[hpoj-devel] Comments on parallel performance, questions on USB

[<rj...@wo...>]

I  Parallel interface performance

A major factor in the poor performance of the new usermode parallel
interface is the use of usermode itself.  I've been through this
before.  You cannot get really good performance of ECP in usermode on
Linux.  Speeds up to 30 KB/sec are easy.  With great difficulty you can
push it to 60 KB/sec.

The 2.4 kernel (and 2.2 backports) provide the "ppdev" module for
implementing customized protocols like 1284.4 in usermode
applications.  It provides basic building blocks like "negotiate" via
ioctl()s.  It provides efficient I/O transfer through the ioctl()
modes and the read()/write() transfers.  I've built customized
protocols using ppdev and gotten over 350KB/sec transfer rates without
any special effort.

There may be some internal reason or Windows port reason for wanting the
whole thing in usermode, but switching to the use of ppdev lets the
development of the 1284.4 protocol remain usermode without losing the
performance of kernel drivers.  It also simplifies extending support
beyond the generic x86 parallel device world.

II Architecture Documentation

When you get around to documenting the structure it would help to have
a high level view of which parts perform what functions.  What is
mandatory for anything to work.  What are the dependencies.  I know
that reading the source eventually reveals this, but having a short
paragraph on each major part would help.

I am planning to attach a G55 to a headless Linux box that hides under
a desk.  I have no need or desire for cute display stuff.  It will
get in the way.  What I want is three things:
 1) a printing daemon with reasonable network status reports.
    Something like CUPS for instance.
 2) Good security.  I do not want anything to run as root.  (With
    ppdev the entire protocol can run as a spooler ID.  I just need to
    give that ID permission to read and write the parallel device.)
    Anything that runs as root requires extensive code review and I do
    not have unlimited time.
 3) A scan -> email.  This is actually where some of the front panel
    features could be nice.  If it lists the usernames that are
    members of a group (e.g. scan users) then the controls on the
    scanner could be used to select the username.  Then the scan can
    be turned into a mail attachment and mailed to that user.  (Again
    avoiding the need for anything to be root.)

This gives me a nice shared light duty printer/scanner/copier.  Which
is what the G55 is for.  I'm rummaging around through code and
emails to figure out how to put this together.

III USB

How stable is the USB these days?  Since I have no desire for cute
graphics, can I improve stability by leaving out some portions?  I
understand that part of the stability problem is inherent in some
buffering bugs internal to the G55.  These seem to be related to
query traffic, so things like eliminating the front panel emulation
might help a lot.

USB is more attractive from a performance and cabling point of view.
In my case, the headless machine parallel port is already in use for
something else, but it has spare USB ports.  So I'm gradually working
along the USB path.

At the moment the G55 is just a copier, attached to nothing, since its
previous Windows support host got moved.

R Horn