[sane-devel] Re: HP SJ5300C

[Major A]

> 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).

Given that USB 1.1 has a maximum payload of 64 bytes per bulk packet,
I don't see a problem with 16kB or even 4kB buffers. Are you sure this
is what causes the slowdown?

> 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.

Good that you mention this -- I'm using libusb for another project
where throughput is really important. I'll have to look into the way
libusb chains URBs.

  Andras

===========================================================================
Major Andras
    e-mail: andras at users.sourceforge.net
    www:    http://andras.webhop.org/
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