stm32h7: refactor systick code

commit 62332841cf87f08728730e5dfa684a19b69ca5f9
Author: Aidan MacDonald <[email protected]>
Date:   Thu Jan 15 01:26:45 2026 +0000

    stm32h7: refactor systick code
    
    Make systick setup less hardcoded, and create a public API
    for use by targets, in preparation for moving system_init()
    into target-specific code.
    
    Improve the implementation of udelay to make it more robust
    against timer wraparound.
    
    Change-Id: I21bb8821cfd1d7e4049fac6e6a4548d80a4276f7

diff --git a/firmware/target/arm/stm32/system-stm32h7.c b/firmware/target/arm/stm32/system-stm32h7.c
index b58fd76322..35c1d2e60d 100644
--- a/firmware/target/arm/stm32/system-stm32h7.c
+++ b/firmware/target/arm/stm32/system-stm32h7.c
@@ -27,25 +27,31 @@
 #include "regs/cortex-m/cm_systick.h"
 #include "regs/stm32h743/dbgmcu.h"
 
-/* EXT timer is 1/8th of CPU clock */
-#define SYSTICK_FREQ            (CPU_FREQ / 8)
-#define SYSTICK_PER_MS          (SYSTICK_FREQ / 1000)
-#define SYSTICK_PER_US          (SYSTICK_FREQ / 1000000)
+/* Assumed initial CPU frequency for calculating systick */
+#ifndef CPUFREQ_INITIAL
+# define CPUFREQ_INITIAL CPU_FREQ
+#endif
+
+/* Tick interval in milliseconds */
+#ifndef SYSTICK_INTERVAL_INITIAL
+# define SYSTICK_INTERVAL_INITIAL (1000 / HZ)
+#endif
 
-/* Max delay is limited by kernel tick interval + safety margin */
-#define SYSTICK_DELAY_MAX_US    (1000000 / HZ / 2)
-#define SYSTICK_DELAY_MAX_MS    (SYSTICK_DELAY_MAX_US / 1000)
+/* Use EXT source which is equal to CPU frequency divided by 8 */
+#define SYSTICK_SOURCE    BV_CM_SYSTICK_CSR_CLKSOURCE_EXT
+#define SYSTICK_PRESCALER 8
+
+/* Convert CPU frequency to number of systick ticks in 1 ms */
+#define CPUFREQ_TO_SYSTICK_PER_MS(f) \
+    ((f) / (SYSTICK_PRESCALER * 1000))
+
+/* SysTick related state */
+static uint32_t systick_per_ms = CPUFREQ_TO_SYSTICK_PER_MS(CPUFREQ_INITIAL);
+static uint32_t systick_interval_in_ms = SYSTICK_INTERVAL_INITIAL;
 
 /* Base address of vector table */
 extern char __vectors_arm[];
 
-static void systick_init(unsigned int interval_in_ms)
-{
-    reg_writef(CM_SYSTICK_RVR, VALUE(SYSTICK_PER_MS * interval_in_ms - 1));
-    reg_writef(CM_SYSTICK_CVR, VALUE(0));
-    reg_writef(CM_SYSTICK_CSR, CLKSOURCE_V(EXT), ENABLE(1));
-}
-
 static void stm_enable_caches(void)
 {
     __discard_idcache();
@@ -56,6 +62,45 @@ static void stm_enable_caches(void)
     arm_isb();
 }
 
+static void stm32_recalc_systick_rvr(void)
+{
+    uint32_t ticks = systick_per_ms * systick_interval_in_ms;
+
+    reg_writef(CM_SYSTICK_RVR, VALUE(ticks - 1));
+}
+
+static void stm32_set_systick_interval(uint32_t interval_in_ms)
+{
+    if (interval_in_ms != systick_interval_in_ms)
+    {
+        systick_interval_in_ms = interval_in_ms;
+        stm32_recalc_systick_rvr();
+    }
+}
+
+void stm32_systick_set_cpu_freq(uint32_t freq)
+{
+    uint32_t ticks_per_ms = CPUFREQ_TO_SYSTICK_PER_MS(freq);
+
+    if (ticks_per_ms != systick_per_ms)
+    {
+        systick_per_ms = ticks_per_ms;
+        stm32_recalc_systick_rvr();
+    }
+}
+
+void stm32_systick_enable(void)
+{
+    stm32_recalc_systick_rvr();
+    reg_writef(CM_SYSTICK_CVR, VALUE(0));
+    reg_writef(CM_SYSTICK_CSR, CLKSOURCE(SYSTICK_SOURCE), ENABLE(1));
+}
+
+void stm32_systick_disable(void)
+{
+    reg_writef(CM_SYSTICK_CSR, ENABLE(0), TICKINT(0));
+}
+
 void system_init(void)
 {
 #if defined(DEBUG)
@@ -72,8 +117,8 @@ void system_init(void)
     /* Initialize system clocks */
     stm_clock_init();
 
-    /* TODO: move this */
-    systick_init(1000/HZ);
+    /* Initialize systick */
+    stm32_systick_enable();
 
     /* Call target-specific initialization */
     gpio_init();
@@ -99,7 +144,8 @@ void system_debug_enable(bool enable)
 
 void tick_start(unsigned int interval_in_ms)
 {
-    (void)interval_in_ms;
+    stm32_set_systick_interval(interval_in_ms);
+    stm32_systick_enable();
 
     reg_writef(CM_SYSTICK_CSR, TICKINT(1));
 }
@@ -110,47 +156,33 @@ void systick_handler(void)
 }
 
 /*
- * NOTE: This assumes that the CPU cannot be reclocked during an interrupt.
- * If that happens, the systick interval and reload value would be modified
- * to maintain the kernel tick interval and the code here will break.
+ * This makes two assumptions:
+ *
+ * 1. the CPU frequency must not change while udelay() is running;
+ *    otherwise the delay time will be wrong.
+ * 2. interrupt handlers should not block execution for more than
+ *    one systick interval; if this happens the delay may be much
+ *    longer than necessary.
  */
-static void __udelay(uint32_t us)
+void udelay(uint32_t us)
 {
+    uint32_t delay_ticks = (us * systick_per_ms / 1000);
     uint32_t start = reg_readf(CM_SYSTICK_CVR, VALUE);
     uint32_t max = reg_readf(CM_SYSTICK_RVR, VALUE);
-    uint32_t delay = us * SYSTICK_PER_US;
 
-    for (;;)
+    while (delay_ticks > 0)
     {
         uint32_t value = reg_readf(CM_SYSTICK_CVR, VALUE);
         uint32_t diff = start - value;
         if (value > start)
             diff += max;
-        if (diff >= delay)
-            break;
-    }
-}
-
-void udelay(uint32_t us)
-{
-    while (us > SYSTICK_DELAY_MAX_US)
-    {
-        __udelay(SYSTICK_DELAY_MAX_US);
-        us -= SYSTICK_DELAY_MAX_US;
-    }
 
-    __udelay(us);
-}
+        if (diff >= delay_ticks)
+            break;
 
-void mdelay(uint32_t ms)
-{
-    while (ms > SYSTICK_DELAY_MAX_MS)
-    {
-        __udelay(SYSTICK_DELAY_MAX_MS * 1000);
-        ms -= SYSTICK_DELAY_MAX_MS;
+        delay_ticks -= diff;
+        start = value;
     }
-
-    __udelay(ms * 1000);
 }
 
 void system_exception_wait(void)
diff --git a/firmware/target/arm/stm32/system-target.h b/firmware/target/arm/stm32/system-target.h
index fa3e206baa..d828b40e16 100644
--- a/firmware/target/arm/stm32/system-target.h
+++ b/firmware/target/arm/stm32/system-target.h
@@ -25,6 +25,15 @@
 #include "cpucache-armv7m.h"
 #include <stdbool.h>
 
+/* Enables the SysTick timer -- SysTick interrupt won't be enabled */
+void stm32_systick_enable(void);
+
+/* Disables the SysTick timer -- also disables SysTick interrupt */
+void stm32_systick_disable(void);
+
+/* Update the CPU frequency to recalibrate SysTick timer */
+void stm32_systick_set_cpu_freq(uint32_t freq);
+
 /* Enable/disable debug clock domain during sleep mode. */
 void system_debug_enable(bool enable);
 
@@ -32,7 +41,7 @@ void system_debug_enable(bool enable);
 void gpio_init(void) INIT_ATTR;
 void fmc_init(void) INIT_ATTR;
 
+/* Busy loop delay based on systick */
 void udelay(uint32_t us);
-void mdelay(uint32_t ms);
 
 #endif /* __STM32_SYSTEM_TARGET_H__ */
-- 
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