mmc: add a file to debugfs for changing host clock at runtime

For debugging power management features it is convenient to have the
possibility of changing the MMC host controller clock at runtime.  This
patch adds a 'clock' file for this under the MMC host root of debugfs.

Usage is as follows:

	# cat /sys/kernel/debug/mmc0/clock
	52000000

	# echo "1000000000" > /sys/kernel/debug/mmc0/clock
	# cat /sys/kernel/debug/mmc0/clock
	52000000

	# echo "48000000" > /sys/kernel/debug/mmc0/clock
	# cat /sys/kernel/debug/mmc0/clock
	48000000

The middle example shows limits being applied by the host driver.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Cc: Adrian Hunter <adrian.hunter@nokia.com>
[cjb: modify changelog language]
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/core/debugfs.c

@@ -134,6 +134,33 @@ static const struct file_operations mmc_ios_fops = {
 	.release	= single_release,
 };
 
+static int mmc_clock_opt_get(void *data, u64 *val)
+{
+	struct mmc_host *host = data;
+
+	*val = host->ios.clock;
+
+	return 0;
+}
+
+static int mmc_clock_opt_set(void *data, u64 val)
+{
+	struct mmc_host *host = data;
+
+	/* We need this check due to input value is u64 */
+	if (val > host->f_max)
+		return -EINVAL;
+
+	mmc_claim_host(host);
+	mmc_set_clock(host, (unsigned int) val);
+	mmc_release_host(host);
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(mmc_clock_fops, mmc_clock_opt_get, mmc_clock_opt_set,
+	"%llu\n");
+
 void mmc_add_host_debugfs(struct mmc_host *host)
 {
 	struct dentry *root;
@@ -150,11 +177,15 @@ void mmc_add_host_debugfs(struct mmc_host *host)
 	host->debugfs_root = root;
 
 	if (!debugfs_create_file("ios", S_IRUSR, root, host, &mmc_ios_fops))
-		goto err_ios;
+		goto err_node;
+
+	if (!debugfs_create_file("clock", S_IRUSR | S_IWUSR, root, host,
+			&mmc_clock_fops))
+		goto err_node;
 
 	return;
 
-err_ios:
+err_node:
 	debugfs_remove_recursive(root);
 	host->debugfs_root = NULL;
 err_root: