Partition the first flash page

SAMD microcontrollers can protect the bootloader flash using
at91samd bootloader [size] command

In processors with big flash like SAMD21G18A the minimum flash partition is
256kB / 16 = 16kB, which is way bigger than minimum bootloader size.

When a "load" is executed during gdb debugging session, if the .text section
starts before 0x4000 (16kB), the entire first flash page gets erased.

This fails if the page is protected via the bootloader command.

This patch partitions the first flash page into logical smaller chunks, so
the "load" command only erases the relevant pages.

If the flash page is smaller/equal than the minimum bootloader size, do nothing.

Author: facchinm

src/flash/nor/at91samd.c

@@ -235,8 +235,14 @@ static int samd_protect_check(struct flash_bank *bank)
 		return res;
 
 	/* Lock bits are active-low */
-	for (int i = 0; i < bank->num_sectors; i++)
-		bank->sectors[i].is_protected = !(lock & (1<<i));
+	int boot_sectors = bank->num_sectors - SAMD_NUM_SECTORS;
+	for (int i = 0; i < bank->num_sectors; i++)	{
+		if (i < boot_sectors) {
+			bank->sectors[i].is_protected = !(lock & (1<<0));
+		} else {
+			bank->sectors[i].is_protected = !(lock & (1<<(i-boot_sectors)));
+		}
+	}
 
 	return ERROR_OK;
 }
@@ -304,17 +310,41 @@ static int samd_probe(struct flash_bank *bank)
 				part->flash_kb, chip->num_pages, chip->page_size);
 	}
 
+	/* Partition the first page into subpages, to allow debugging if boot sector is protected
+	 * The minimum boot page allowed is 2kb */
+	int boot_sectors = chip->sector_size / 2048;
+	int boot_offset = 0;
+
+	if (boot_sectors == 1) {
+		boot_sectors = 0;
+	}
+
 	/* Allocate the sector table */
-	bank->num_sectors = SAMD_NUM_SECTORS;
+	if (boot_sectors > 0) {
+		LOG_INFO("SAMD: partitioning the first flash page into %d subpages",
+				boot_sectors);
+		bank->num_sectors = SAMD_NUM_SECTORS + boot_sectors - 1;
+		boot_offset = chip->sector_size;
+	} else {
+		bank->num_sectors = SAMD_NUM_SECTORS;
+	}
+
 	bank->sectors = calloc(bank->num_sectors, sizeof((bank->sectors)[0]));
 	if (!bank->sectors)
 		return ERROR_FAIL;
 
-	/* Fill out the sector information: all SAMD sectors are the same size and
-	 * there is always a fixed number of them. */
-	for (int i = 0; i < bank->num_sectors; i++) {
+	/* Will be skipped if SAMD_BOOT_SECTORS == 0 */
+	for (int i = 0; i < boot_sectors; i++) {
+		bank->sectors[i].size = chip->sector_size / boot_sectors;
+		bank->sectors[i].offset = i * chip->sector_size / boot_sectors;
+		/* mark as unknown */
+		bank->sectors[i].is_erased = -1;
+		bank->sectors[i].is_protected = -1;
+	}
+
+	for (int i = boot_sectors; i < bank->num_sectors; i++) {
 		bank->sectors[i].size = chip->sector_size;
-		bank->sectors[i].offset = i * chip->sector_size;
+		bank->sectors[i].offset = ((i - boot_sectors) * chip->sector_size ) + boot_offset;
 		/* mark as unknown */
 		bank->sectors[i].is_erased = -1;
 		bank->sectors[i].is_protected = -1;