Re: [Xenomai-help] Analogy cmd_write example explanation

[Jan Kiszka <jan.kiszka@domain.hid>]

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Alexis Berlemont wrote:
> Hi,
> 
> Jan Kiszka 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.
>>
>> Right, this stickiness of RTDM syscalls to the current execution mode
>> conceptually dates back to the days we had users with services provided
>> to both domain. This model is deprecated (e.g. RTnet dropped resources
>> allocation from RT domain, only supports NRT now).
>>
>> So I've no concerns making the call conforming - but... I guess this
>> would just paper over a design issue (or misunderstanding?) in Analogy.
>> Why does it depend on shadow vs. non-shadow at all? The explanations I
>> read so far were not clarifying the reason. Is there any Analogy service
>> that is provided to both domains (via different implementations)? If
>> yes, why?
>>
> 
> Analogy provides all the services in both domains:
> - the instructions services are synchronous operations; no explicit
> rescheduling are triggered, consequently, the implementations of the
> various kinds of instructions displays are the same in both mode;
> - the commands services allow asynchronous operations; so the handlers
> behind the read / write fops are the same for RT and NRT entries;
> however, the event management code relies on rtdm_event in one case and
> on rtdm_nrtsig + waitqueues in the other case.
> 
> The main part of the "event" code is located in a4l_wait_sync and
> a4l_signal_sync.
> 
> Which design issue do you have in mind ?

I was concerned about resource allocating services for both domains. For
that reason, you should drop open_rt & close_rt callbacks even if they
may not allocate anything at analogy level ATM, they do at RTDM level.
RT resource allocation of RTDM drivers is deprecated (I will remove the
corresponding callbacks for Xenomai 3).

> 
> Concerning your question: why ?
> 
> Not all the acquisition system users ask for determinism; working with
> real-time tasks may not be mandatory. So all they need to know is the
> Analogy API.
> 
> Furthermore, many acquisition boards have an impressive set of
> configuration parameters which are accessed via the operations described
> above. One may be interested in scripting a part of the user application
> so as to gain development time. I don't want to lose flexibility by
> confining Analogy to real-time tasks.

I can't asses ATM how much this actually buys the typical Analogy user,
specifically given that it still takes a Xenomai kernel + user land to
use it and shadowing low-prio (SCHED_OTHER) tasks is feasible as well today.

But you should at least warn the user that select() is not working that
way. It remains restricted to RT tasks as there is no Linux "shadow" FD
for the RTDM one.

Jan


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