Very Computer

by Ma » Fri, 30 Apr 2004 20:54:39

I want to synchronize a user space thread to an external event that
generates an interrupt. I thought of using the following approach: the
ISR that treats the interrupt does a quick processing (such as data
capture and buffering) and then sends data to the user space thread
through a message queue or signals to the thread waiting on a
semaphore and it gets the buffered data from shared memory or some
similar mechanism. So far I haven't succeded in finding a way to do
it. I've tryed to write a module which uses semaphores (<linux/sem.h>)
but aparently there is no way to use SysV IPC primitives in kernel
modules.

I would appreciate having hints on how to do it or pointers to
documentation and example code if available.

Thank you very much in advance for your help.

Elder.

by **Michael N. Mora** » Fri, 30 Apr 2004 21:41:55

> I want to synchronize a user space thread to an external event that
> generates an interrupt. I thought of using the following approach: the
> ISR that treats the interrupt does a quick processing (such as data
> capture and buffering) and then sends data to the user space thread
> through a message queue or signals to the thread waiting on a
> semaphore and it gets the buffered data from shared memory or some
> similar mechanism. So far I haven't succeded in finding a way to do
> it. I've tryed to write a module which uses semaphores (<linux/sem.h>)
> but aparently there is no way to use SysV IPC primitives in kernel
> modules.

> I would appreciate having hints on how to do it or pointers to
> documentation and example code if available.

> Thank you very much in advance for your help.

> Elder.

A user-space signal is the equivalent of an interrupt.
When user-space drivers are involved, I like to use
"send_sig_info()" to send a signal to the user-space
task to indicate changes in the state of the driver.

I like to model the user/kernel space interface to
look like a hardware device in terms of interrupt/signal
handling, with ioctl's to acknowledge/enable/disable
signals/interrupts from the driver. However, I'm an
embedded type that is used to that kind of thing ;-)

Getting the data to/from kernel space is another
issue, typically done by the read/write driver
interface, or by using mmap().

--
Michael N. Moran (h) 770 516 7918
5009 Old Field Ct. (c) 678 521 5460
Kennesaw, GA, USA 30144 http://mnmoran.org

"... abstractions save us time working, but they don't
save us time learning."
Joel Spolsky, The Law of Leaky Abstractions

The Beatles were wrong: 1 & 1 & 1 is 1

by **Herman Bruyninck** » Fri, 30 Apr 2004 22:45:56

Quote:> I want to synchronize a user space thread to an external event that
> generates an interrupt.

Maybe the ongoing work about D-Bus can give you inspiration:
<http://freedesktop.org/Software/dbus>
One of its motivations was/is exactly to be able to do more driver stuff in
user space, and I got the impression that that was what you are looking
for...

Herman
--
K.U.Leuven, Mechanical Eng., Mechatronics & Robotics Research Group
<http://people.mech.kuleuven.ac.be/~bruyninc> Tel: +32 16 322480

by **Dan Kege** » Sat, 01 May 2004 01:11:10

>> I want to synchronize a user space thread to an external event that
>> generates an interrupt. I thought of using the following approach: the
>> ISR that treats the interrupt does a quick processing (such as data
>> capture and buffering) and then sends data to the user space thread
>> through a message queue or signals to the thread waiting on a
>> semaphore and it gets the buffered data from shared memory or some
>> similar mechanism. So far I haven't succeded in finding a way to do
>> it. I've tryed to write a module which uses semaphores (<linux/sem.h>)
>> but aparently there is no way to use SysV IPC primitives in kernel
>> modules.

> A user-space signal is the equivalent of an interrupt.
> When user-space drivers are involved, I like to use
> "send_sig_info()" to send a signal to the user-space
> task to indicate changes in the state of the driver. ...

> Getting the data to/from kernel space is another
> issue, typically done by the read/write driver
> interface, or by using mmap().

What kind of time resolution do you need?

One other option is to use netlink sockets. They're
pretty easy. One drawback is netlink socket addresses
are somewhat precious, a little like signal numbers.

- Dan

by **Wolfgang Müe** » Sat, 01 May 2004 02:32:08

Hello Elder,

Quote:> I want to synchronize a user space thread to an external event that
> generates an interrupt. I thought of using the following approach: the
> ISR that treats the interrupt does a quick processing (such as data
> capture and buffering) and then sends data to the user space thread

- Let your driver register a character interface
- from your user space thread, do a synchronous read.
- in your interrupt routine, copy the data and wakeup the
waiting thread.

Very common situation. Works without any problem for me.

best regards

Wolfgang Mes

by Ma » Sat, 01 May 2004 21:22:18

> What kind of time resolution do you need?

I presume this question was asked to me.

I am just evaluating how well would Linux (either 2.4 with preemption
and low latency patches or 2.6 with preemption enabled) perform for my
application. Worst case interrupt latency of 400us and thread
activation latency of 4ms are more than acceptable in my application.

Regards.

Elder.

by Ma » Sat, 01 May 2004 21:27:17

> > I want to synchronize a user space thread to an external event that
> > generates an interrupt.

> Maybe the ongoing work about D-Bus can give you inspiration:
> <http://freedesktop.org/Software/dbus>
> One of its motivations was/is exactly to be able to do more driver stuff in
> user space, and I got the impression that that was what you are looking
> for...

> Herman

I had a quick look at it. My requirements are much more modest and I'd
rather use native linux resources instead. Still I will look at its
code for some tips.

Elder.

by **Chesney Chris** » Sun, 02 May 2004 06:09:27

X-No-Archive:yes

Quote:>I would appreciate having hints on how to do it or pointers to
>documentation and example code if available.

What you'll need to do is create a device driver with an entry in /dev,
and then set up a poll()/select() call within it. These can be used to
inform a user space application that some data has arrived to be
processed.

Your user space code can open the device and use the above functions
which among other things provide the ability to block a thread until an
interrupt is received, etc. Much more elegant than signal processing.

The free book "Linux Device Drivers" covers this subject in detail.

--

"Jokes mentioning ducks were considered particularly funny." - cnn.com

by **Chesney Chris** » Sun, 02 May 2004 06:11:01

X-No-Archive:yes

Quote:>> What kind of time resolution do you need?

>I presume this question was asked to me.

>I am just evaluating how well would Linux (either 2.4 with preemption
>and low latency patches or 2.6 with preemption enabled) perform for my
>application. Worst case interrupt latency of 400us and thread
>activation latency of 4ms are more than acceptable in my application.

Supposedly the low-latency patch can give worst-case latency of around
150us, but you'd need to check out how this fits on your architecture.

--

"Jokes mentioning ducks were considered particularly funny." - cnn.com

by **Michael Schnel** » Tue, 04 May 2004 19:34:00

Quote:
> I want to synchronize a user space thread to an external event that
> generates an interrupt.

Look at rtc.c in the Kernel source. They provide a blocking read to the
application to have it wait for an event. Her some data is provided to
the application, too.

-Michael

by **Michael Schnel** » Tue, 04 May 2004 19:35:35

Quote:> Supposedly the low-latency patch can give worst-case latency of around
> 150us, but you'd need to check out how this fits on your architecture.

Nope. Linux can't guarantee any worst-case latency at all (I did find
more than 100 msek with the low-latency patch in very rare instances).

-Michael

by **Chesney Chris** » Tue, 04 May 2004 22:05:14

X-No-Archive:yes

A certain Michael Schnell, of comp.arch.embedded "fame", writes :

Quote:>> Supposedly the low-latency patch can give worst-case latency of around
>> 150us, but you'd need to check out how this fits on your architecture.

>Nope. Linux can't guarantee any worst-case latency at all (I did find
>more than 100 msek with the low-latency patch in very rare instances).

That's why I was quite careful not to use the word "guarantee".

Clearly if you need hard real time performance, you need to look at
another OS.

--

"Jokes mentioning ducks were considered particularly funny." - cnn.com

by **Guy Maco** » Wed, 05 May 2004 00:57:20

Quote:
>A certain Michael Schnell, of comp.arch.embedded "fame", writes :

>>> Supposedly the low-latency patch can give worst-case latency of around
>>> 150us, but you'd need to check out how this fits on your architecture.

>>Nope. Linux can't guarantee any worst-case latency at all (I did find
>>more than 100 msek with the low-latency patch in very rare instances).

>That's why I was quite careful not to use the word "guarantee".

>Clearly if you need hard real time performance, you need to look at
>another OS.

"Hard real-time means no surprises or silent failures as system
configuration changes or load increases. Deadlines will still
be met. For example, the worst case delay on a 1 millisecond
thread on the HP Pavilion Notebook (AMD K7) is 12 microseconds.
-http://www.fsmlabs.com/products/rtlinuxpro/

"RTAI's microkernel approach guarantees that the data-acquisition
task will take place on schedule, even while the previously acquired
and calculated result is written to disk."
-http://www.elecdesign.com/Articles/Index.cfm?ArticleID=3816&pg=2

"Lineo Solutions, Inc. (Lineo) demonstrated ... hard real-time
support with Linux .... Deterministic time metrics of the systems
demonstrated ... is as follows.

Interrupt Response Time 5 microseconds

Task Latency Time 4 microseconds

Periodic Task Latency Time 20 microseconds

Timer Clock Accuracy 1 microsecond, Jitter 1 microsecond)"

- http://www.lineo.co.jp/eng/news-events/press-release/20040109.html

"RTAI - the Realtime Linux Application Interface for Linux ... lets
you write applications with strict timing constraints for your
favourite operating system."
-http://www.aero.polimi.it/~rtai/

"KURT-Linux: Kansas University Real-Time Linux: Microsecond timing
resolution and event-driven real-time scheduling..."
-http://www.ittc.ukans.edu/kurt/documents-noframes.html

--
Guy Macon, Electronics Engineer & Project Manager for hire.
Remember Doc Brown from the _Back to the Future_ movies? Do you
have an "impossible" engineering project that only someone like
Doc Brown can solve? My resume is at http://www.guymacon.com/

by **Chesney Chris** » Wed, 05 May 2004 04:12:47

X-No-Archive:yes

A certain Guy Macon, of comp.arch.embedded "fame", writes :

Quote:>"RTAI's microkernel approach guarantees that the data-acquisition
>task will take place on schedule, even while the previously acquired
> and calculated result is written to disk."
> -http://www.elecdesign.com/Articles/Index.cfm?ArticleID=3816&pg=2

http://www.aero.polimi.it/~rtai/documentation/articles/guide.html

"What is RTAI?

RTAI means Real Time Application Interface. Strictly speaking, it is not
a real time operating system, such as VXworks or QNX. It is based on the
Linux kernel, providing the ability to make it fully pre-emptable. "

Your serve.

[BTW, isn't RTAI a kernel - a microkernel - all by itself ? You could
say that this isn't actually Linux. Linux runs essentially as a process
above RTAI. This isn't necessary in an OS such as, for example,
Greenhills Integrity, which provides guaranteed latency and other
goodies.]

--

"Jokes mentioning ducks were considered particularly funny." - cnn.com

Very Computer

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux

Synchronizing user space threads with kernel space in linux