[CXP] Discussing the RTDM specification

Fino Meng fino.meng at linux.intel.com
Sat Dec 19 05:53:49 CET 2020

On Fri, Dec 18, 2020 at 03:19:44PM +0100, Philippe Gerum via Xenomai wrote:
> This wiki page [1] 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
>   move.


> - 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.
> - rtdm_mutex_timedlock() has dubious semantics. Hitting a timeout
>   condition on grabbing a mutex either means that:
>   * the mutex was already locked on entry to the call if timeout ==
>   * the mutex was not released within the allotted time, which had to be
>     long enough to prevent early shots. This means that something is
>     most likely going really wrong in the software, since a mutex is
>     supposed to cover fairly short, hopefully simple sections of code,
>     exhibiting an obvious exit path.
>   In the first case, we would be better off providing
>   rtdm_trylock_mutex() which has clearer semantics, starting from 3.3,
>   adding it to the CXP. The ship is most likely wrecked already in the
>   second case, so using a timeout condition as a way towards recovery is
>   unlikely to succeed at this point anyway.


rtdm timer can be used to implement some kind of timeout wakeup feature,
orthogonal to mutex API.

for beginners, simple rules are easy to remember:

spinlock: thread will busy waiting, no sleep, occopying CPU;
mutex: thread will go to sleep, abdicate CPU;

BR fino

> [1] https://gitlab.denx.de/Xenomai/xenomai/-/wikis/CXP_RTDM
> -- 
> Philippe.

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