partition: DOS: Add parameter for the NT disk Signature

The NT disk Signature is a unique 32-bit value in the MBR allowing
to identify a MSDOS partitioned disk. The signature can be used to
specify the Linux rootfs.
This patch adds support for the NT disk Signature in the form of a
device parameter which can both be read and written.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

common/partitions/dos.c

@@ -16,6 +16,8 @@
 #include <disks.h>
 #include <init.h>
 #include <asm/unaligned.h>
+#include <dma.h>
+#include <linux/err.h>
 
 #include "parser.h"
 
@@ -40,6 +42,74 @@ static int disk_guess_size(struct device_d *dev, struct partition_entry *table)
 	return (int)size;
 }
 
+static void *read_mbr(struct block_device *blk)
+{
+	void *buf = dma_alloc(SECTOR_SIZE);
+	int ret;
+
+	ret = block_read(blk, buf, 0, 1);
+	if (ret) {
+		free(buf);
+		return NULL;
+	}
+
+	return buf;
+}
+
+static int write_mbr(struct block_device *blk, void *buf)
+{
+	int ret;
+
+	ret = block_write(blk, buf, 0, 1);
+	if (ret)
+		return ret;
+
+	return block_flush(blk);
+}
+
+struct disk_signature_priv {
+	uint32_t signature;
+	struct block_device *blk;
+};
+
+static int dos_set_disk_signature(struct param_d *p, void *_priv)
+{
+	struct disk_signature_priv *priv = _priv;
+	struct block_device *blk = priv->blk;
+	void *buf;
+	__le32 *disksigp;
+	int ret;
+
+	buf = read_mbr(blk);
+	if (!buf)
+		return -EIO;
+
+	disksigp = buf + 0x1b8;
+
+	*disksigp = cpu_to_le32(priv->signature);
+
+	ret = write_mbr(blk, buf);
+
+	free(buf);
+
+	return ret;
+}
+
+static int dos_get_disk_signature(struct param_d *p, void *_priv)
+{
+	struct disk_signature_priv *priv = _priv;
+	struct block_device *blk = priv->blk;
+	void *buf;
+
+	buf = read_mbr(blk);
+	if (!buf)
+		return -EIO;
+
+	priv->signature = le32_to_cpup((__le32 *)(buf + 0x1b8));
+
+	return 0;
+}
+
 /**
  * Check if a DOS like partition describes this block device
  * @param blk Block device to register to
@@ -55,6 +125,7 @@ static void dos_partition(void *buf, struct block_device *blk,
 	struct partition pentry;
 	uint8_t *buffer = buf;
 	int i;
+	struct disk_signature_priv *dsp;
 
 	table = (struct partition_entry *)&buffer[446];
 
@@ -74,6 +145,23 @@ static void dos_partition(void *buf, struct block_device *blk,
 			dev_dbg(blk->dev, "Skipping empty partition %d\n", i);
 		}
 	}
+
+	dsp = xzalloc(sizeof(*dsp));
+	dsp->blk = blk;
+
+	/*
+	 * This parameter contains the NT disk signature. This allows to
+	 * to specify the Linux rootfs using the following syntax:
+	 *
+	 *   root=PARTUUID=ssssssss-pp
+	 *
+	 * where ssssssss is a zero-filled hex representation of the 32-bit
+	 * signature and pp is a zero-filled hex representation of the 1-based
+	 * partition number.
+	 */
+	dev_add_param_int(blk->dev, "nt_signature",
+			dos_set_disk_signature, dos_get_disk_signature,
+			&dsp->signature, "%08x", dsp);
 }
 
 static struct partition_parser dos = {