To: linux-kernel@vger.kernel.org
From: d@teklibre.com (Dave Taht)
Subject: Frontier USB Drivers (Was: Staging tree status for the .32 kernel merge)
Date: Sat, 12 Sep 2009 13:40:28 -0600
Organization: Teklibre - http://www.teklibre.com
Message-ID: <8763bodk9f.fsf@mahal.sjds.teklibre.org>
References: <20090903041429.GA29875@kroah.com>
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Greg KH <greg@kroah.com> writes:

> 	- frontier.  A nice driver, again, should not be hard to get
> 	  merged into the main tree, if someone wants an easy project...

Thank you for your kind words.

Mild correction, it's actually two drivers, one for the Frontier Designs
Tranzport, the other is for the Frontier Designs Alphatrack. The
interfaces are very similar in most respects.

I would certainly like to see the frontier drivers escape staging and
become part of the regular kernel. The tranzport sub-driver in
particular, has been in daily use in my studio for nearly two years now,
and people like Dave Phillips are also happy with it. (at least the
kernel portion, userspace needs more work)

In looking over the drivers/staging/frontier/TODO list for these
drivers, I would love to get some help and feedback on the remaining,
minor, problems:

	- review by the USB developer community

DEEPLY Desired, see below for details. 

- Missing usb ids for lsusb.

Although I had submitted the requisite usb ids to the maintainer
I'm not sure if they ever made it into the kernel. I am not presently
tracking the tip of kernel development however.

- Stolen USB minor device numbers

I reused some uncommon usb minor numbers in these drivers. Presumably
new ones need to be applied for or has the world gone dynamic these days?

Haven't the foggiest idea how to apply for minor numbers. I just googled
for the right procedure. Oh, wait, there's this greg KH guy that assigns
those. Greg!  Need two minor numbers for these devices! We've talked
about it, a couple times...

It's possible to plug more than one Alphatrack into a system. I'm not
sure how that's supposed to work, numberwise.

	- checkpatch.pl clean

I submitted patches to this during the last kernel round that made the
alphatrack and tranzport drivers checkpatch.pl clean for all except one
error message that I didn't understand, which I noted at the time.

$ /home/d/src/git/linux-2.6/scripts/checkpatch.pl --file alphatrack.c
WARNING: mutexes are preferred for single holder semaphores
#681: FILE: alphatrack.c:681:
+	init_MUTEX(&dev->sem);

total: 0 errors, 1 warnings, 895 lines checked


***But it IS a single holder semaphore!***

	- sparse clean

I don't know a darn thing about sparse. I'm willing to learn but...

	- possibly just port to userspace with libusb

No. The original version of this was written for libusb. Unless libusb
has improved dramatically, it was simply incapable of keeping up with
interrupts, particularly the high number of interrupts the scroll wheel
would generate. When you have a keyboard-like device, missing one key-up
event is disastrous. When the tranzport is controlling a real-time audio
session, with live musicians and backing tracks all playing at once,
when you hit that key and spin that knob it better do what you expect,
every time. Thus, these became kernel drivers.

I also tried writing this in UIO, but that lacked adequate support for
USB disconnect events. Thus, these drivers became very small drivers
that merely handled interrupts, usb related events, and kept a
ringbuffer around of backlogged events.

I wrote these drivers back around 2.6.17 and today is the first time
I've subscribed to read lkml since 2.6.22....

	- fix userspace interface to be sane

This is the biggest open question I have. Right now the drivers just
handle the interrupts, and queue up the data in a ringbuffer, sending the
events in the exact same format as received from the device, and vice versa.

Coming up with an abstract means to handle a very specialized device - a
control surface that simultaneously is a LCD screen, a bunch of LEDS, a
scroll wheel, and 20+ buttons that can be pressed in various
combinations, would kind of involve reinventing a char I/O based X11,
and I don't really feel that much or any of that needs to happen in
kernel space.

(the alphatrack is worse, it has a slider with feedback and buttons that
have touch sensitivity, and a special key that remaps the sliders and
buttons to another keymap entirely)

There are a lot of "surfaces" out there in the music world, with all
kinds of combinations of buttons and knobs and gee-gaws, etc, but coming
up with adaquate abstractions for them all is an effort on the scale of
X11, with a considerably smaller user-base, and more rapidly shifting
market.

Far better, I thought, to just handle the RT critical portions of the
code in the kernel and hand off the rest of the chaos to userspace.

But certainly, some guidance and thought on this matter would be welcomed.

	- review by the USB developer community

I'd like that. My test or production userspace code doesn't test poll
for example. My inexperienced eyeballs look at the poll stub and say
that it can't possibly work, and I worry that there are situations
involving suspend or hubs or inotify or some other desirable set of
usb calls, or something that has changed since I wrote the thing back in
2.6.17 days. that actually matters in newer versions of the kernel.

So if someone truly expert in USB would PLEASE review this code I'd
appreciate it very much. I can be available via irc or email for further
discussions, and I will track lkml for as long as it takes.

Stuff that is not in the TODO that needs to be done.

Documentation 

udev line

-- 

Dave (AKA "Mike") Taht 
Postcards from the Bleeding Edge (http://the-edge.blogspot.com)

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