fat: fatwrite: fix the command for FAT12

The u-boot command fatwrite empties FAT clusters from the beginning
till the end of the file.
Specifically for FAT12 it fails to detect the end of the file and goes
beyond the file bounds thus corrupting the file system.

Additionally, FAT entry chaining-up into a file is not implemented
for FAT12.

The users normally workaround this by re-formatting the partition as
FAT16/FAT32, like here:
https://github.com/FEDEVEL/openrex-uboot-v2015.10/issues/1

The patch fixes the bounds of a file and FAT12 entries chaining into
a file, including EOF markup.

Signed-off-by: Philipp Skadorov <philipp.skadorov@savoirfairelinux.com>

fs/fat/fat_write.c

@@ -467,11 +467,12 @@ get_long_file_name(fsdata *mydata, int curclust, __u8 *cluster,
 }
 
 /*
- * Set the entry at index 'entry' in a FAT (16/32) table.
+ * Set the entry at index 'entry' in a FAT (12/16/32) table.
  */
 static int set_fatent_value(fsdata *mydata, __u32 entry, __u32 entry_value)
 {
-	__u32 bufnum, offset;
+	__u32 bufnum, offset, off16;
+	__u16 val1, val2;
 
 	switch (mydata->fatsize) {
 	case 32:
@@ -482,6 +483,10 @@ static int set_fatent_value(fsdata *mydata, __u32 entry, __u32 entry_value)
 		bufnum = entry / FAT16BUFSIZE;
 		offset = entry - bufnum * FAT16BUFSIZE;
 		break;
+	case 12:
+		bufnum = entry / FAT12BUFSIZE;
+		offset = entry - bufnum * FAT12BUFSIZE;
+		break;
 	default:
 		/* Unsupported FAT size */
 		return -1;
@@ -521,6 +526,45 @@ static int set_fatent_value(fsdata *mydata, __u32 entry, __u32 entry_value)
 	case 16:
 		((__u16 *) mydata->fatbuf)[offset] = cpu_to_le16(entry_value);
 		break;
+	case 12:
+		off16 = (offset * 3) / 4;
+
+		switch (offset & 0x3) {
+		case 0:
+			val1 = cpu_to_le16(entry_value) & 0xfff;
+			((__u16 *)mydata->fatbuf)[off16] &= ~0xfff;
+			((__u16 *)mydata->fatbuf)[off16] |= val1;
+			break;
+		case 1:
+			val1 = cpu_to_le16(entry_value) & 0xf;
+			val2 = (cpu_to_le16(entry_value) >> 4) & 0xff;
+
+			((__u16 *)mydata->fatbuf)[off16] &= ~0xf000;
+			((__u16 *)mydata->fatbuf)[off16] |= (val1 << 12);
+
+			((__u16 *)mydata->fatbuf)[off16 + 1] &= ~0xff;
+			((__u16 *)mydata->fatbuf)[off16 + 1] |= val2;
+			break;
+		case 2:
+			val1 = cpu_to_le16(entry_value) & 0xff;
+			val2 = (cpu_to_le16(entry_value) >> 8) & 0xf;
+
+			((__u16 *)mydata->fatbuf)[off16] &= ~0xff00;
+			((__u16 *)mydata->fatbuf)[off16] |= (val1 << 8);
+
+			((__u16 *)mydata->fatbuf)[off16 + 1] &= ~0xf;
+			((__u16 *)mydata->fatbuf)[off16 + 1] |= val2;
+			break;
+		case 3:
+			val1 = cpu_to_le16(entry_value) & 0xfff;
+			((__u16 *)mydata->fatbuf)[off16] &= ~0xfff0;
+			((__u16 *)mydata->fatbuf)[off16] |= (val1 << 4);
+			break;
+		default:
+			break;
+		}
+
+		break;
 	default:
 		return -1;
 	}
@@ -529,7 +573,7 @@ static int set_fatent_value(fsdata *mydata, __u32 entry, __u32 entry_value)
 }
 
 /*
- * Determine the next free cluster after 'entry' in a FAT (16/32) table
+ * Determine the next free cluster after 'entry' in a FAT (12/16/32) table
  * and link it to 'entry'. EOC marker is not set on returned entry.
  */
 static __u32 determine_fatent(fsdata *mydata, __u32 entry)
@@ -651,6 +695,8 @@ static void flush_dir_table(fsdata *mydata, dir_entry **dentptr)
 		set_fatent_value(mydata, dir_newclust, 0xffffff8);
 	else if (mydata->fatsize == 16)
 		set_fatent_value(mydata, dir_newclust, 0xfff8);
+	else if (mydata->fatsize == 12)
+		set_fatent_value(mydata, dir_newclust, 0xff8);
 
 	dir_curclust = dir_newclust;
 
@@ -670,16 +716,13 @@ static int clear_fatent(fsdata *mydata, __u32 entry)
 {
 	__u32 fat_val;
 
-	while (1) {
+	while (!CHECK_CLUST(entry, mydata->fatsize)) {
 		fat_val = get_fatent_value(mydata, entry);
 		if (fat_val != 0)
 			set_fatent_value(mydata, entry, 0);
 		else
 			break;
 
-		if (fat_val == 0xfffffff || fat_val == 0xffff)
-			break;
-
 		entry = fat_val;
 	}
 
@@ -750,7 +793,9 @@ set_contents(fsdata *mydata, dir_entry *dentptr, __u8 *buffer,
 		*gotsize += actsize;
 
 		/* Mark end of file in FAT */
-		if (mydata->fatsize == 16)
+		if (mydata->fatsize == 12)
+			newclust = 0xfff;
+		else if (mydata->fatsize == 16)
 			newclust = 0xffff;
 		else if (mydata->fatsize == 32)
 			newclust = 0xfffffff;