[CXP] Discussing the RTDM specification
jan.kiszka at siemens.com
Wed Dec 23 10:45:35 CET 2020
On 18.12.20 15:19, Philippe Gerum via Xenomai wrote:
> This wiki page  contains a draft proposal about specifying which
> services from the current RTDM interface should be part of the Common
> Xenomai Platform. Some proposals for deprecation stand out:
> - I suspect that only very few RTDM drivers are actually handling
> requests from other kernel-based drivers in real world applications,
> at least not enough to justify RTDM codifying these rare cases into a
> common interface (rtdm_open, rtdm_read, rtdm_write etc).
> In other words, although I would agree that a few particular drivers
> might want to export a couple of services to kernel-based clients in
> order to provide them some sort of backchannel, it seems wrong to
> require RTDM drivers to provide a kernel interface which would match
> their user interface in the same terms. For these specific cases, ad
> hoc code in these few drivers should be enough.
> Besides, I believe that most kernel->kernel request paths implemented
> by in-tree RTDM drivers have never been tested for years, if ever.
> Meanwhile, this kernel->kernel API introduces a basic exception case
> into many RTDM and driver code paths, e.g. for differentiating kernel
> vs user buffers, for only very few use cases.
> For these reasons, I would suggest to deprecate the kernel->kernel API
> from RTDM starting from 3.3, excluding it from the CXP in the same
That's fine with me. The idea was once that something like bus drivers
would appear, but that never happened.
> - RTDM_EXECUTE_ATOMICALLY() and related calls relying on the Cobalt big
> lock must go. For SMP scalability reasons, this big lock was
> eliminated from the EVL core, which means that all the attached
> semantics will not hold there. Serializing access to shared resources
> should be guaranteed by resource-specific locking, not by a giant
> traffic light like the big lock implements.
This is more complicated: RTDM_EXECUTE_ATOMICALLY was in fact deprecated
long ago, but users were migrated to cobalt_atomic_enter/leave which may
now make it look like we no longer need this. Maybe this is already the
case when using rtdm_waitqueue*, but let's convert everyone first.
> - rtdm_mutex_timedlock() has dubious semantics. Hitting a timeout
> condition on grabbing a mutex either means that:
I think you are missing the use cases:
(which goes long with timeout sequences)
In fact, all that could be replaced with
but that is what we want to replace as well...
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