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SubjectRe: Stupid user with user-space questions, matrix LED driving with user space code only.
On Sun, 2013-02-17 at 18:05 -0800, Greg KH wrote: 
> On Sun, Feb 17, 2013 at 02:37:24PM +0000, Jonathan Andrews wrote:
> > From a user perspective it seems a bit crap to have to change the kernel
> > if you have a workload that preemption is harmful to.
> > In the case of something like the Raspberry Pi changing the kernel if
> > the distribution has not done the work for me sounds like real effort.
> > The kernel is tied to binary obscurity from broadcom... To build I need
> > a working cross compiler, toolchain, kernel sources, Pi specific patches
> > then to get everything in the correct place on an SD card containing two
> > filesystems. Its possible but its not going to "just work" at my skill
> > level....
> As you can not boot a kernel on the RPI platform just yet,
> there's very little that the community can do here to help
> you out.
Somebody could stick in the code to enable/disable preemption at runtime
on PREEMPT compiled kernels :-) - Then all I have to do is wait for it
to filter down to the Raspian kernel maintainers, i'm patient ;-) ?

In all seriousness I assume preemption has a timer and an interrupt
vector, cant the timer simply be enabled/disabled leaving just the
kernel call mechanism untouched. IE a "preemption" kernel that is now
not preempting ...... OK, OK - I have many other egg sucking suggestions
but am I the only one who wants the functionality ?

Seems a half step to add all this wizzy real-time code to the kernel
then stop users doing real-time in user space by allowing the schedular
to yield during what a user wishes to be a citical section (from a
timing point of view not the atomic point of view).

What about a yield alignment mechanism for user space. IE the process
calls the kernel with a request "schedule me first after a yeild" - then
the process at least has whatever the timer granularity is to do
something timing critical... add a flag to ignore or defer interrupts
and you have a semi 'hard-realtime' behaviour for user space, allowing
user space to grab small chunks of real time. Yes a nasty looking
facility for SMP intel servers but really useful for embedded.

> I suggest you go take this up with the developers whom you got
> this specific kernel build from, there's nothing we can do here about
> it.
I suspected it was not going to be simple. As I suspect my feature
request is not that simple but if you don't ask........


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