Jan Kiszka escreveu:
I'm not very sure about that. It can work for most situations but there
could be one situation where it would crash just because it was chosen
to have a little smaller code size and a little speed gain... I would
not like to think in the consequences of a crash in the driver while a
RT-system is working on real world...

And I would not like to think of having instrumented RTOS cores and a
drivers in the field. This would rather indicate to me that someone did
not do his/her homework: testing and debugging the system before
delivering it.
Ok, I'm not really interested on this topic for continue discussing it. It is fine for me to do that checks on my own code before calling rtdm_mmap_to_user...
Via any invocation, just check the handler prototypes:
rtdm_open_handler_t, rtdm_close_handler_t, rtdm_ioctl_handler_t, ...
Now I've found them! I think I was a bit lazy at that time... :) Thanks!

Yes, but it would be simpler for new developers used to the V4L2 API to
migrate to a similar RT-Video interface. Anyway, as I've already said, I
don't think some users would do any mmap during runtime, but maybe for
some reconfiguration for some reason and they would like to make this in
a RT-context. I can't really imagine a practical situation from the
moment, but I think it is still possible to happen...

Ok, but then you can still offer mmap via secondary mode even to RT
But I would lose the determinism...
It's a corner case, and mmap will never have a deterministic
execution time without changing the vmm rather fundamentally, so the
user can't expect such behaviour. If (s)he does so, the application
design is broken anyway.
I agree. That is why I'm trying to provide an alternative solution. It is not quite a mmap, but it is similar...

That is something I could change in my driver... Making the mmap on the
VIDIOC_REQBUFS ioctl request. But, as a driver could return a different
number of buffers than the requested, the user could not be satisfied
with the returned buffers and the mmap would have been useless... I
still don't know how will the final design be like, but I'm fine with
your already provided rtdm_mmap_to_user solution. But I still want to
keep the API as closer as possible to V4L2 for facilitating the users to
migrate to real-time while taking advantage of the already available
V4L2 documentation and examples all over the web and just doing the
minimal needed modifications.

Ok, I see the idea behind it, and under those constraints it does make
sense to provide some wrapper for mmap over the RT capturing device.

I think I now got your goals completely:

 1. keep the interface of V4L2
... as closer as possible.
 2. completely provide it a deterministic way
I'm not really worried about that, but if I could do it in some not very sofisticated way, I would like to provide such behaviour. But I recognize that for must situations the user won't need deterministic mmap.
I can understand the first goal. Keeping the V4L2 interface for a
potential RTDM frame grabbing device profile can make sense if it
doesn't complicate the rt-driver internals too much or stresses it

But I still have some doubts that aiming for 2. regarding mmap is worth
the effort. The problem is that you cannot predict how many buffers have
to be pre-mapped in order to provide them with bounded delay via some
rt-mmap. I wouldn't spent time on this.
This could be another module parameter. If the user is not satisfied with the default limits, (s)he could override them. But I really don't pretend to spent more time on this... I'm very worried about my deadline... :( I still have too much to do in 4 months... :((
Ok, but even if you decide to let rt-mmap be non-deterministic, you
still need some means to prevent the scenario you described above.
Actually, all you need is some callback when the mapped memory block was
actually released (munmap/application exit). Such a callback can be
registered inside the rtdm_mmap handler as a vma_fileop.
I have never worked with vma_fileop... I would need to learn it first.
 It will run in
non-RT, and could be used to mark the related hardware buffer as finally
free for re-allocation.
Now, I did realize that there is one more problem on my current design. If the user application exits or is terminated, I'm not sure if the close handler is called if the user forgot/was not able to. If it is not, the buffer would be marked as used until I reloaded the driver... Is the close handler invocated on application termination?
I will draft some extension of the current
rtdm_mmap_to_user function, but likely not before tomorrow evening or so.
Thank you.



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