device driver / char module interrupt vector -> user space code
Dear Kernel List,
I am writing a char module for a PCI stage controller and want to add the following functionality;
The device will generate an interrupt (or software trigger) and I want this to run a bit of user code with relatively latency. (<1ms). I am unclear how to do this while still separating the user from the kernel code and maintaining security - would this usually be handled by issuing a signal to the user space process? if so how and what latency can I expect?
Thanks matt
(more info: the stage controller moves little mechanical platforms in an experiment, as the platforms reach certain positions (determined by the hardware or the driver) user code must be run to perform some action (say take a measurement). The stages move continuously so timing errors map to positional errors. I could do all this in kernel space by linking the various drivers controlling the various hardware together under a super-driver for the entire experiment, however, it would be much nicer to achieve this with separate modules and user code).
Quoted 8 lines · SA
I am writing a char module for a PCI stage controller and want to add the following functionality;
The device will generate an interrupt (or software trigger) and I want this to run a bit of user code with relatively latency. (<1ms). I am unclear how to do this while still separating the user from the kernel code and maintaining security - would this usually be handled by issuing a signal to the user space process? if so how and what latency can I expect?
You could deliver a signal, or if appropriate you can have a system call that blocks until the IRQ. To get good reliable latency, mlockall() the process you need to be real time, and set it to a real time scheduling priority.
Quoted 10 lines · Alan Cox
Quoted 8 lines · SA
I am writing a char module for a PCI stage controller and want to add the following functionality;
The device will generate an interrupt (or software trigger) and I want this to run a bit of user code with relatively latency. (<1ms). I am unclear how to do this while still separating the user from the kernel code and maintaining security - would this usually be handled by issuing a signal to the user space process? if so how and what latency can I expect?
Quoted 4 lines
You could deliver a signal, or if appropriate you can have a system call that blocks until the IRQ. To get good reliable latency,
mlockall()the process you need to be real time, and set it to a real time scheduling priority.
If you noticed the discussion about signal delivery and context switch latency, a syscall (can be ioctl on special device) waiting for the event would be faster than signal.
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Jan 'Bulb' Hudec <[email protected]>
Quoted 10 lines · Alan Cox
Quoted 8 lines · SA
I am writing a char module for a PCI stage controller and want to add the following functionality;
The device will generate an interrupt (or software trigger) and I want this to run a bit of user code with relatively latency. (<1ms). I am unclear how to do this while still separating the user from the kernel code and maintaining security - would this usually be handled by issuing a signal to the user space process? if so how and what latency can I expect?
Quoted 4 lines
You could deliver a signal, or if appropriate you can have a system call that blocks until the IRQ. To get good reliable latency,
mlockall()the process you need to be real time, and set it to a real time scheduling priority.
To get <1ms latencies I suspect you will also need the preemtive kernel and lock-break patches (http://www.tech9.net/rml/linux/) and if you're running on an x86, changing the HZ value in linux/asm/param.h from 100 to 1000 will also be a good idea IMHO.
Of course, this requires re-compiling and replacing the kernel which may or may not be apropriate. AFAIK both the patches and the HZ value change are already in the 2.5.x development kernels already, but not everyone are running *that* version quite yet... :-D
Gilad.
Signature · 6 lines
Gilad Ben-Yossef <[email protected]> Code mangler, senior coffee drinker and VP SIGSEGV Qlusters ltd. "You got an EMP device in the server room? That is so cool." -- from a hackers-il thread on paranoia