flash: Add optional verify-after-write feature

Sometimes it might make sense to verify the written data to NOR flash.
This patch adds this feature. To enable this verify-after-write, you
need to define CONFIG_FLASH_VERIFY in your board config header.

Please note that this option is useless in nearly all cases,
since such flash programming errors usually are detected earlier
while unprotecting/erasing/programming. Please only enable
this option if you really know what you are doing.

Signed-off-by: Stefan Roese <sr@denx.de>

README

@@ -3248,6 +3248,15 @@ Configuration Settings:
 		digits and dots.  Recommended value: 45 (9..1) for 80
 		column displays, 15 (3..1) for 40 column displays.
 
+- CONFIG_FLASH_VERIFY
+		If defined, the content of the flash (destination) is compared
+		against the source after the write operation. An error message
+		will be printed when the contents are not identical.
+		Please note that this option is useless in nearly all cases,
+		since such flash programming errors usually are detected earlier
+		while unprotecting/erasing/programming. Please only enable
+		this option if you really know what you are doing.
+
 - CONFIG_SYS_RX_ETH_BUFFER:
 		Defines the number of Ethernet receive buffers. On some
 		Ethernet controllers it is recommended to set this value

common/flash.c

@@ -149,6 +149,9 @@ flash_write (char *src, ulong addr, ulong cnt)
 	flash_info_t *info_first = addr2info (addr);
 	flash_info_t *info_last  = addr2info (end );
 	flash_info_t *info;
+	__maybe_unused char *src_orig = src;
+	__maybe_unused char *addr_orig = (char *)addr;
+	__maybe_unused ulong cnt_orig = cnt;
 
 	if (cnt == 0) {
 		return (ERR_OK);
@@ -185,6 +188,14 @@ flash_write (char *src, ulong addr, ulong cnt)
 		addr += len;
 		src  += len;
 	}
+
+#if defined(CONFIG_FLASH_VERIFY)
+	if (memcmp(src_orig, addr_orig, cnt_orig)) {
+		printf("\nVerify failed!\n");
+		return ERR_PROG_ERROR;
+	}
+#endif /* CONFIG_SYS_FLASH_VERIFY_AFTER_WRITE */
+
 	return (ERR_OK);
 #endif /* CONFIG_SPD823TS */
 }