL PROTECTED]
> >
> >
>
> --
> [EMAIL PROTECTED]
> [EMAIL PROTECTED] USB project
> http://www.etla.net/~n_hibma/
>
>
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