Clear STM32L4 PER,MER,PG bits

The controller doesn't clear them automatically when the operation
finishes, so to avoid getting errors we need to clear them manually
before we request another operation.

src/stlink-common.c

@@ -80,6 +80,10 @@
 #define STM32L4_FLASH_CR_STRT   16      /* Start command */
 #define STM32L4_FLASH_CR_BKER   11      /* Bank select for page erase */
 #define STM32L4_FLASH_CR_PNB    3       /* Page number (8 bits) */
+// Bits requesting flash operations (useful when we want to clear them)
+#define STM32L4_FLASH_CR_OPBITS                                     \
+    ((1lu<<STM32L4_FLASH_CR_PG) | (1lu<<STM32L4_FLASH_CR_PER)       \
+    | (1lu<<STM32L4_FLASH_CR_MER1) | (1lu<<STM32L4_FLASH_CR_MER1))
 // Page is fully specified by BKER and PNB
 #define STM32L4_FLASH_CR_PAGEMASK (0x1fflu << STM32L4_FLASH_CR_PNB)
 
@@ -263,6 +267,7 @@ static void set_flash_cr_pg(stlink_t *sl) {
         stlink_write_debug32(sl, FLASH_F4_CR, x);
     } else if (sl->chip_id == STM32_CHIPID_L4) {
         uint32_t x = read_flash_cr(sl);
+        x &=~ STM32L4_FLASH_CR_OPBITS;
         x |= (1 << STM32L4_FLASH_CR_PG);
         stlink_write_debug32(sl, STM32L4_FLASH_CR, x);
     } else {
@@ -282,14 +287,8 @@ static void __attribute__((unused)) clear_flash_cr_pg(stlink_t *sl) {
 }
 
 static void set_flash_cr_per(stlink_t *sl) {
-    if (sl->chip_id == STM32_CHIPID_L4) {
-    	uint32_t n = read_flash_cr(sl);
-	n |= (1lu << STM32L4_FLASH_CR_PER);
-    	stlink_write_debug32(sl, STM32L4_FLASH_CR, n);
-    } else {
-    	const uint32_t n = 1 << FLASH_CR_PER;
-    	stlink_write_debug32(sl, FLASH_CR, n);
-    }
+    const uint32_t n = 1 << FLASH_CR_PER;
+    stlink_write_debug32(sl, FLASH_CR, n);
 }
 
 static void __attribute__((unused)) clear_flash_cr_per(stlink_t *sl) {
@@ -303,12 +302,12 @@ static void set_flash_cr_mer(stlink_t *sl) {
             (sl->chip_id == STM32_CHIPID_F446))
         stlink_write_debug32(sl, FLASH_F4_CR,
                 stlink_read_debug32(sl, FLASH_F4_CR) | (1 << FLASH_CR_MER));
-    else if (sl->chip_id == STM32_CHIPID_L4)
-	stlink_write_debug32(sl, STM32L4_FLASH_CR,
-		stlink_read_debug32(sl, STM32L4_FLASH_CR) |
-			(1lu << STM32L4_FLASH_CR_MER1) |
-			(1lu << STM32L4_FLASH_CR_MER2));
-    else
+    else if (sl->chip_id == STM32_CHIPID_L4) {
+        uint32_t x = stlink_read_debug32(sl, STM32L4_FLASH_CR);
+        x &=~ STM32L4_FLASH_CR_OPBITS;
+        x |= (1lu << STM32L4_FLASH_CR_MER1) | (1lu << STM32L4_FLASH_CR_MER2);
+        stlink_write_debug32(sl, STM32L4_FLASH_CR, x);
+    } else
         stlink_write_debug32(sl, FLASH_CR,
                 stlink_read_debug32(sl, FLASH_CR) | (1 << FLASH_CR_MER));
 }
@@ -434,6 +433,7 @@ static inline void write_flash_cr_snb(stlink_t *sl, uint32_t n) {
 
 static inline void write_flash_cr_bker_pnb(stlink_t *sl, uint32_t n) {
     uint32_t x = read_flash_cr(sl);
+    x &=~ STM32L4_FLASH_CR_OPBITS;
     x &=~ STM32L4_FLASH_CR_PAGEMASK;
     x |= (n << STM32L4_FLASH_CR_PNB);
     x |= (1lu << STM32L4_FLASH_CR_PER);