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
> linux-usb@suse.com mailing list.
>
> ~Randy
>
> > -----Original Message-----
> > From: Karel Srsen [mailto:Karel.Srsen@asicentrum.cz]
> >
> > 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 Karel.Srsen@asicentrum.cz please.
> >
> > Karel
>
>
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>
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