hw/timer/armv7m_systick: Rewrite 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") -- ptimer.

Rewrite the systick timer to use a ptimer rather than
a raw QEMU timer.

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.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201015151829.14656-3-peter.maydell@linaro.org

1 files changed, 2 insertions, 1 deletions

diff --git a/include/hw/timer/armv7m_systick.h b/include/hw/timer/armv7m_systick.h
index 97cb345ddb..84496faaf9 100644
--- a/include/hw/timer/armv7m_systick.h
+++ b/include/hw/timer/armv7m_systick.h
@@ -14,6 +14,7 @@
 
 #include "hw/sysbus.h"
 #include "qom/object.h"
+#include "hw/ptimer.h"
 
 #define TYPE_SYSTICK "armv7m_systick"
 
@@ -27,7 +28,7 @@ struct SysTickState {
     uint32_t control;
     uint32_t reload;
     int64_t tick;
-    QEMUTimer *timer;
+    ptimer_state *ptimer;
     MemoryRegion iomem;
     qemu_irq irq;
 };