clk: Add write operation for clk_parent debugfs node

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>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Michael Turquette <mturquette@baylibre.com>
Link: https://lore.kernel.org/r/20211013172042.10884-1-semen.protsenko@linaro.org
[sboyd@kernel.org: Collapse ifdefs]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk.c

@@ -3217,6 +3217,42 @@ 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;
@@ -3282,8 +3318,12 @@ static void clk_debug_create_one(struct clk_core *core, struct dentry *pdentry)
 #ifdef CLOCK_ALLOW_WRITE_DEBUGFS
 	debugfs_create_file("clk_prepare_enable", 0644, root, core,
 			    &clk_prepare_enable_fops);
-#endif
 
+	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);