On 04/02/2012 06:18 PM, Jan Kiszka wrote:
> On 2012-04-02 18:08, Gilles Chanteperdrix wrote:
>> On 04/02/2012 05:54 PM, Jan Kiszka wrote:
>>> On 2012-04-02 17:39, Gilles Chanteperdrix wrote:
>>>> On 02/24/2012 03:40 PM, Philippe Gerum wrote:
>>>>> On 02/24/2012 01:28 PM, Gilles Chanteperdrix wrote:
>>>>>> On 02/20/2012 05:46 PM, Philippe Gerum wrote:
>>>>>>> On 02/19/2012 08:43 PM, Gilles Chanteperdrix wrote:
>>>>>>>>
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> restarting the ipipe-core is the good opportunity to look a bit at our
>>>>>>>> current state and think about what we could improve. On ARM, at least,
>>>>>>>> the thing we could improve is the timer subsystem. A long time ago,
>>>>>>>> linux has switched to a system allowing to select at run-time which
>>>>>>>> timer to use, and we do not really benefit from this, what timer we use
>>>>>>>> is selected at compile time, resulting in a massive ifdefery on arm,
>>>>>>>> and
>>>>>>>> even on x86, we have to select at compile time whether using the 8254
>>>>>>>> or
>>>>>>>> APIC, whereas we could decide to use whatever linux is using, even say
>>>>>>>> HPET, without any compilation option. That is assuming we want to move
>>>>>>>> the x86 timer-specific code from xenomai to I-pipe.
>>>>>>>>
>>>>>>>> The idea to reach this goal would be to add some ipipe specific members
>>>>>>>> to the struct clock_event_device, the way we do for the interrupt
>>>>>>>> controller:
>>>>>>>> - a CLOCK_EVT_FEAT_IPIPE would signal that the clockevent device is
>>>>>>>> ipipe capable, meaning that it implements the following callback
>>>>>>>> - ipipe_steal would be called when stealing the timer, we could decide
>>>>>>>> to call this callback either as part of ipipe_request_timer, or with a
>>>>>>>> specific ipipe_steal_timer call, currently we simply set
>>>>>>>> __ipipe_mach_timerstolen to 1, but it is pure luck that we never needed
>>>>>>>> something more complicated, besides, we may want to start a timer that
>>>>>>>> was not started by linux so, we would put its initialization here;
>>>>>>>> - ipipe_stolen would record whether the timer is stolen
>>>>>>>> - ipipe_min_delta_ticks, ipipe_max_delta_ticks would be used by the
>>>>>>>> ipipe_set_next_event callback
>>>>>>>> - ipipe_ack would be called to ack the timer interrupt the way we
>>>>>>>> currently do it currently on arm with __ipipe_mach_acktimer
>>>>>>>> - ipipe_set_next_event would program the next timer shot, the way it is
>>>>>>>> currently done in __ipipe_mach_set_dec.
>>>>>>>>
>>>>>>>> All this is pretty straightforward, but there is still a question: how
>>>>>>>> does ipipe_request_timer select a timer. The idea is that on platform
>>>>>>>> without PIC muting, it is probably more efficient to use the same timer
>>>>>>>> for linux and xenomai. But on platforms with PIC muting, we could want
>>>>>>>> to select a different timer for linux and xenomai, but how do we find
>>>>>>>> it, what if linux has not selected a timer because it is unusable on
>>>>>>>> that platform?
>>>>>>>
>>>>>>> Checking the clock device mode for CLOCK_EVT_MODE_SHUTDOWN, and falling
>>>>>>> back to sharing the active kernel tick device + disabling PIC muting?
>>>>>>
>>>>>> OK. Another question is: do we want to move x86 timer handling from
>>>>>> xenomai to ipipe. If not, we have to find a way to support the two
>>>>>> possible configurations. What we could do is using the timer name in
>>>>>> ipipe_request_tickdev: if a timer name is supplied, we keep the old
>>>>>> implementation, if no timer name is supplied, we use the newho
>>>>>> implementation which looks for the best candidate with the
>>>>>> CLOCK_EVT_FEAT_IPIPE bit.
>>>>>>
>>>>>>
>>>>>
>>>>> Yes, makes sense. At any rate, handling the real-time timer the way we
>>>>> want for Xenomai directly from the pipeline is the only sane option. We
>>>>> only have to be careful about backward compatibility of the newest core
>>>>> pipelines with 2.6.x, until we stop maintaining this branch in favor of
>>>>> 3.x. We may also move ipipe_request_tckdev() to the compat module fully,
>>>>> removing it from the current API if that makes sense.
>>>>>
>>>>
>>>> For those who would like to follow, the result, a bit different from what
>>>> was originally planned is the interface implemented by this file:
>>>> http://git.xenomai.org/?p=ipipe-gch.git;a=blob;f=kernel/ipipe/timer.c;h=b9936469652d8fe998157d155fda77df81f0425a;hb=52d36aa86d5c5d463d86d384ad717f26ec96ef8c
>>>>
>>>> And ARM and x86 architectures were re-factored over this interface. As
>>>> an example, the implementation of x86 timers is in this patch:
>>>> http://git.xenomai.org/?p=ipipe-gch.git;a=commitdiff;h=52d36aa86d5c5d463d86d384ad717f26ec96ef8c;hp=675a8ed4365eb1f23b098f913caf40e4dc792b80
>>>>
>>>> 8254, local APIC and HPET in legacy mode were tested, even selected
>>>> at run-time. Only per-cpu HPET remains to be tested (the hardware
>>>> I have does not support it).
>>>
>>> FWI, QEMU (w/ or w/o KVM) can emulated enough HPET timers, also with MSI
>>> support, but that was broken in I-pipe last time I checked. Use -global
>>> hpet.timers=4 (or more for more CPUs) and -global hpet.msi=on.
>>
>> Ah thanks. How was it broken? MSI were broken? As far as I understood
>> MSI support for HPET are required for linux to select per-cpu HPET.
>
> I-pipe was not aware of Linux using the HPET as per-CPU timersource. So
> things fell apart once Linux switched. And, yes, there must be MSI
> support to make Linux use them as per-CPU timers.
Ok. It is supposed to work with the refactoring then.
--
Gilles.
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