This patch series rewrites our implementation of the armv7m systick timer to use ptimers.
The armv7m systick timer is a 24-bit decrementing, wrap-on-zero, clear-on-write counter. Our current implementation has various bugs and dubious workarounds in it (for instance see https://bugs.launchpad.net/qemu/+bug/1872237). We have an implementation of a simple decrementing counter and we put a lot of effort into making sure it handles the interesting corner cases (like "spend a cycle at 0 before reloading"), so rather than trying to fix these all over again in systick's hand-rolled countdown code it's much simpler to just rewrite it to use a ptimer. Unfortunately this is a migration compatibility break, which will affect all M-profile boards. Among other bugs, this fixes https://bugs.launchpad.net/qemu/+bug/1872237 : now writes to SYST_CVR when the timer is enabled correctly do nothing; when the timer is enabled via SYST_CSR.ENABLE, the ptimer code will (because of POLICY_NO_IMMEDIATE_RELOAD) arrange that after one timer tick the counter is reloaded from SYST_RVR and then counts down from there, as the architecture requires. Side note: the trace from the test program in LP1872237 won't look quite like it does on the hardware: under QEMU the "waiting for 1000 ms" debug printing generally reports a SYST_CVR value of 0. This is because QEMU's emulated CPU is comparatively fast and our systick has a hard-wired value of 1MHz for the frequency of the 'external reference clock', which means that execution of the guest code reaches "read SYST_CVR" before the first tick of the timer clock after enabling of the timer (which is where the reload of SYST_CVR from SYST_RVR is required). The exception is the first iteration, where the time QEMU takes to translate the guest code is enough that the timer tick happens before the register read. You can also get the timer tick to win the race by fiddling around with the -icount option (which effectively is slowing down the emulated CPU speed). Some day we should model both the 'system_clock_scale' (ie the CPU clock frequency) and the 'external reference clock' as QEMU clock source/sinks so that board code can specify the correct reference clock frequency. Patch 1 is a minor tweak to the ptimer code to suppress a spurious warning message for the "timer callback function disabled the ptimer" case, which the systick timer uses. Patch 2 is the actual conversion. thanks -- PMM Peter Maydell (2): hw/core/ptimer: Support ptimer being disabled by timer callback hw/timer/armv7m_systick: Rewrite to use ptimers include/hw/timer/armv7m_systick.h | 3 +- hw/core/ptimer.c | 4 + hw/timer/armv7m_systick.c | 124 +++++++++++++----------------- 3 files changed, 58 insertions(+), 73 deletions(-) -- 2.20.1
