char/nvram: Allow the set_checksum and initialize ioctls to be omitted

The drivers/char/nvram.c module has previously supported only RTC "CMOS"
NVRAM, for which it provides appropriate checksum ioctls. Make these
ioctls optional so the module can be re-used with other kinds of NVRAM.

The ops struct methods that implement the ioctls now return error
codes so that a multi-platform kernel binary can do the right thing when
running on hardware without a suitable NVRAM.
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/nvram.c

@@ -136,16 +136,25 @@ static void __nvram_set_checksum(void)
 	__nvram_write_byte(sum & 0xff, PC_CKS_LOC + 1);
 }
 
-#if 0
-void nvram_set_checksum(void)
+static long pc_nvram_set_checksum(void)
 {
-	unsigned long flags;
+	spin_lock_irq(&rtc_lock);
+	__nvram_set_checksum();
+	spin_unlock_irq(&rtc_lock);
+	return 0;
+}
 
-	spin_lock_irqsave(&rtc_lock, flags);
+static long pc_nvram_initialize(void)
+{
+	ssize_t i;
+
+	spin_lock_irq(&rtc_lock);
+	for (i = 0; i < NVRAM_BYTES; ++i)
+		__nvram_write_byte(0, i);
 	__nvram_set_checksum();
-	spin_unlock_irqrestore(&rtc_lock, flags);
+	spin_unlock_irq(&rtc_lock);
+	return 0;
 }
-#endif  /*  0  */
 
 static ssize_t pc_nvram_get_size(void)
 {
@@ -156,6 +165,8 @@ const struct nvram_ops arch_nvram_ops = {
 	.read_byte      = pc_nvram_read_byte,
 	.write_byte     = pc_nvram_write_byte,
 	.get_size       = pc_nvram_get_size,
+	.set_checksum   = pc_nvram_set_checksum,
+	.initialize     = pc_nvram_initialize,
 };
 EXPORT_SYMBOL(arch_nvram_ops);
 #endif /* CONFIG_X86 */
@@ -241,51 +252,50 @@ static ssize_t nvram_misc_write(struct file *file, const char __user *buf,
 static long nvram_misc_ioctl(struct file *file, unsigned int cmd,
 			     unsigned long arg)
 {
-	int i;
+	long ret = -ENOTTY;
 
 	switch (cmd) {
-
 	case NVRAM_INIT:
 		/* initialize NVRAM contents and checksum */
 		if (!capable(CAP_SYS_ADMIN))
 			return -EACCES;
 
+		if (arch_nvram_ops.initialize != NULL) {
 			mutex_lock(&nvram_mutex);
-		spin_lock_irq(&rtc_lock);
-
-		for (i = 0; i < NVRAM_BYTES; ++i)
-			__nvram_write_byte(0, i);
-		__nvram_set_checksum();
-
-		spin_unlock_irq(&rtc_lock);
+			ret = arch_nvram_ops.initialize();
 			mutex_unlock(&nvram_mutex);
-		return 0;
-
+		}
+		break;
 	case NVRAM_SETCKS:
 		/* just set checksum, contents unchanged (maybe useful after
 		 * checksum garbaged somehow...) */
 		if (!capable(CAP_SYS_ADMIN))
 			return -EACCES;
 
+		if (arch_nvram_ops.set_checksum != NULL) {
 			mutex_lock(&nvram_mutex);
-		spin_lock_irq(&rtc_lock);
-		__nvram_set_checksum();
-		spin_unlock_irq(&rtc_lock);
+			ret = arch_nvram_ops.set_checksum();
 			mutex_unlock(&nvram_mutex);
-		return 0;
-
-	default:
-		return -ENOTTY;
 		}
+		break;
+	}
+	return ret;
 }
 
 static int nvram_misc_open(struct inode *inode, struct file *file)
 {
 	spin_lock(&nvram_state_lock);
 
+	/* Prevent multiple readers/writers if desired. */
 	if ((nvram_open_cnt && (file->f_flags & O_EXCL)) ||
-	    (nvram_open_mode & NVRAM_EXCL) ||
-	    ((file->f_mode & FMODE_WRITE) && (nvram_open_mode & NVRAM_WRITE))) {
+	    (nvram_open_mode & NVRAM_EXCL)) {
+		spin_unlock(&nvram_state_lock);
+		return -EBUSY;
+	}
+
+	/* Prevent multiple writers if the set_checksum ioctl is implemented. */
+	if ((arch_nvram_ops.set_checksum != NULL) &&
+	    (file->f_mode & FMODE_WRITE) && (nvram_open_mode & NVRAM_WRITE)) {
 		spin_unlock(&nvram_state_lock);
 		return -EBUSY;
 	}

include/linux/nvram.h

@@ -31,6 +31,8 @@ struct nvram_ops {
 	void            (*write_byte)(unsigned char, int);
 	ssize_t         (*read)(char *, size_t, loff_t *);
 	ssize_t         (*write)(char *, size_t, loff_t *);
+	long            (*initialize)(void);
+	long            (*set_checksum)(void);
 };
 
 extern const struct nvram_ops arch_nvram_ops;