[PATCH v6] clk: Add write operation for clk_parent debugfs node

[Sam Protsenko <semen.protsenko@linaro.org>]

Useful for testing mux clocks. One can write the index of the parent to
be set into clk_parent node, starting from 0. Example

    # cd /sys/kernel/debug/clk/mout_peri_bus
    # cat clk_possible_parents
      dout_shared0_div4 dout_shared1_div4
    # cat clk_parent
      dout_shared0_div4
    # echo 1 > clk_parent
    # cat clk_parent
      dout_shared1_div4

CLOCK_ALLOW_WRITE_DEBUGFS has to be defined in drivers/clk/clk.c in
order to use this feature.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
Changes in v6:
  - Added R-b tag by Geert Uytterhoeven
  - Removed curly braces around 'else' block (now delta is minimal and
    the code looks better)

Changes in v5:
  - Used kstrtou8_from_user() API
  - Got rid of code duplication
  - Fixed typo in commit message
  - Added R-b tag by Andy Shevchenko

Changes in v4:
  - Fixed the commit title

Changes in v3:
  - Removed unwanted changes added by mistake

Changes in v2:
  - Merged write() function into existing 'clk_parent' file
  - Removed 'if (val >= core->num_parents)' check
  - Removed incorrect usage of IS_ERR_OR_NULL()

 drivers/clk/clk.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 65508eb89ec9..bf1eadfeaee0 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -3214,6 +3214,42 @@ static int current_parent_show(struct seq_file *s, void *data)
 }
 DEFINE_SHOW_ATTRIBUTE(current_parent);
 
+#ifdef CLOCK_ALLOW_WRITE_DEBUGFS
+static ssize_t current_parent_write(struct file *file, const char __user *ubuf,
+				    size_t count, loff_t *ppos)
+{
+	struct seq_file *s = file->private_data;
+	struct clk_core *core = s->private;
+	struct clk_core *parent;
+	u8 idx;
+	int err;
+
+	err = kstrtou8_from_user(ubuf, count, 0, &idx);
+	if (err < 0)
+		return err;
+
+	parent = clk_core_get_parent_by_index(core, idx);
+	if (!parent)
+		return -ENOENT;
+
+	clk_prepare_lock();
+	err = clk_core_set_parent_nolock(core, parent);
+	clk_prepare_unlock();
+	if (err)
+		return err;
+
+	return count;
+}
+
+static const struct file_operations current_parent_rw_fops = {
+	.open		= current_parent_open,
+	.write		= current_parent_write,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+#endif
+
 static int clk_duty_cycle_show(struct seq_file *s, void *data)
 {
 	struct clk_core *core = s->private;
@@ -3281,6 +3317,12 @@ static void clk_debug_create_one(struct clk_core *core, struct dentry *pdentry)
 			    &clk_prepare_enable_fops);
 #endif
 
+#ifdef CLOCK_ALLOW_WRITE_DEBUGFS
+	if (core->num_parents > 1)
+		debugfs_create_file("clk_parent", 0644, root, core,
+				    &current_parent_rw_fops);
+	else
+#endif
 	if (core->num_parents > 0)
 		debugfs_create_file("clk_parent", 0444, root, core,
 				    &current_parent_fops);
-- 
2.30.2