99ddaaa9d7
Multi-wraps ----------- Previously, on every new timeout, we programmed registers LR=timeout and CMP=0. The counter than counted from LR down to 0, triggered the IRQ, jumped back to LR, and counted down again. If one installed small timeouts (< 1000 us), it was likely that the counter wrapped multiple times before we were able to read it out. Initially, this was not a big issue as the additional wraps were simply ignored and the amount of time lost through this was not big. But when we want to do correct rate limitation, multiple wraps cause an overflow in the additional calculations, and this has a big effect on the resulting time value. Thus, we now program the counter to start from ~0 and count down to 0. We set CMP=~0-timeout so that the timer still triggers the IRQ at the right time. The counter continues counting down after the IRQ has triggered until we install a new timeout. We do not consider anymore that the counter wraps. The maximum timeout is set to half the maximum counter value, so, we should be able to install a new timeout before the counter wraps. Rate limit for time updates --------------------------- In the time span between two interrupts we have to remember how many ticks we have already added to the time value. This is because at each call of curr_time we can only see how many ticks have passed since the last call of schedule_timeout and not since the last call of curr_time. But we want to limit the rate of time updates in curr_time. With the member for ticks that were already added since the last call to schedule_timeout we can then calculate how many are yet to be added. |
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This source-code repository contains genuine low-level OS components and interfaces of Genode. It solely depends on the framework's base API.