cfi: Check for blank before erase

Added an optional check in the CFI driver to evaluate if the sector is
already blank before issuing an erase command.  Improves erase time by
over a factor of 10 if already blank.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Stefan Roese <sr@denx.de>

drivers/mtd/cfi_flash.c

@@ -1078,6 +1078,32 @@ int flash_erase (flash_info_t * info, int s_first, int s_last)
 
 	for (sect = s_first; sect <= s_last; sect++) {
 		if (info->protect[sect] == 0) { /* not protected */
+#ifdef CONFIG_SYS_FLASH_CHECK_BLANK_BEFORE_ERASE
+			int k;
+			int size;
+			int erased;
+			u32 *flash;
+
+			/*
+			 * Check if whole sector is erased
+			 */
+			size = flash_sector_size(info, sect);
+			erased = 1;
+			flash = (u32 *)info->start[sect];
+			/* divide by 4 for longword access */
+			size = size >> 2;
+			for (k = 0; k < size; k++) {
+				if (flash_read32(flash++) != 0xffffffff) {
+					erased = 0;
+					break;
+				}
+			}
+			if (erased) {
+				if (flash_verbose)
+					putc(',');
+				continue;
+			}
+#endif
 			switch (info->vendor) {
 			case CFI_CMDSET_INTEL_PROG_REGIONS:
 			case CFI_CMDSET_INTEL_STANDARD: