Wolfgang Mauerer wrote: > Gilles Chanteperdrix wrote: >> Wolfgang Mauerer wrote: >>> (This is the Xenomai part of the mechanism, please see the ipipe >>> mailing list for the patches that provide the required basis >>> infrastructure) >>> >>> This patch series extends Xenomai with a new clock, CLOCK_HOST_REALTIME. >>> It allows for sharing NTP-corrected real time timestamps between >>> Linux/ipipe and Xenomai. The data are also available in userspace and can >>> be read without switching to kernel mode. Notice, however, that the new >>> clock only enables to read to time, but cannot serve as a full time basis. >>> Some changes to the ipipe layer are required as basis. >>> >>> In contrast to the initial approach, we don't use a transactional mechanism >>> to copy the information over from Linux, but use classical synchronisation. >>> The code can be compiled in conditionally for both, ipipe and Xenomai. When >>> disabled by architectures that don't support apt clock sources, there is >>> no runtime-overhead associated with the feature. >>> >>> Some points that may require further discussion: >>> >>> - POSIX only specifies a few clock_ids, and these have already been >>> extended by the Linux kernel. We use the maximum id (16) for the new >>> clock, but it might also make sense to use 7 (CLOCK_MONOTONIC_COARSE+1) >>> or 4 (CLOCK_THREAD_CPUTIME_ID+1). >>> >>> - The current implementation deals with x86_64's TSC. Support for other >>> architectures can be added. Additionally, the user has to make sure that >>> the TSC clock source remains active once selected. To implement >>> deactivation (e.g., when the Linux clock source is changed), more >>> ipipe hooks would be required, though. >>> >>> There are two alternatives including other architectures: >>> >>> * We can create a new clocksource that abstracts the per-architecture >>> differences, and use this clocksource as basis for >>> Xenomai. Essentially, this means mapping all desired >>> non-x86-Clocksources to the interface offered in this patch. >>> This requires more changes in the ipipe layer than variant B, namely, >>> * We can create a union in struct xnvdso of all arch-specific clock >>> datasets and introduce feature flags like XNVDSO_FEAT_HOSTRT_X86, >>> XNVDSO_FEAT_HOSTRT_WHATEVER. The reader-side code then needs to >>> match the data provided, which requires more changes on the >>> Xenomai side. >> The dataset is the same on all architectures, since we provide the same >> "clocksource abstraction" on the 5 architectures we support: a TSC >> emulation. So, a simple approach is simply cut-and-paste the x86 >> update_vsyscall code for other architectures, another approach is to put >> this code in a wrapper which we call on all architectures. > > this disables the possibility of using non-TSC time sources that > can also be accessed from userland. That's certainly not a requirement > for us, I'd just like to mention it.
This would only make sense on x86, as other architectures seem to have only one clock-source. And since other clocksources than TSC have a higher overhead, I do not think it is likely that we would like to use them. Anyway, we can start with TSC and if the need arises, fix that later. > > We also need to ensure that the host has not switched away from TSC > because in this case, the NTP correction values delivered from Linux > are for a different clocksource than the one used by Xenomai. > So we need to detect a switch from TSC to non-TSC, which can only be > done in arch-specific code -- I don't see a generic way to know which > clocksources are based on the TSC and which one are not. Well, all architectures call ipipe_request_tickdev, so, we can check if the clock pointer passed to update_vsyscall is the same as the one returned by ipipe_request_tickdev. Do not know if this is really possible, I would need to check more closely how ipipe_request_tickdev works. >> This update_vsyscall code would call ipipe_dispatch_event to pass the >> data to Xenomai. >> > okay. That seems to have dropped from my radar, as I don't recall having > any issues with this requirement. I am sure that I always thought that it should be done this way. But maybe I did not say it... Sorry if that is the case. -- Gilles. _______________________________________________ Xenomai-core mailing list Xenomaifirstname.lastname@example.org https://mail.gna.org/listinfo/xenomai-core