On Wed, Jun 15, 2005, Alan Stern <[EMAIL PROTECTED]> wrote:
> On Wed, 15 Jun 2005, Pete Zaitcev wrote:
> > Makes sense. I don't need an extra call, everything works as it is.
> > How about this:
> >
> > diff -urp -X dontdiff linux-2.6.12-rc6/drivers/usb/core/message.c
> > linux-2.6.12-rc6-lem/drivers/usb/core/message.c
> > --- linux-2.6.12-rc6/drivers/usb/core/message.c 2005-06-06
> > 13:44:37.000000000 -0700
> > +++ linux-2.6.12-rc6-lem/drivers/usb/core/message.c 2005-06-15
> > 12:13:52.000000000 -0700
> > @@ -658,8 +658,12 @@ static int usb_string_sub(struct usb_dev
> > {
> > int rc;
> >
> > - /* Try to read the string descriptor by asking for the maximum
> > - * possible number of bytes */
> > + /*
> > + * Try to read the string descriptor by asking for the maximum
> > + * possible number of bytes.
> > + * A request for an odd number of bytes of a UTF-16 string
> > + * may seem odd, but this is what Windows does, so let's be safe.
> > + */
> > rc = usb_get_string(dev, langid, index, buf, 255);
> >
> > /* If that failed try to read the descriptor length, then
> >
> > Signed-off-by: Pete Zaitcev <[EMAIL PROTECTED]>
>
> That looks good. Someone at Microsoft may have been thinking of the
> bLength value in the descriptor header. Since it's a single byte, it
> can't hold anything larger than 255.
I can't say why Windows went with a value of 255 here, but I can say
that I have received a couple of reports of devices that only treat the
length as an 8 bit field. I suspect the behavior of these devices is
specifically because Windows only reads 255 bytes.
libusb used to use 256 byte reads as well and since 256 is actually 9
bits, it caused some devices to think we were requesting 0 bytes.
So there is real world evidence that using a read of 255 is the best for
compatibility :)
JE
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