[LinuxPPS] multiple access to /dev/ppsN entries

[Ben Gardiner]

Hello all,

I am in the midst of designing a timing subsystem for a linux based project.
I think that the LinuxPPS project is a very useful addition to the kernel
and I would like to incorporate it into this design. First, my thanks to you
for creating and especially pushing this code upstream.

I _think_ that I need to have multiple processes accessing the same
/dev/ppsN entry -- I'm hoping that you will offer your expert opinion as to
whether this is possible.

It looks like the pps_cdev_open() in 2.6.33 (
http://lxr.linux.no/#linux+v2.6.33/drivers/pps/pps.c#L197 ) is not
implementing any access control and that the 'atomic_t usage' member of
pps_device ( http://lxr.linux.no/linux+*/include/linux/pps_kernel.h#L65 ) is
there to facilitate ref counting so that there can be multiple opens of
/dev/pps device nodes.

However, my experience with multiple access to a device node is non-existent
and I'm not in a position to experiment with LinuxPPS core yet. I would
apreciate your expert opinion as to whether multiple access from userspace
to the same PPS source is possible.

Best Regards,

Ben Gardiner
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