[ECOS] from ISR to thread

[Schmidt Henning Larsen <HenningLS at danfoss dot com>]

Hi

I'm looking for some of the advantages and disadvantages there is in Ecos.
One of the great constrains I've found I eCos is that it is difficult to get
continues data from ISR to a thread.
I will here describe what I think is the problem, and I hope someone will
give it a comment.

In operating systems that allow us to signal on a counting semaphore in
ISR's and doesn't have DSR's. It will be simple to get data from ISR to
thread.

Pseudo code for such a RTOS:

Semaphore: sem
Hardware: hw
Ringbuffer: rbuf

Threadcode:
{
  while(1)
  {
    sem.wait()
    data = rbuf.getdata()
    doSomething(data)
  }
}

ISR code:
{
  newData = hw.data // read data from hardware
  rbuf.putdata(newData) // put in ringbuffer
  sem.signal()
  interrupt_acknowledge()
}


This will work, even if we get a new interrupt when the thread is
"doingsomething", because it is a counting semaphore.
If we do it the same way in eCos, using a dsr and a condition variable it
will look as follows:


Condition variable: cond
Hardware data: hw
Ringbuffer: rbuf

Threadcode:
{
  while(1)
  {
    cyg_drv_cond_wait(cond)
    data = rbuf.getdata()
    doSomething(data)
  }
}

ISR code:
{
  rbuf.putdata(hw.data)
  cyg_drv_interrupt_mask() // don't allow interrupt on this device
  cyg_drv_interrupt_acknowledge()
  return cyg_isr_call_dsr
}

DSR code
{
  cyg_drv_cond_signal(cond)
  cyg_drv_interrupt_unmask() // allow interrupt on this device
}


This will not work, because if a new interrupt appears while the thread is
"doingSomething", the thread will not be notified because it hasn't returned
to the condition variable. When the tread returns to the condition variable
it has missed the notify, and don't know that it actually should run once
more. The problem is that we can't call a counting semaphore.

If we assign yet another thread we can solve the problem this way:


Condition variable: cond
Semaphore: sem
Hardware: hw
Ringbuffer: rbuf

WorkingThread code:
{
  while(1)
  {
    cyg_semaphore_wait(sem)
    data = rbuf.getdata()
    doSomething(data)
  }
}

ISR code:
{
  rbuf.putdata(hw.data)
  cyg_drv_interrupt_mask() // don't allow interrupt on this device
  cyg_drv_interrupt_acknowledge() //allow other interrupts to appear
  return cyg_isr_call_dsr // tells the kernel to run the dsr
}

DSR code:
{
  cyg_drv_cond_signal(cond)
}

extra thread code:
{
  while(1)
  {
    cyg_drv_cond_wait(cond)
    cyg_semaphore_post(sem)
    cyg_drv_interrupt_unmask() // allow interrupt on this device
  }
}


This should work, but there are some problems:
It will introduce a very long interrupt latency on the device.
Large overhead ( DSR, extra thread, both a condition variable and a
semaphore )
Another way to solve the problem, is to be absolute sure that the
workingthread has returned to the condition variable before a new interrupt
appears, but that will be a great constrain to the way we write our program.

My conclusion is: 
ECos doesn't have a useful synchronization mechanism, in situations where we
will allow interrupts to appear, while we are processing data from previous
interrupts on the same source/device.

I hope someone can tell me if I'm wrong.

Henning