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Date:   Thu, 17 Dec 2020 20:51:34 -0800
From:   Drew Fustini <drew@...gleboard.org>
To:     linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Drew Fustini <drew@...gleboard.org>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Pantelis Antoniou <pantelis.antoniou@...aro.org>,
        Pantelis Antoniou <pantelis.antoniou@...il.com>,
        Jason Kridner <jkridner@...gleboard.org>,
        Robert Nelson <robertcnelson@...gleboard.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Tony Lindgren <tony@...mide.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: [RFC PATCH v2] pinctrl: add helper to expose pinctrl state in debugfs

BeagleBoard.org [0] currently uses an out-of-tree driver called
bone-pinmux-helper [1] developed by Pantelis Antoniou [2] back in 2013.
The driver assists users of our BeagleBone and PocketBeagle boards in
rapid prototyping by allowing them to change at run-time between defined
set of pinctrl states [3] for each pin on the expansion connectors [4].
This is achieved by exposing a 'state' file in sysfs for each pin which
is used by our 'config-pin' utility [5].

Our goal is to eliminate all out-of-tree drivers for BeagleBoard.org
boards and thus I have been working to replace bone-pinmux-helper with a
new driver that could be acceptable upstream. My understanding is that
debugfs, unlike sysfs, could be the appropriate mechanism to expose such
functionality.

Here is an example of how pin P9.14 on the BeagleBone Black expansion
connector [6] would be represented in device tree:

&am33xx_pinmux {
	/* P9_14 (ZCZ ball U14) gpio1_18 */
	P9_14_default_pin: pinmux_P9_14_default_pin { pinctrl-single,pins = <
		AM33XX_IOPAD(0x0848, PIN_OUTPUT_PULLDOWN | INPUT_EN | MUX_MODE7) >; };
	P9_14_gpio_pin: pinmux_P9_14_gpio_pin { pinctrl-single,pins = <
		AM33XX_IOPAD(0x0848, PIN_OUTPUT | INPUT_EN | MUX_MODE7) >; };
	P9_14_gpio_pu_pin: pinmux_P9_14_gpio_pu_pin { pinctrl-single,pins = <
		AM33XX_IOPAD(0x0848, PIN_OUTPUT_PULLUP | INPUT_EN | MUX_MODE7) >; };
	P9_14_gpio_pd_pin: pinmux_P9_14_gpio_pd_pin { pinctrl-single,pins = <
		AM33XX_IOPAD(0x0848, PIN_OUTPUT_PULLDOWN | INPUT_EN | MUX_MODE7) >; };
	P9_14_gpio_input_pin: pinmux_P9_14_gpio_input_pin { pinctrl-single,pins = <
		AM33XX_IOPAD(0x0848, PIN_INPUT | MUX_MODE7) >; };
	P9_14_pwm_pin: pinmux_P9_14_pwm_pin { pinctrl-single,pins = <
		AM33XX_IOPAD(0x0848, PIN_OUTPUT_PULLDOWN | INPUT_EN | MUX_MODE6) >; };
};

&ocp {
	/* P9_14 (ZCZ ball U14) */
	P9_14_pinmux {
		compatible = "pinctrl,state-helper";
		status = "okay";
		pinctrl-names = "default", "gpio", "gpio_pu", "gpio_pd", "gpio_input", "pwm";
		pinctrl-0 = <&P9_14_default_pin>;
		pinctrl-1 = <&P9_14_gpio_pin>;
		pinctrl-2 = <&P9_14_gpio_pu_pin>;
		pinctrl-3 = <&P9_14_gpio_pd_pin>;
		pinctrl-4 = <&P9_14_gpio_input_pin>;
		pinctrl-5 = <&P9_14_pwm_pin>;
	};
};

I used the compatible string "pinctrl,state-helper" but would appreciate
advice on how to best name this. Should I create a new vendor prefix?

The P9_14_pinmux entry would cause pinctrl-state-helper to be probed.
The driver would create the corresponding pinctrl state file in debugfs
for the pin.  Here is an example of how the state can be read and
written from userspace:

root@...glebone:~# cat /sys/kernel/debug/pinctrl/pinctrl_state/ocp:P9_14_pinmux/state
default
root@...glebone:~# echo pwm > /sys/kernel/debug/pinctrl/pinctrl_state/ocp:P9_14_pinmux/state
root@...glebone:~# cat /sys/kernel/debug/pinctrl/pinctrl_state/ocp:P9_14_pinmux/state
pwm

I would very much appreciate feedback on both this general concept, and
also specific areas in which the code should be changed to be acceptable
upstream.

Thank you!
Drew

[0] http://beagleboard.org/latest-images
[1] https://github.com/beagleboard/linux/blob/5.4/drivers/misc/cape/beaglebone/bone-pinmux-helper.c
[2] https://github.com/RobertCNelson/linux-dev/blob/master/patches/drivers/ti/gpio/0001-BeagleBone-pinmux-helper.patch
[3] https://github.com/beagleboard/BeagleBoard-DeviceTrees/blob/v5.4.x-ti-overlays/src/arm/am335x-bone-common-univ.dtsi#L2084
[4] https://github.com/beagleboard/beaglebone-black/wiki/System-Reference-Manual#section-7-1
[5] https://github.com/beagleboard/bb.org-overlays/blob/master/tools/beaglebone-universal-io/config-pin
[6] https://beagleboard.org/Support/bone101/#headers

Cc: Pantelis Antoniou <pantelis.antoniou@...sulko.com>
Cc: Pantelis Antoniou <pantelis.antoniou@...aro.org>
Cc: Pantelis Antoniou <pantelis.antoniou@...il.com>
Cc: Jason Kridner <jkridner@...gleboard.org>
Cc: Robert Nelson <robertcnelson@...gleboard.org>
Cc: Linus Walleij <linus.walleij@...aro.org>
Cc: Tony Lindgren <tony@...mide.com>
Cc: Andy Shevchenko <andy.shevchenko@...il.com>
Signed-off-by: Drew Fustini <drew@...gleboard.org>
---
v2 changes:
- add RFC: comments in places where I would appreciate feedback
- remove panto@...oniou-consulting.com as it is bouncing
- add alternative email addresses for Pantelis
- update pinctrl_state_read() to use kasprintf() based on Andy's advice
- update pinctrl_state_write() to use memdup_user_nul()
- call debugfs_lookup() to use existing "pinctrl" dir as parent
- create subdir "pinctrl_state" to be parent of per-pin directories
- add pinctrl_state_helper_init() and pinctrl_state_helper_exit() to
handle creating and cleaning up debugfs_root ("pinctrl_state")

 drivers/pinctrl/Kconfig                |  10 +
 drivers/pinctrl/Makefile               |   1 +
 drivers/pinctrl/pinctrl-state-helper.c | 241 +++++++++++++++++++++++++
 3 files changed, 252 insertions(+)
 create mode 100644 drivers/pinctrl/pinctrl-state-helper.c

diff --git a/drivers/pinctrl/Kconfig b/drivers/pinctrl/Kconfig
index 8828613c4e0e..4faed5c8c83b 100644
--- a/drivers/pinctrl/Kconfig
+++ b/drivers/pinctrl/Kconfig
@@ -255,6 +255,16 @@ config PINCTRL_SINGLE
 	help
 	  This selects the device tree based generic pinctrl driver.
 
+config PINCTRL_STATE_HELPER
+	tristate "Helper to expose pinctrl state to debugfs"
+	depends on OF
+	depends on HAS_IOMEM
+	select GENERIC_PINCTRL_GROUPS
+	select GENERIC_PINMUX_FUNCTIONS
+	select GENERIC_PINCONF
+	help
+	  This selects the device tree based generic pinctrl driver.
+
 config PINCTRL_SIRF
 	bool "CSR SiRFprimaII pin controller driver"
 	depends on ARCH_SIRF
diff --git a/drivers/pinctrl/Makefile b/drivers/pinctrl/Makefile
index 1731b2154df9..156c356dbd3f 100644
--- a/drivers/pinctrl/Makefile
+++ b/drivers/pinctrl/Makefile
@@ -34,6 +34,7 @@ obj-$(CONFIG_PINCTRL_RZA1)	+= pinctrl-rza1.o
 obj-$(CONFIG_PINCTRL_RZA2)	+= pinctrl-rza2.o
 obj-$(CONFIG_PINCTRL_RZN1)	+= pinctrl-rzn1.o
 obj-$(CONFIG_PINCTRL_SINGLE)	+= pinctrl-single.o
+obj-$(CONFIG_PINCTRL_STATE_HELPER)	+= pinctrl-state-helper.o
 obj-$(CONFIG_PINCTRL_SIRF)	+= sirf/
 obj-$(CONFIG_PINCTRL_SX150X)	+= pinctrl-sx150x.o
 obj-$(CONFIG_ARCH_TEGRA)	+= tegra/
diff --git a/drivers/pinctrl/pinctrl-state-helper.c b/drivers/pinctrl/pinctrl-state-helper.c
new file mode 100644
index 000000000000..81f9932d6ec5
--- /dev/null
+++ b/drivers/pinctrl/pinctrl-state-helper.c
@@ -0,0 +1,241 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Pinmux state helper driver
+ *
+ * Copyright (C) 2013 Pantelis Antoniou <panto@...oniou-consulting.com>
+ * Copyright (C) 2020 Drew Fustini <drew@...gleboard.org>
+ */
+
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/of.h>
+#include <linux/of_device.h>
+#include <linux/pinctrl/pinctrl.h>
+#include <linux/fs.h>
+#include <linux/path.h>
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+#include <linux/dcache.h>
+#include "core.h"
+
+#define DRIVER_NAME	"pinctrl_state_helper"
+/* RFC: is there a better directory name? */
+#define DEBUGFS_DIR	"pinctrl_state"
+#define STATE_NAME_MAX	64
+
+static struct dentry *debugfs_root;
+
+struct pinctrl_state_helper_priv {
+	struct device *dev;
+	struct pinctrl *pinctrl;
+	struct dentry *pin_dentry;
+	/* RFC: should this be dynamically allocated? */
+	char selected_state_name[STATE_NAME_MAX];
+};
+
+static ssize_t pinctrl_state_read(struct file *file, char __user *user_buf,
+				  size_t cnt, loff_t *ppos)
+{
+	struct pinctrl_state_helper_priv *priv;
+	struct seq_file *sfile;
+	char *state_name;
+	ssize_t len;
+	char *buf;
+
+	if (*ppos != 0)
+		return 0;
+
+	sfile = file->private_data;
+	if (!sfile)
+		return -ENODEV;
+
+	priv = sfile->private;
+	if (!priv)
+		return -ENODEV;
+
+	state_name = priv->selected_state_name;
+	if (state_name == NULL || strlen(state_name) == 0)
+		state_name = "none";
+
+	buf = kasprintf(GFP_KERNEL, "%s\n", state_name);
+	if (!buf)
+		return -ENOMEM;
+
+	if (cnt < strlen(buf)) {
+		kfree(buf);
+		return -ENOSPC;
+	}
+
+	len = simple_read_from_buffer(user_buf, cnt, ppos, buf, strlen(buf));
+	kfree(buf);
+	return len;
+}
+
+static ssize_t pinctrl_state_write(struct file *file, const char __user *user_buf,
+				   size_t cnt, loff_t *ppos)
+{
+	int err;
+	char *state_name;
+	struct seq_file *sfile;
+	struct pinctrl *p;
+	struct pinctrl_state *state;
+	struct pinctrl_state_helper_priv *priv;
+
+	if (*ppos != 0)
+		return -EINVAL;
+
+	if (cnt == 0)
+		return 0;
+
+	state_name = memdup_user_nul(user_buf, cnt);
+	if (IS_ERR(state_name))
+		return PTR_ERR(state_name);
+
+	if (state_name[cnt - 1] == '\n')
+		state_name[cnt - 1] = '\0';
+
+	sfile = file->private_data;
+	priv = sfile->private;
+	p = devm_pinctrl_get(priv->dev);
+	state = pinctrl_lookup_state(p, state_name);
+	if (IS_ERR(state)) {
+		err = PTR_ERR_OR_ZERO(state);
+		goto err_no_state;
+	}
+
+	err = pinctrl_select_state(p, state);
+	if (err != 0) {
+		err = -EINVAL;
+		goto err_no_state;
+	}
+	strncpy(priv->selected_state_name, state_name, STATE_NAME_MAX);
+
+	kfree(state_name);
+	return cnt;
+
+err_no_state:
+	kfree(state_name);
+	return err;
+}
+
+static int pinctrl_state_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, NULL, inode->i_private);
+}
+
+static const struct file_operations pinctrl_state_ops = {
+	.owner = THIS_MODULE,
+	.open = pinctrl_state_open,
+	.read = pinctrl_state_read,
+	.write = pinctrl_state_write,
+	.llseek = no_llseek,
+	.release = single_release,
+};
+
+static int pinctrl_state_helper_probe(struct platform_device *pdev)
+{
+	int err;
+	struct pinctrl_state *state;
+	struct pinctrl_state_helper_priv *priv;
+	struct dentry *pin_dir;
+	struct device *dev = &pdev->dev;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (priv == NULL) {
+		dev_err(dev, "Failed to allocate priv");
+		return -ENOMEM;
+	}
+
+	platform_set_drvdata(pdev, priv);
+
+	priv->pinctrl = devm_pinctrl_get(dev);
+	if (IS_ERR(priv->pinctrl)) {
+		dev_err(dev, "Failed to get pinctrl");
+		err = PTR_ERR_OR_ZERO(priv->pinctrl);
+		goto err_no_pinctrl;
+	}
+
+	strcpy(priv->selected_state_name, PINCTRL_STATE_DEFAULT);
+	state = pinctrl_lookup_state(priv->pinctrl, priv->selected_state_name);
+	if (IS_ERR(state)) {
+		dev_err(dev, "Failed to lookup default state");
+		err = PTR_ERR_OR_ZERO(state);
+		goto err_no_state;
+	}
+
+	err = pinctrl_select_state(priv->pinctrl, state);
+	if (err != 0) {
+		dev_err(dev, "Failed to select default state");
+		goto err_no_state;
+	}
+
+	pin_dir = debugfs_create_dir(dev_name(dev), debugfs_root);
+	priv->dev = dev;
+	priv->pin_dentry = debugfs_create_file("state", 0600, pin_dir, priv, &pinctrl_state_ops);
+
+	return 0;
+
+err_no_state:
+	devm_pinctrl_put(priv->pinctrl);
+err_no_pinctrl:
+	devm_kfree(dev, priv);
+	return err;
+}
+
+static int pinctrl_state_helper_remove(struct platform_device *pdev)
+{
+	struct pinctrl_state_helper_priv *priv;
+
+	priv = platform_get_drvdata(pdev);
+	devm_pinctrl_put(priv->pinctrl);
+	debugfs_remove(priv->pin_dentry);
+
+	return 0;
+}
+
+static const struct of_device_id helper_of_match[] = {
+	{
+		/* RFC: keep generic or make begalebone specific? */
+		.compatible = "pinctrl,state-helper",
+	},
+	{ },
+};
+MODULE_DEVICE_TABLE(of, helper_of_match);
+
+static struct platform_driver pinctrl_state_helper_driver = {
+	.probe		= pinctrl_state_helper_probe,
+	.remove		= pinctrl_state_helper_remove,
+	.driver = {
+		.name           = "pinctrl_state_helper",
+		.owner          = THIS_MODULE,
+		.of_match_table	= helper_of_match,
+	},
+};
+
+static int pinctrl_state_helper_init(void)
+{
+	int err;
+	struct dentry *pinctrl_dir;
+
+	pinctrl_dir = debugfs_lookup("pinctrl", NULL);
+	debugfs_root = debugfs_create_dir(DEBUGFS_DIR, pinctrl_dir);
+	if (IS_ERR(debugfs_root) || !debugfs_root) {
+		pr_err("Failed to create debugfs directory %s", DEBUGFS_DIR);
+		return -EINVAL;
+	}
+
+	err = platform_driver_register(&pinctrl_state_helper_driver);
+	return err;
+}
+module_init(pinctrl_state_helper_init);
+
+static void pinctrl_state_helper_exit(void)
+{
+	debugfs_remove_recursive(debugfs_root);
+	platform_driver_unregister(&pinctrl_state_helper_driver);
+}
+module_exit(pinctrl_state_helper_exit);
+
+MODULE_AUTHOR("Drew Fustini <drew@...gleboard.org>");
+MODULE_DESCRIPTION("Helper driver exposes pinctrl state to debugfs");
+MODULE_LICENSE("GPL v2");
-- 
2.25.1

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