[sane-devel] Re: HP SJ5300C

[Sergey Vlasov]

On Mon, 07 Apr 2003 17:28:30 +0200 (CEST)
Rene Rebe <rene.rebe@gmx.net> wrote:

> I wouldn't like to mess with scanner.c. libusb just works fine and is
> portable. You also mentioned that scanner.c might be deprecated for
> kernel 2.6 or so ...

At least with Mustek BearPaw 2400TA, there is a difference between using
scanner.c or libusb: libusb is slower.  Usually this does not matter -
but at some resolutions scanning becomes unusably slow with libusb (lots
of backtracking, even on Athlon XP 2000+), while the same settings with
/dev/usb/scanner0 gives at most 2-3 backtrackings.  This is because the
GT68xx chips have a very small buffer - only 16KB.

Looking at the code, I see that scanner.c uses 32KB buffer size for
read, but devio.c (which is used by libusb) uses one-page buffers (4KB
on x86).

At least with Linux this can be improved - even beyond the scanner.c
capabilities.  usbdevfs has USBDEVFS_SUBMITURB, which allow asynchronous
URB submitting, and even seems to allow queueing more than one URB for
the same pipe.  (The Windows kernel driver for GT68xx seems to use this:
the usbsnoop logs shows that it submits 5 URBs at once and then receives
the scanned data using them as a buffer ring.)  However, this feature is
not currently supported by libusb.