driver: timer: npcx: bypass timer counter reading issue

Originally, when the timer's source clock is 32.768 kHz, the timer driver
uses two consecutive reads to ensure the timer reading is correct.
However, it is not robust enough due to an asynchronous timing issue in
the chip. The workaround is to add at least two NOPs between the
LDR and CMP instructions. This commit implements the workaround in the
assembly code to ensure it is not affected by the compiler toolchain
or optimization flags.

Signed-off-by: Jun Lin <[email protected]>

drivers/timer/npcx_itim_timer.c

@@ -216,15 +216,18 @@ static inline uint32_t npcx_itim_get_evt_cyc32(void)
 {
 	uint32_t cnt1, cnt2;
 
-	cnt1 = evt_tmr->ITCNT32;
-	/*
-	 * Wait for two consecutive equal values are read since the source clock
-	 * of event timer is 32KHz.
-	 */
-	while ((cnt2 = evt_tmr->ITCNT32) != cnt1) {
-		cnt1 = cnt2;
-	}
-
+	__asm__ volatile(
+		"ldr    %[c2], [%[tmr], %[itcnt32_off]]\n\t"
+		".read_itim_cnt_loop_%=:\n\t"
+		"mov    %[c1], %[c2]\n\t"
+		"ldr    %[c2], [%[tmr], %[itcnt32_off]]\n\t"
+		"nop\n\t"
+		"nop\n\t"
+		"cmp    %[c1], %[c2]\n\t"
+		"bne    .read_itim_cnt_loop_%=\n\t"
+		: [c1] "=&r"(cnt1), [c2] "=&r"(cnt2)
+		: [tmr] "r"(evt_tmr), [itcnt32_off] "i"(offsetof(struct itim32_reg, ITCNT32))
+		: "memory");
 	/* Return current value of 32-bit counter of event timer  */
 	return cnt2;
 }