From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4BB50C8B.2000405@domain.hid> References: <4B97BA0C.9000702@domain.hid> <4B9AD0DE.4020103@domain.hid> <1268472523.27899.135.camel@domain.hid> <4B9BB9B1.5050003@domain.hid> <1268498034.27899.167.camel@domain.hid> <4B9C2100.6090806@domain.hid> <1268584465.27899.197.camel@domain.hid> <4BB4F857.5020906@domain.hid> <4BB50C8B.2000405@domain.hid> Content-Type: text/plain; charset="UTF-8" Date: Thu, 01 Apr 2010 23:22:20 +0200 Message-ID: <1270156940.2418.403.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-help] Analogy cmd_write example explanation List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Alexis Berlemont , xenomai@xenomai.org On Thu, 2010-04-01 at 23:13 +0200, Jan Kiszka wrote: > Gilles Chanteperdrix wrote: > > Philippe Gerum wrote: > >> On Sun, 2010-03-14 at 00:34 +0100, Alexis Berlemont wrote: > >>> Philippe Gerum wrote: > >>>> On Sat, 2010-03-13 at 17:13 +0100, Alexis Berlemont wrote: > >>>>> Hi, > >>>>> > >>>>> Philippe Gerum wrote: > >>>>>> On Sat, 2010-03-13 at 00:40 +0100, Alexis Berlemont wrote: > >>>>>>> Hi, > >>>>>>> > >>>>>>> Sorry for answering so late. I took a few days off far from any internet > >>>>>>> connection. > >>>>>>> > >>>>>>> It seems you sent many mails related with Analogy. Many thanks for your > >>>>>>> interest. I have not read all of them yet. However, I am beginning by > >>>>>>> this one (which seems unanswered). The answer is quick and easy :) > >>>>>>> > >>>>>>> Daniele Nicolodi wrote: > >>>>>>>> Hello. I'm looking into the analogy cmd_write example. > >>>>>>>> > >>>>>>>> I'm not sure I understand the reason for the rt_task_set_mode() function > >>>>>>>> call into the data acquisition loop (lines 413 or 464 in the code > >>>>>>>> shipped with xenomai 2.5.1). > >>>>>>>> > >>>>>>>> I do not understand why we have to set the primary mode at every > >>>>>>>> iteration, when we set it before for the task (line 380). > >>>>>>>> > >>>>>>>> Is it because the dump_function() uses system calls that can make the > >>>>>>>> task to switch to secondary mode, or there is a deeper reason I'm missing? > >>>>>>>> > >>>>>>> You are right. The dumping routine triggers a switch to secondary mode. > >>>>>>> That is why, the program switches back to primary mode after. > >>>>>> This is wrong. The Xenomai core will switch your real-time thread to > >>>>>> primary mode automatically when running a4l_insn* calls that end up > >>>>>> invoking rt_dev_ioctl(), since you did declare a real-time entry point > >>>>>> for this one. > >>>>>> > >>>>> I don't understand. I thought that rt_dev_ioctl() triggered an > >>>>> __rtdm_ioctl syscall, which, according to the rtdm systab, is declared > >>>>> with the flags "__xn_exec_current | __xn_exec_adaptive". > >>>>> > >>>>> So as __rt_dev_ioctl (the kernel handler behind the ioctl syscall) will > >>>>> return -ENOSYS neither in RT nor in NRT mode (because analogy declares > >>>>> both RT and NRT fops entries), I thought there was no automatic > >>>>> mode-switching. > >>>> The point is that your ioctl_nrt handler should return -ENOSYS when it > >>>> detects that the current request should be processed by the converse > >>>> domain, to trigger the switch to primary mode. This is why the adaptive > >>>> tag is provided in the first place. > >>> The problem is that rtdm does not provide any function to know whether > >>> the thread is shadowed. We just have rtdm_in_rt_context() which tells us > >>> whether the thread is RT or not. If it is NRT, we cannot distinguish a > >>> Linux thread from a Xenomai one. > >>> > >>> I thought with a little patch like this in ksrc/skins/rtdm/core.c, we > >>> could force -ENOSYS if the calling thread was a Xenomai NRT thread: > >>> > >>> diff --git a/ksrc/skins/rtdm/core.c b/ksrc/skins/rtdm/core.c > >>> index 8677c47..cc0cfe9 100644 > >>> --- a/ksrc/skins/rtdm/core.c > >>> +++ b/ksrc/skins/rtdm/core.c > >>> @@ -423,6 +423,9 @@ do { \ > >>> \ > >>> if (rtdm_in_rt_context()) \ > >>> ret = ops->operation##_rt(context, user_info, args); \ > >>> + else if (xnshadow_thread(user_info) != NULL && \ > >>> + ops->operation##_rt != (void *)rtdm_no_support) \ > >>> + ret = -ENOSYS; \ > >>> else \ > >>> ret = ops->operation##_nrt(context, user_info, args); \ > >>> \ > >> No, this would be a half-working kludge. But I think you have pinpointed > >> a more general issue with RTDM: syscalls should be tagged as both > >> adaptive and conforming, instead of bearing the __xn_exec_current bit. > >> Actually, we do want the current domain to change when it is not the > >> most appropriate, which __xn_exec_current prevents so far. > >> > >> What we rather want is to have shadows migrating to primary mode when > >> running rtdm_ioctl, since this is the preferred mode of operation for > >> Xenomai threads, so that ioctl_rt is always invoked first when present, > >> giving an opportunity to forward the request to secondary mode by > >> returning -ENOSYS. Conforming calls always enforce the preferred runtime > >> mode, i.e. primary for Xenomai shadows, secondary for plain Linux tasks. > >> That logic applies to all RTDM syscalls actually. > >> > >> __xn_exec_current allows application code to infer that the RTDM driver > >> might behave differently depending on the current runtime mode of the > >> calling thread, which is very much error-prone, and likely not what was > >> envisioned initially. > > > > Argh.... The switchtest driver is relying on __xn_exec_current to have > > context switches occur precisely in the mode we want. __xn_exec_adaptive > > introduce more context switches around which we can not place separate > > checks for fpu context, so, in short, breaks it badly. Fixing this > > requires turning the switchtest driver into a skin with its own syscalls. > > > > Note the sequence which occurs when a shadowed thread running in > > secondary mode calls an ioctl for which only an nrt implementation occurs: > > the thread is hardened to handle the ioctl > > ioctl_rt is called which returns -ENOSYS > > the thread is relaxed > > ioctl_nrt is called > > > > It boils down to putting an rt_task_set_mode(PRIMARY) before each rtdm > > syscall made by a thread with a shadow, and in fact seems to result in > > as bad a result. Is it really what we want? The __xn_exec_current bit > > resulted in a more lazy behaviour. > > > > Also note that, at least when using the posix skin, almost all threads > > have shadows, and only the priority makes the difference between a > > really critical thread, and non critical threads with the null priority. > > So, this will happen all the time. > > Right. Actually, we only need the conforming property for the (fairly > rare) case that a service is provided in both flavors. Wrong. You want conforming because real-time is the preferred mode of operations for real-time threads. > And if such a > service is hidden behind a single IOCTL command, it's even not possible > to detect this at RTDM level. We do need help from the driver or its > user space library, or we need to give them the tools to do the adaptive > switch themselves. > > So three options: > - Implement adaptive switching inside RTDM > o For all major functions != ioctl, we need to check for a shadow > thread and the availability of an RT handler => switch > o For IOCTLs we need an extra bit, likely in the IOC_TYPE namespace > to indicate the availability of an RT handler. This comes at the > risk of colliding with existing drivers that do not use the > standard IOCTL encoding scheme. > - Push adaptive switching into the driver > o That means providing the information if the caller could be > switched to RT context. > - Push adaptive switching into the helper library > o That means exporting a service that performs a switch if the > caller is switchable, but currently relaxed. > > The second option is the least invasive one, I think. The first one is > most transparent, but also more fragile and complex than the rest. Three > is likely not Philippe's favorite as it would introduce yet another > explicit mode switching mechanism. > > Opinions? Other ideas? > > Jan > -- Philippe.