FreeRTOS vs Zephyr/Mynewt task blocked state

1
votes

I cannot seem to find any info on this question, so I thought I'd ask here. (No reply here: https://lists.zephyrproject.org/pipermail/zephyr-devel/2017-June/007743.html)

When a driver (eg. SPI or UART) is invoked through FreeRTOS using the vendor HAL, then there are two options for waiting upon completion:

1) Interrupt 2) busy-waiting

My question is this:

If the driver is invoked using busy-waiting; Does FreeRTOS then have any knowledge of the busy-waiting (occuring in the HAL Driver)? Does the task still get a time slot allocated (for doing busy-waiting). Is this how it works? (Presuming FreeRTOS task has a preemptive scheduler)

Now in Zephyr (and probably Mynewt), I can see that when the driver is called, Zephyr keeps track of the calling task, which is then suspended (blocked state) until finished. Then the driver interrupt routine it puts the calling thread into the run-queue, when ready to proceed. This way no cycles are waisted. Is this correct understood?

Thanks Anders

1
rtosfreertos

1 Answers

0
votes

I don't understand your question. In FreeRTOS, if a driver is implemented to perform a busy wait (i.e. the driver has no knowledge of the multithreading, so is not event driven, and instead uses a busy wait that uses all CPU time) then the RTOS scheduler has no idea that is happening, so will schedule the task just as if it would any other task. Therefore, if the task is the highest priority ready state task it will use all the CPU time, and if there are other tasks of equal priority, it will share the CPU time with those tasks.

On the other hand, if the driver is written to make use of an RTOS (be that Zephr, FreeRTOS, or any other) then it can make use of the RTOS primitives to create a much more efficient event driven execution pattern. I can't see how the different schedulers you mention will behave any differently in this respect. For example, how can Zephr know that a task it didn't know the application writer was going to create was going to call a library function it had no previous knowledge of, and that the library function was going to use a busy wait?