Tim Wescott <tim@seemywebsite.now> writes:
> On 05/28/2010 03:46 PM, Randy Yates wrote:
>> On May 28, 5:23 pm, D Yuniskis<not.going.to...@seen.com> wrote:
>>> Hi Tim,
>>>
>>>
>>>
>>> Tim Wescott wrote:
>>>> On 05/28/2010 12:41 PM, Steve Pope wrote:
>>>>> Randy Yates<ya...@ieee.org> wrote:
>>>
>>>>>> The SEM module in DSP/BIOS maintains a non-negative count of the number
>>>>>> of times it has been "posted". Then when a pend occurs, the process
>>>>>> either a) blocks if count = 0, or b) decrements count and resumes.
>>>
>>>>>> I have one task T1 that must run to completion before other tasks (T2,
>>>>>> ..., TN) run. It *seems* this would be a good use of a semaphore;
>>>>>> create a semaphore SEM_T1, then have each task T2, ..., TN pend on
>>>>>> SEM_T1. Then when T1 completes, it posts to SEM_T1.
>>>
>>>>>> However, this won't work with DSP/BIOS semaphores. What will happen is
>>>>>> that the first task that pended, say, T2, will get unblocked when T1
>>>>>> completes, but since there was only one pend by T1, none of the other
>>>>>> T3-TN will unblock.
>>>
>>>>>> How would you solve this problem in DSP/BIOS?
>>>
>>>>> I find the following very useful in an RTOS:
>>>
>>>>> Partition those tasks which you wish to initiated from interrupt
>>>>> events into a finite set of priority levels (the fewer the better).
>>>
>>>>> Within each level each task is preceded with common code which
>>>>> implements the following sequence of operations (which must be made
>>>>> uninterruptable):
>>>
>>>>> (1) Is there another task of the same level already running?
>>>
>>>>> (2) If so, place the current task at the end of the queue for
>>>>> this level, and return from interrupt.
>>>
>>>>> (3) If not, lower priority and start running the current task.
>>>
>>>>> And at the end of the task:
>>>
>>>>> (4) Raise priority
>>>
>>>>> (5) If another task for this level is queued, execute it.
>>>>> Otherwise, if the queue is empty, return from interrupt.
>>>
>>>>> Whether you do this with semaphores is an implementation detail.
>>>
>>>>> What you don't want to do is have tasks queueing or executing other
>>>>> tasks which are from a _different_ priority level.
>>>
>>>>> Applying this to your example, T1 is higher priority than T2,
>>>>> T3 etc. which are all at the same (but lower) priority level.
>>>
>>>>> So, when T1 gets to step (3), it lowers priority enough to
>>>>> enough to allow other T1-level tasks to run - but not T2, T3
>>>>> etc. tasks. Then after T1 gets to step (5), all tasks queued
>>>>> at the T2, T3 level can potentially run.
>>>
>>>>> (Note that if another T1 task does interrupt T1, it only
>>>>> gets queued, it does not pre-empt T1.)
>>>
>>>>> Fundamentally you need a queue of tasks at each priority level,
>>>>> rather than individual tasks reaching across levels to
>>>>> start or stop things.
>>>
>>>> This sounds way complicated. Are you doing this within the context of
>>>> an RTOS? If so, why in heck can't you just use as many fixed priorities
>>>> as you need to get the job done?
>>>
>>> Because it isn't an issue of "priorities". He wants an
>>> explicit interlock between the execution of T1 and T2..TN.
>>>
>>> Either let T1 *start* T2..TN, *resume* them, *or* use
>>> a counting semaphore, event flag, condition variable, mutexes,
>>> etc. as I described in my other post.
>>>
>>> E.g., my Init() task runs at the absolute lowest priority
>>> in the system. When it is done setting things up, it becomes
>>> the "idle" task.
>>
>> Again, bingo! You've hit it on the head - that's exactly my scenario
>> too. I have an Init() task that needs to run before everything else
>> starts up.
>
> Is this an RTOS that's integrated into your C environment and comes up
> "invisibly", or does it need to be called explicitly? If I'm using an
> RTOS that needs to be started from main() I'll often do all of my
> initialization there, then kick off the RTOS.
Oh, so you want to know what's REALLY going on? ... :)
Yes, I had my initialization initially in main() also. The problem is
that one of the things that needs initializing uses the SPI bus, and the
processor is so damned fast (2 ns instruction cycle) some type of delay
is required between SPI bus transactions.
So instead of attempting to write a delay routine, whose accuracy in this
day and age is going to be subject to things like compiler optimization
flags, code cache hits, etc., I thought it would be much more elegant to
utilize the OS's TSK_sleep() function (which is based on hardware timer
ticks).
So, even though my Init() task was running at the highest priority, the
TSK_sleep() calls block and allow lower-priority tasks to run. Hence my
need for a "gating" semaphore.
--
Randy Yates % "My Shangri-la has gone away, fading like
Digital Signal Labs % the Beatles on 'Hey Jude'"
mailto://yates@ieee.org %
http://www.digitalsignallabs.com % 'Shangri-La', *A New World Record*, ELO