Add STM32F103 port

Add a fully functional STM32F1 port, currently mostly targeting STM32F103
microcontrollers. This requires an 8 MHz XTAL. The maximum possible step rate
is around 282K steps per second.

This uses stm32flash to burn the firmware. The bootloader needs to be started
by setting BOOT0 to 1 and resetting the MCU. There is no automatic bootloader,
unlike on Arduino.

Signed-off-by: Grigori Goronzy <greg@kinoho.net>

src/stm32f1/timer.c

@@ -0,0 +1,188 @@
+// STM32F1 timer interrupt scheduling code.
+//
+// Copyright (C) 2018 Grigori Goronzy <greg@kinoho.net>
+//
+// This file may be distributed under the terms of the GNU GPLv3 license.
+
+#include "autoconf.h"
+#include "board/misc.h" // timer_from_us
+#include "stm32f1xx.h"
+#include "stm32f1xx.h"
+#include "stm32f1xx_ll_bus.h"
+#include "stm32f1xx_ll_tim.h"
+#include "command.h" // shutdown
+#include "board/irq.h" // irq_save
+#include "sched.h" // sched_timer_dispatch
+
+
+/****************************************************************
+ * Low level timer code
+ ****************************************************************/
+
+DECL_CONSTANT(CLOCK_FREQ, CONFIG_CLOCK_FREQ);
+
+union u32_u {
+    struct { uint8_t b0, b1, b2, b3; };
+    struct { uint16_t lo, hi; };
+    uint32_t val;
+};
+
+static inline uint16_t
+timer_get(void)
+{
+    return LL_TIM_GetCounter(TIM2);
+}
+
+static inline void
+timer_set(uint16_t next)
+{
+    LL_TIM_OC_SetCompareCH1(TIM2, next);
+}
+
+static inline void
+timer_repeat_set(uint16_t next)
+{
+    LL_TIM_OC_SetCompareCH2(TIM2, next);
+    LL_TIM_ClearFlag_CC2(TIM2);
+}
+
+// Activate timer dispatch as soon as possible
+void
+timer_kick(void)
+{
+    timer_set(timer_get() + 50);
+    LL_TIM_ClearFlag_CC1(TIM2);
+}
+
+static struct timer wrap_timer;
+
+void
+timer_reset(void)
+{
+    sched_add_timer(&wrap_timer);
+}
+DECL_SHUTDOWN(timer_reset);
+
+void
+timer_init(void)
+{
+    irqstatus_t flag = irq_save();
+    LL_APB1_GRP1_EnableClock(LL_APB1_GRP1_PERIPH_TIM2);
+    LL_TIM_SetPrescaler(TIM2, __LL_TIM_CALC_PSC(SystemCoreClock, CONFIG_CLOCK_FREQ));
+    LL_TIM_SetCounter(TIM2, 0);
+    LL_TIM_SetAutoReload(TIM2, 0xFFFF);
+    LL_TIM_EnableIT_CC1(TIM2);
+    LL_TIM_CC_EnableChannel(TIM2, LL_TIM_CHANNEL_CH1);
+    LL_TIM_CC_EnableChannel(TIM2, LL_TIM_CHANNEL_CH2);
+    NVIC_EnableIRQ(TIM2_IRQn);
+    NVIC_SetPriority(TIM2_IRQn, 0);
+    timer_kick();
+    timer_repeat_set(timer_get() + 50);
+    timer_reset();
+    LL_TIM_EnableCounter(TIM2);
+    irq_restore(flag);
+}
+DECL_INIT(timer_init);
+
+
+/****************************************************************
+ * 32bit timer wrappers
+ ****************************************************************/
+
+static uint16_t timer_high;
+
+// Return the current time (in absolute clock ticks).
+uint32_t
+timer_read_time(void)
+{
+    irqstatus_t flag = irq_save();
+    union u32_u calc = { .val = timer_get() };
+    calc.hi = timer_high;
+    if (unlikely(LL_TIM_IsActiveFlag_UPDATE(TIM2))) {
+        irq_restore(flag);
+        if (calc.b1 < 0xff)
+            calc.hi++;
+        return calc.val;
+    }
+    irq_restore(flag);
+    return calc.val;
+}
+
+// Timer that runs every cycle - allows 16bit comparison optimizations
+static uint_fast8_t
+timer_event(struct timer *t)
+{
+    union u32_u *nextwake = (void*)&wrap_timer.waketime;
+    if (LL_TIM_IsActiveFlag_UPDATE(TIM2)) {
+        // Hardware timer has overflowed - update overflow counter
+        LL_TIM_ClearFlag_UPDATE(TIM2);
+        timer_high++;
+        *nextwake = (union u32_u){ .hi = timer_high, .lo = 0x8000 };
+    } else {
+        *nextwake = (union u32_u){ .hi = timer_high + 1, .lo = 0x0000 };
+    }
+    return SF_RESCHEDULE;
+}
+static struct timer wrap_timer = {
+    .func = timer_event,
+    .waketime = 0x8000,
+};
+
+#define TIMER_IDLE_REPEAT_TICKS timer_from_us(500)
+#define TIMER_REPEAT_TICKS timer_from_us(100)
+
+#define TIMER_MIN_TRY_TICKS timer_from_us(1)
+#define TIMER_DEFER_REPEAT_TICKS timer_from_us(5)
+
+// Hardware timer IRQ handler - dispatch software timers
+void __visible __aligned(16)
+TIM2_IRQHandler(void)
+{
+    irq_disable();
+    LL_TIM_ClearFlag_CC1(TIM2);
+    uint16_t next;
+    for (;;) {
+        // Run the next software timer
+        next = sched_timer_dispatch();
+
+        for (;;) {
+            int16_t diff = timer_get() - next;
+            if (likely(diff >= 0)) {
+                // Another timer is pending - briefly allow irqs and then run it
+                irq_enable();
+                if (unlikely(LL_TIM_IsActiveFlag_CC2(TIM2)))
+                    goto check_defer;
+                irq_disable();
+                break;
+            }
+
+            if (likely(diff <= -TIMER_MIN_TRY_TICKS))
+                // Schedule next timer normally
+                goto done;
+
+            irq_enable();
+            if (unlikely(LL_TIM_IsActiveFlag_CC2(TIM2)))
+                goto check_defer;
+            irq_disable();
+            continue;
+
+        check_defer:
+            // Check if there are too many repeat timers
+            irq_disable();
+            uint16_t now = timer_get();
+            if ((int16_t)(next - now) < (int16_t)(-timer_from_us(1000)))
+                try_shutdown("Rescheduled timer in the past");
+            if (sched_tasks_busy()) {
+                timer_repeat_set(now + TIMER_REPEAT_TICKS);
+                next = now + TIMER_DEFER_REPEAT_TICKS;
+                goto done;
+            }
+            timer_repeat_set(now + TIMER_IDLE_REPEAT_TICKS);
+            timer_set(now);
+        }
+    }
+
+done:
+    timer_set(next);
+    irq_enable();
+}