jlaitine commented on PR #15938: URL: https://github.com/apache/nuttx/pull/15938#issuecomment-2703630963
> Regarding the problem you described, the original implementation of aligning the tick scale also has this problem. Let's take an example. Assume that the frequency is `62500000`, `USEC_PER_TICK = 10`, `cycle_per_tick = 625`. > > tick |--------------------|-----------t0---t1--| tick5662--------------5663---------------5664 > > let `t_0 = 3539791 (625 * 5663 + 416)` be the time when we call `sleep(1tick)`. let `t_1 = 3539876 (625 * 5664 - 124)` be the time when we call arm64_tick_start(1tick). > > We calculate the next expiration cycle via: `arm64_arch_timer_count() / priv->cycle_per_tick * priv->cycle_per_tick +ticks * priv->cycle_per_tick` That is: `3539876 / 625 * 625 + 1 * 625 = 3540000` (exact tick 5664). The time difference between next expiration time and `t0` is `3540000 - 3539791 = 209`. This makes us only sleep for `209` cycles to trigger the timer, instead of at least 625 cycles to trigger the timer, causing the timer fired prematurely. > > In the case of using relative timer: The next expiration cycle is calculate by: `arm64_arch_timer_count() + ticks * priv->frequency / TICK_PER_SEC = 3539876 + 1 * 62500000 / 100000 = 3539876 + 625 = 3540501`. Which is 710 cycles next to the `t_0`, and greater than 1 tick. Satisfy the requirement of sleeping for at least 1 tick. Yes, I see what you want to achieve. As I said, you could follow the example of our risc-v implementation: https://github.com/tiiuae/nuttx/blob/6838f8c595716e49feacd5ddc6b569a5a4bf3118/arch/risc-v/src/common/riscv_mtimer.c#L225 Just calculate first the absolute tick on which you want to wake up ( current tick + amount of ticks to sleep). Then convert this absolute tick value to timer compare value. Then your every tick start is properly aligned to a tick boundary, and you don't experience the rounding error - only the variance of consecutive tick times. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: commits-unsubscr...@nuttx.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org
