Add "pinmux-select" to debugfs which will activate a function and group
when "<function-name group-name>" are written to the file. The write
operation pinmux_select() handles this by checking that the names map to
valid selectors and then calling ops->set_mux().

The existing "pinmux-functions" debugfs file lists the pin functions
registered for the pin controller. For example:

function: pinmux-uart0, groups = [ pinmux-uart0-pins ]
function: pinmux-mmc0, groups = [ pinmux-mmc0-pins ]
function: pinmux-mmc1, groups = [ pinmux-mmc1-pins ]
function: pinmux-i2c0, groups = [ pinmux-i2c0-pins ]
function: pinmux-i2c1, groups = [ pinmux-i2c1-pins ]
function: pinmux-spi1, groups = [ pinmux-spi1-pins ]

To activate function pinmux-i2c1 and group pinmux-i2c1-pins:

echo "pinmux-i2c1 pinmux-i2c1-pins" > pinmux-select

Signed-off-by: Drew Fustini <redacted>
---
 drivers/pinctrl/pinmux.c | 107 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)

diff --git a/drivers/pinctrl/pinmux.c b/drivers/pinctrl/pinmux.c
index c651b2db0925..23fa32f0a067 100644
--- a/drivers/pinctrl/pinmux.c
+++ b/drivers/pinctrl/pinmux.c

@@ -673,6 +673,111 @@ void pinmux_show_setting(struct seq_file *s,
 DEFINE_SHOW_ATTRIBUTE(pinmux_functions);
 DEFINE_SHOW_ATTRIBUTE(pinmux_pins);
 
+#define PINMUX_MAX_NAME 64
+static ssize_t pinmux_select(struct file *file, const char __user *user_buf,
+				   size_t len, loff_t *ppos)
+{
+	struct seq_file *sfile = file->private_data;
+	struct pinctrl_dev *pctldev = sfile->private;
+	const struct pinmux_ops *pmxops = pctldev->desc->pmxops;
+	const char *const *groups;
+	char *buf, *fname, *gname;
+	unsigned int num_groups;
+	int fsel, gsel, ret;
+
+	if (len > (PINMUX_MAX_NAME * 2)) {
+		dev_err(pctldev->dev, "write too big for buffer");
+		return -EINVAL;
+	}
+
+	buf = kzalloc(PINMUX_MAX_NAME * 2, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	fname = kzalloc(PINMUX_MAX_NAME, GFP_KERNEL);
+	if (!fname) {
+		ret = -ENOMEM;
+		goto free_buf;
+	}
+
+	gname = kzalloc(PINMUX_MAX_NAME, GFP_KERNEL);
+	if (!buf) {
+		ret = -ENOMEM;
+		goto free_fname;
+	}
+
+	ret = strncpy_from_user(buf, user_buf, PINMUX_MAX_NAME * 2);
+	if (ret < 0) {
+		dev_err(pctldev->dev, "failed to copy buffer from userspace");
+		goto free_gname;
+	}
+	buf[len-1] = '\0';
+
+	ret = sscanf(buf, "%s %s", fname, gname);
+	if (ret != 2) {
+		dev_err(pctldev->dev, "expected format: <function-name> <group-name>");
+		goto free_gname;
+	}
+
+	fsel = pinmux_func_name_to_selector(pctldev, fname);
+	if (fsel < 0) {
+		dev_err(pctldev->dev, "invalid function %s in map table\n", fname);
+		ret = -EINVAL;
+		goto free_gname;
+	}
+
+	ret = pmxops->get_function_groups(pctldev, fsel, &groups, &num_groups);
+	if (ret) {
+		dev_err(pctldev->dev, "no groups for function %d (%s)", fsel, fname);
+		goto free_gname;
+
+	}
+
+	ret = match_string(groups, num_groups, gname);
+	if (ret < 0) {
+		dev_err(pctldev->dev, "invalid group %s", gname);
+		goto free_gname;
+	}
+
+	ret = pinctrl_get_group_selector(pctldev, gname);
+	if (ret < 0) {
+		dev_err(pctldev->dev, "failed to get group selectorL %s", gname);
+		goto free_gname;
+	}
+	gsel = ret;
+
+	ret = pmxops->set_mux(pctldev, fsel, gsel);
+	if (ret) {
+		dev_err(pctldev->dev, "set_mux() failed: %d", ret);
+		goto free_gname;
+	}
+
+	return len;
+
+free_gname:
+	devm_kfree(pctldev->dev, gname);
+free_fname:
+	devm_kfree(pctldev->dev, fname);
+free_buf:
+	devm_kfree(pctldev->dev, buf);