[PATCH RFC] gpio: Add Virtual Aggregator GPIO Driver

[Geert Uytterhoeven <geert+renesas@xxxxxxxxx>]

GPIO controllers are exported to userspace using /dev/gpiochip*
character devices.  Access control to these devices is provided by
standard UNIX file system permissions, on an all-or-nothing basis:
either a GPIO controller is accessible for a user, or it is not.
Currently no mechanism exists to control access to individual GPIOs.

Hence add a virtual GPIO driver to aggregate existing GPIOs (up to 32),
and expose them as a new gpiochip.  This is useful for implementing
access control, and assigning a set of GPIOs to a specific user.
Furthermore, it would simplify and harden exporting GPIOs to a virtual
machine, as the VM can just grab the full virtual GPIO controller, and
no longer needs to care about which GPIOs to grab and which not,
reducing the attack surface.

Virtual GPIO controllers are instantiated by writing to the "new_device"
attribute file in sysfs:

    $ echo "<gpiochipA> <gpioA1> [<gpioA2> ...]"
           "[, <gpiochipB> <gpioB1> [<gpioB2> ...]] ...]"
            > /sys/bus/platform/drivers/gpio-virt-agg/new_device

Likewise, virtual GPIO controllers can be destroyed after use:

    $ echo gpio-virt-agg.<N> \
            > /sys/bus/platform/drivers/gpio-virt-agg/delete_device

Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
---
Aggregating GPIOs and exposing them as a new gpiochip was suggested in
response to my proof-of-concept for GPIO virtualization with QEMU[1][2].

Sample session on r8a7791/koelsch:

  - Disable the leds node in arch/arm/boot/dts/r8a7791-koelsch.dts

  - Create virtual aggregators:

    $ echo "e6052000.gpio 19 20" \
            > /sys/bus/platform/drivers/gpio-virt-agg/new_device

    gpio-virt-agg gpio-virt-agg.0: GPIO 0 => e6052000.gpio/19
    gpio-virt-agg gpio-virt-agg.0: GPIO 1 => e6052000.gpio/20
    gpiochip_find_base: found new base at 778
    gpio gpiochip8: (gpio-virt-agg.0): added GPIO chardev (254:8)
    gpiochip_setup_dev: registered GPIOs 778 to 779 on device: gpiochip8 (gpio-virt-agg.0)

    $ echo "e6052000.gpio 21, e6050000.gpio 20 21 22" \
            > /sys/bus/platform/drivers/gpio-virt-agg/new_device

    gpio-virt-agg gpio-virt-agg.1: GPIO 0 => e6052000.gpio/21
    gpio-virt-agg gpio-virt-agg.1: GPIO 1 => e6050000.gpio/20
    gpio-virt-agg gpio-virt-agg.1: GPIO 2 => e6050000.gpio/21
    gpio-virt-agg gpio-virt-agg.1: GPIO 3 => e6050000.gpio/22
    gpiochip_find_base: found new base at 774
    gpio gpiochip9: (gpio-virt-agg.1): added GPIO chardev (254:9)
    gpiochip_setup_dev: registered GPIOs 774 to 777 on device: gpiochip9 (gpio-virt-agg.1)

  - Adjust permissions on /dev/gpiochip[89] (optional)

  - Control LEDs:

    $ gpioset gpiochip8 0=0 1=1	# LED6 OFF, LED7 ON
    $ gpioset gpiochip8 0=1 1=0	# LED6 ON, LED7 OFF
    $ gpioset gpiochip9 0=0	# LED8 OFF
    $ gpioset gpiochip9 0=1	# LED8 ON

  - Destroy virtual aggregators:

    $ echo gpio-virt-agg.0 \
            > /sys/bus/platform/drivers/gpio-virt-agg/delete_device
    $ echo gpio-virt-agg.1 \
            > /sys/bus/platform/drivers/gpio-virt-agg/delete_device

Thanks for your comments!

References:
  - [1] "[PATCH QEMU POC] Add a GPIO backend"
	(https://lore.kernel.org/linux-renesas-soc/20181003152521.23144-1-geert+renesas@xxxxxxxxx/)
  - [2] "Getting To Blinky: Virt Edition / Making device pass-through
	 work on embedded ARM"
	(https://fosdem.org/2019/schedule/event/vai_getting_to_blinky/)
---
 drivers/gpio/Kconfig         |   9 +
 drivers/gpio/Makefile        |   1 +
 drivers/gpio/gpio-virt-agg.c | 390 +++++++++++++++++++++++++++++++++++
 3 files changed, 400 insertions(+)
 create mode 100644 drivers/gpio/gpio-virt-agg.c

diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index f1f02dac324e52b6..8aff4d9626dee110 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -1475,3 +1475,12 @@ config GPIO_MOCKUP
 	  it.
 
 endif
+
+config GPIO_VIRT_AGG
+	tristate "GPIO Virtual Aggregator"
+	depends on GPIOLIB
+	help
+	  This enabled the GPIO Virtual Aggregator, which provides a way to
+	  aggregate existing GPIOs into a new virtual GPIO device.
+	  This is useful for assigning a collection of GPIOs to a user, or
+	  exported them to a virtual machine.
diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
index 0a494052c1e845ee..32e885b7f3aa4eee 100644
--- a/drivers/gpio/Makefile
+++ b/drivers/gpio/Makefile
@@ -152,6 +152,7 @@ obj-$(CONFIG_GPIO_UCB1400)		+= gpio-ucb1400.o
 obj-$(CONFIG_GPIO_UNIPHIER)		+= gpio-uniphier.o
 obj-$(CONFIG_GPIO_VF610)		+= gpio-vf610.o
 obj-$(CONFIG_GPIO_VIPERBOARD)		+= gpio-viperboard.o
+obj-$(CONFIG_GPIO_VIRT_AGG)		+= gpio-virt-agg.o
 obj-$(CONFIG_GPIO_VR41XX)		+= gpio-vr41xx.o
 obj-$(CONFIG_GPIO_VX855)		+= gpio-vx855.o
 obj-$(CONFIG_GPIO_WHISKEY_COVE)		+= gpio-wcove.o
diff --git a/drivers/gpio/gpio-virt-agg.c b/drivers/gpio/gpio-virt-agg.c
new file mode 100644
index 0000000000000000..20e5f22beed9d385
--- /dev/null
+++ b/drivers/gpio/gpio-virt-agg.c
@@ -0,0 +1,390 @@
+// SPDX-License-Identifier: GPL-2.0-only
+//
+// GPIO Virtual Aggregator
+//
+// Copyright (C) 2019 Glider bvba
+
+#include <linux/gpio/driver.h>
+#include <linux/idr.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/platform_device.h>
+
+#include "gpiolib.h"
+
+#define DRV_NAME	"gpio-virt-agg"
+#define MAX_GPIOS	32
+
+struct gpio_virt_agg_entry {
+	struct platform_device *pdev;
+};
+
+struct gpio_virt_agg_priv {
+	struct gpio_chip chip;
+	struct gpio_desc *desc[MAX_GPIOS];
+};
+
+static DEFINE_MUTEX(gpio_virt_agg_lock);	/* protects idr */
+static DEFINE_IDR(gpio_virt_agg_idr);
+
+static int gpio_virt_agg_get_direction(struct gpio_chip *chip,
+				       unsigned int offset)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+
+	return gpiod_get_direction(priv->desc[offset]);
+}
+
+static int gpio_virt_agg_direction_input(struct gpio_chip *chip,
+					 unsigned int offset)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+
+	return gpiod_direction_input(priv->desc[offset]);
+}
+
+static int gpio_virt_agg_direction_output(struct gpio_chip *chip,
+					  unsigned int offset, int value)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+
+	return gpiod_direction_output(priv->desc[offset], value);
+}
+
+static int gpio_virt_agg_get(struct gpio_chip *chip, unsigned int offset)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+
+	return gpiod_get_value(priv->desc[offset]);
+}
+
+static int gpio_virt_agg_get_multiple(struct gpio_chip *chip,
+				      unsigned long *mask, unsigned long *bits)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+	DECLARE_BITMAP(values, MAX_GPIOS) = { 0 };
+	struct gpio_desc *desc[MAX_GPIOS];
+	unsigned int i, j = 0;
+	int ret;
+
+	for_each_set_bit(i, mask, priv->chip.ngpio)
+		desc[j++] = priv->desc[i];
+
+	ret = gpiod_get_array_value(j, desc, NULL, values);
+	if (ret)
+		return ret;
+
+	for_each_set_bit(i, mask, priv->chip.ngpio)
+		__assign_bit(i, bits, test_bit(j++, values));
+
+	return 0;
+}
+
+static void gpio_virt_agg_set(struct gpio_chip *chip, unsigned int offset,
+			      int value)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+
+	gpiod_set_value(priv->desc[offset], value);
+}
+
+static void gpio_virt_agg_set_multiple(struct gpio_chip *chip,
+				       unsigned long *mask,
+				       unsigned long *bits)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+	DECLARE_BITMAP(values, MAX_GPIOS);
+	struct gpio_desc *desc[MAX_GPIOS];
+	unsigned int i, j = 0;
+
+	for_each_set_bit(i, mask, priv->chip.ngpio) {
+		__assign_bit(j, values, test_bit(i, bits));
+		desc[j++] = priv->desc[i];
+	}
+
+	gpiod_set_array_value(j, desc, NULL, values);
+}
+
+static int gpio_virt_agg_set_config(struct gpio_chip *chip,
+				    unsigned int offset, unsigned long config)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+
+	chip = priv->desc[offset]->gdev->chip;
+	if (chip->set_config)
+		return chip->set_config(chip, offset, config);
+
+	// FIXME gpiod_set_transitory() expects success if not implemented
+	return -ENOTSUPP;
+}
+
+static int gpio_virt_agg_init_valid_mask(struct gpio_chip *chip)
+{
+	struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
+	unsigned int i;
+
+	for (i = 0; i < priv->chip.ngpio; i++) {
+		if (gpiochip_line_is_valid(priv->desc[i]->gdev->chip,
+					   gpio_chip_hwgpio(priv->desc[i])))
+			set_bit(i, chip->valid_mask);
+	}
+
+	return 0;
+}
+
+static int gpiochip_match_label(struct gpio_chip *chip, void *data)
+{
+	return !strcmp(chip->label, data);
+}
+
+static struct gpio_chip *gpiochip_find_by_label(const char *label)
+{
+	return gpiochip_find((void *)label, gpiochip_match_label);
+}
+
+static ssize_t new_device_store(struct device_driver *driver, const char *buf,
+				size_t count)
+{
+	struct gpio_virt_agg_entry *gva;
+	struct platform_device *pdev;
+	int res, id;
+
+	gva = kzalloc(sizeof(*gva), GFP_KERNEL);
+	if (!gva)
+		return -ENOMEM;
+
+	mutex_lock(&gpio_virt_agg_lock);
+	id = idr_alloc(&gpio_virt_agg_idr, gva, 0, 0, GFP_KERNEL);
+	mutex_unlock(&gpio_virt_agg_lock);
+
+	if (id < 0) {
+		res = id;
+		goto free_gva;
+	}
+
+	/* kernfs guarantees string termination, so count + 1 is safe */
+	pdev = platform_device_register_data(NULL, DRV_NAME, id, buf,
+					     count + 1);
+	if (IS_ERR(pdev)) {
+		res = PTR_ERR(pdev);
+		goto remove_idr;
+	}
+
+	gva->pdev = pdev;
+	return count;
+
+remove_idr:
+	mutex_lock(&gpio_virt_agg_lock);
+	idr_remove(&gpio_virt_agg_idr, id);
+	mutex_unlock(&gpio_virt_agg_lock);
+free_gva:
+	kfree(gva);
+	return res;
+}
+
+static DRIVER_ATTR_WO(new_device);
+
+static ssize_t delete_device_store(struct device_driver *driver,
+				   const char *buf, size_t count)
+{
+	struct gpio_virt_agg_entry *gva;
+	int id;
+
+	if (strncmp(buf, DRV_NAME ".", strlen(DRV_NAME ".")))
+		return -EINVAL;
+
+	id = simple_strtoul(buf + strlen(DRV_NAME "."), NULL, 10);
+
+	mutex_lock(&gpio_virt_agg_lock);
+	gva = idr_remove(&gpio_virt_agg_idr, id);
+	mutex_unlock(&gpio_virt_agg_lock);
+
+	if (!gva) {
+		pr_info("Cannot find %s.%d\n", DRV_NAME, id);
+		return -ENOENT;
+	}
+
+	platform_device_unregister(gva->pdev);
+	kfree(gva);
+	return count;
+}
+static DRIVER_ATTR_WO(delete_device);
+
+static struct attribute *gpio_virt_agg_attrs[] = {
+	&driver_attr_new_device.attr,
+	&driver_attr_delete_device.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(gpio_virt_agg);
+
+static int gpio_virt_agg_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	const char *param = dev_get_platdata(dev);
+	struct gpio_virt_agg_priv *priv;
+	const char *label = NULL;
+	struct gpio_chip *chip;
+	struct gpio_desc *desc;
+	unsigned int offset;
+	int error, i;
+	char *s;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv) {
+		error = -ENOMEM;
+		goto fail;
+	}
+
+	for (i = 0; i < MAX_GPIOS; i++) {
+		if (*param == '\0' || *param == '\n')
+			break;
+
+		if (*param == ',') {
+			if (label) {
+				devm_kfree(dev, label);
+				label = NULL;
+			}
+			for (param++; *param == ' '; param++) ;
+		}
+
+		if (!label) {
+			s = strchr(param, ' ');
+			if (!s) {
+				dev_info(dev, "Missing gpiochip\n");
+				error = -EINVAL;
+				goto fail;
+			}
+			label = devm_kasprintf(dev, GFP_KERNEL, "%.*s",
+					       (int)(s - param), param);
+			if (!label) {
+				error = -ENOMEM;
+				goto fail;
+			}
+
+			chip = gpiochip_find_by_label(label);
+			if (!chip) {
+				dev_info(dev, "Cannot find gpiochip %s\n",
+					 label);
+				error = -ENODEV;
+				goto fail;
+			}
+
+			for (param = s + 1; *param == ' '; param++) ;
+		}
+
+		offset = simple_strtoul(param, &s, 10);
+
+		desc = gpiochip_get_desc(chip, offset);
+		if (IS_ERR(desc)) {
+			error = PTR_ERR(desc);
+			dev_info(dev, "Cannot get GPIO %s/%u: %d\n", label,
+				 offset, error);
+			goto fail;
+		}
+
+		error = gpiod_request(desc, dev_name(dev));
+		if (error) {
+			dev_info(dev, "Cannot request GPIO %s/%u: %d\n", label,
+				 offset, error);
+			goto fail;
+		}
+
+		dev_dbg(dev, "GPIO %u => %s/%u\n", i, label, offset);
+		priv->desc[i] = desc;
+
+		if (gpiod_cansleep(desc))
+			priv->chip.can_sleep = true;
+		if (desc->gdev->chip->set_config)
+			priv->chip.set_config = gpio_virt_agg_set_config;
+		if (desc->gdev->chip->need_valid_mask) {
+			priv->chip.need_valid_mask = true;
+			priv->chip.init_valid_mask =
+				gpio_virt_agg_init_valid_mask;
+		}
+
+		for (param = s; *param == ' '; param++) ;
+	}
+	if (i == MAX_GPIOS)
+		dev_warn(&pdev->dev,
+			 "Too many gpios specified, truncating to %u\n",
+			 MAX_GPIOS);
+
+	priv->chip.label = dev_name(dev);
+	priv->chip.parent = dev;
+	priv->chip.owner = THIS_MODULE;
+	priv->chip.get_direction = gpio_virt_agg_get_direction;
+	priv->chip.direction_input = gpio_virt_agg_direction_input;
+	priv->chip.direction_output = gpio_virt_agg_direction_output;
+	priv->chip.get = gpio_virt_agg_get;
+	priv->chip.get_multiple = gpio_virt_agg_get_multiple;
+	priv->chip.set = gpio_virt_agg_set;
+	priv->chip.set_multiple = gpio_virt_agg_set_multiple;
+	priv->chip.base = -1;
+	priv->chip.ngpio = i;
+	platform_set_drvdata(pdev, priv);
+
+	error = gpiochip_add_data(&priv->chip, priv);
+	if (error)
+		goto fail;
+
+	return 0;
+
+fail:
+	while (i-- > 0)
+		gpiod_free(priv->desc[i]);
+
+	return error;
+}
+
+static int gpio_virt_agg_remove(struct platform_device *pdev)
+{
+	struct gpio_virt_agg_priv *priv = platform_get_drvdata(pdev);
+	unsigned int i;
+
+	gpiochip_remove(&priv->chip);
+
+	for (i = 0; i < priv->chip.ngpio; i++)
+		gpiod_free(priv->desc[i]);
+
+	return 0;
+}
+
+static struct platform_driver gpio_virt_agg_driver = {
+	.probe = gpio_virt_agg_probe,
+	.remove = gpio_virt_agg_remove,
+	.driver = {
+		.name = DRV_NAME,
+		.groups = gpio_virt_agg_groups,
+	},
+};
+
+static int __init gpio_virt_agg_init(void)
+{
+	return platform_driver_register(&gpio_virt_agg_driver);
+}
+module_init(gpio_virt_agg_init);
+
+static int __exit gpio_virt_agg_idr_remove(int id, void *p, void *data)
+{
+	struct gpio_virt_agg_entry *gva = p;
+
+	platform_device_unregister(gva->pdev);
+	kfree(gva);
+	return 0;
+}
+
+static void __exit gpio_virt_agg_exit(void)
+{
+	mutex_lock(&gpio_virt_agg_lock);
+	idr_for_each(&gpio_virt_agg_idr, gpio_virt_agg_idr_remove, NULL);
+	idr_destroy(&gpio_virt_agg_idr);
+	mutex_unlock(&gpio_virt_agg_lock);
+
+	platform_driver_unregister(&gpio_virt_agg_driver);
+}
+module_exit(gpio_virt_agg_exit);
+
+MODULE_AUTHOR("Geert Uytterhoeven <geert+renesas@xxxxxxxxx>");
+MODULE_DESCRIPTION("GPIO Virtual Aggregator");
+MODULE_LICENSE("GPL v2");
-- 
2.17.1