gpio: Remove VLA from gpiolib

The new challenge is to remove VLAs from the kernel (see https://lkml.org/lkml/2018/3/7/621) to eventually turn on -Wvla.

Using a kmalloc array is the easy way to fix this but kmalloc is still more expensive than stack allocation. Introduce a fast path with a fixed size stack array to cover most chip with gpios below some fixed amount. The slow path dynamically allocates an array to cover those chips with a large number of gpios.

Reviewed-and-tested-by Lukas Wunner <[email protected]>

Signed-off-by Lukas Wunner <[email protected]>

Signed-off-by Laura Abbott <[email protected]>

---
v6: Introduce a config option for setting the fast path GPIOs because there are too many combinations to make the arch default workable. I went with a default of 512 in the Kconfig.

Also to other points: I don't think the warning should be triggerable from userspace, it should only happen on probe. I also think only memsetting half the array is more likely to be error prone. We can change it if there is significant overhead.

---
  drivers/gpio/Kconfig          | 10 +++++
  drivers/gpio/gpiolib.c        | 76 +++++++++++++++++++++++++++--------
  drivers/gpio/gpiolib.h        |  2 +-
  include/linux/gpio/consumer.h | 10 +++--
  4 files changed, 76 insertions(+), 22 deletions(-)

drivers/gpio/Kconfig +10 -0

index 68d812b38be7..2855b5c5c8ca 100644
@@ -22,6 +22,16 @@ menuconfig GPIOLIB
 
  if GPIOLIB
 
+config GPIOLIB_FASTPATH_LIMIT
+	int "Maximum number of GPIOs for fast path"
+	default 512
+	help
+	   This adjusts the point at which certain APIs will switch from
+	   using a statically allocated fixed size buffer to a dynamically
+	   allocated buffer. This is a trade-off in stackspace vs. speed.
+	   You shouldn't need to change this unless you really need to
+	   optimize one of those two.
+
  config OF_GPIO
  	def_bool y
  	depends on OF

drivers/gpio/gpiolib.c +59 -17

index d66de67ef307..f7ce546796e0 100644
@@ -61,6 +61,11 @@ static struct bus_type gpio_bus_type = {
  	.name = "gpio",
  };
 
+/*
+ * Number of GPIOs to use for the fast path in set array
+ */
+#define FASTPATH_NGPIO CONFIG_GPIOLIB_FASTPATH_LIMIT
+
  /* gpio_lock prevents conflicts during gpio_desc[] table updates.
   * While any GPIO is requested, its gpio_chip is not removable;
   * each GPIO's "requested" flag serves as a lock and refcount.

@@ -399,12 +404,11 @@ static long linehandle_ioctl(struct file *filep, unsigned int cmd,
  			vals[i] = !!ghd.values[i];
 
  		/* Reuse the array setting function */
-		gpiod_set_array_value_complex(false,
+		return gpiod_set_array_value_complex(false,
  					      true,
  					      lh->numdescs,
  					      lh->descs,
  					      vals);
-		return 0;
  	}
  	return -EINVAL;
  }

@@ -1192,6 +1196,10 @@ int gpiochip_add_data_with_key(struct gpio_chip *chip, void *data,
  		goto err_free_descs;
  	}
 
+	if (chip->ngpio > FASTPATH_NGPIO)
+		chip_warn(chip, "line cnt %d is greater than fast path cnt %d\n",
+		chip->ngpio, FASTPATH_NGPIO);
+
  	gdev->label = kstrdup_const(chip->label ?: "unknown", GFP_KERNEL);
  	if (!gdev->label) {
  		status = -ENOMEM;

@@ -2662,16 +2670,28 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
 
  	while (i < array_size) {
  		struct gpio_chip *chip = desc_array[i]->gdev->chip;
-		unsigned long mask[BITS_TO_LONGS(chip->ngpio)];
-		unsigned long bits[BITS_TO_LONGS(chip->ngpio)];
+		unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)];
+		unsigned long *mask, *bits;
  		int first, j, ret;
 
+		if (likely(chip->ngpio <= FASTPATH_NGPIO)) {
+			memset(fastpath, 0, sizeof(fastpath));
+			mask = fastpath;
+			bits = fastpath + BITS_TO_LONGS(FASTPATH_NGPIO);
+		} else {
+			mask = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
+					   sizeof(*mask),
+					   can_sleep ? GFP_KERNEL : GFP_ATOMIC);
+			if (!mask)
+				return -ENOMEM;
+			bits = mask + BITS_TO_LONGS(chip->ngpio);
+		}
+
  		if (!can_sleep)
  			WARN_ON(chip->can_sleep);
 
  		/* collect all inputs belonging to the same chip */
  		first = i;
-		memset(mask, 0, sizeof(mask));
  		do {
  			const struct gpio_desc *desc = desc_array[i];
  			int hwgpio = gpio_chip_hwgpio(desc);

@@ -2682,8 +2702,11 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
  			 (desc_array[i]->gdev->chip == chip));
 
  		ret = gpio_chip_get_multiple(chip, mask, bits);
-		if (ret)
+		if (ret) {
+			if (mask != fastpath)
+				kfree(mask);
  			return ret;
+		}
 
  		for (j = first; j < i; j++) {
  			const struct gpio_desc *desc = desc_array[j];

@@ -2695,6 +2718,9 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
  			value_array[j] = value;
  			trace_gpio_value(desc_to_gpio(desc), 1, value);
  		}
+
+		if (mask != fastpath)
+			kfree(mask);
  	}
  	return 0;
  }

@@ -2878,7 +2904,7 @@ static void gpio_chip_set_multiple(struct gpio_chip *chip,
  	}
  }
 
-void gpiod_set_array_value_complex(bool raw, bool can_sleep,
+int gpiod_set_array_value_complex(bool raw, bool can_sleep,
  				   unsigned int array_size,
  				   struct gpio_desc **desc_array,
  				   int *value_array)

@@ -2887,14 +2913,26 @@ void gpiod_set_array_value_complex(bool raw, bool can_sleep,
 
  	while (i < array_size) {
  		struct gpio_chip *chip = desc_array[i]->gdev->chip;
-		unsigned long mask[BITS_TO_LONGS(chip->ngpio)];
-		unsigned long bits[BITS_TO_LONGS(chip->ngpio)];
+		unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)];
+		unsigned long *mask, *bits;
  		int count = 0;
 
+		if (likely(chip->ngpio <= FASTPATH_NGPIO)) {
+			memset(fastpath, 0, sizeof(fastpath));
+			mask = fastpath;
+			bits = fastpath + BITS_TO_LONGS(FASTPATH_NGPIO);
+		} else {
+			mask = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
+					   sizeof(*mask),
+					   can_sleep ? GFP_KERNEL : GFP_ATOMIC);
+			if (!mask)
+				return -ENOMEM;
+			bits = mask + BITS_TO_LONGS(chip->ngpio);
+		}
+
  		if (!can_sleep)
  			WARN_ON(chip->can_sleep);
 
-		memset(mask, 0, sizeof(mask));
  		do {
  			struct gpio_desc *desc = desc_array[i];
  			int hwgpio = gpio_chip_hwgpio(desc);

@@ -2925,7 +2963,11 @@ void gpiod_set_array_value_complex(bool raw, bool can_sleep,
  		/* push collected bits to outputs */
  		if (count != 0)
  			gpio_chip_set_multiple(chip, mask, bits);
+
+		if (mask != fastpath)
+			kfree(mask);
  	}
+	return 0;
  }
 
  /**

@@ -3000,13 +3042,13 @@ EXPORT_SYMBOL_GPL(gpiod_set_value);
   * This function should be called from contexts where we cannot sleep, and will
   * complain if the GPIO chip functions potentially sleep.
   */
-void gpiod_set_raw_array_value(unsigned int array_size,
+int gpiod_set_raw_array_value(unsigned int array_size,
  			 struct gpio_desc **desc_array, int *value_array)
  {
  	if (!desc_array)
-		return;
-	gpiod_set_array_value_complex(true, false, array_size, desc_array,
-				      value_array);
+		return -EINVAL;
+	return gpiod_set_array_value_complex(true, false, array_size,
+					desc_array, value_array);
  }
  EXPORT_SYMBOL_GPL(gpiod_set_raw_array_value);
 

@@ -3326,14 +3368,14 @@ EXPORT_SYMBOL_GPL(gpiod_set_value_cansleep);
   *
   * This function is to be called from contexts that can sleep.
   */
-void gpiod_set_raw_array_value_cansleep(unsigned int array_size,
+int gpiod_set_raw_array_value_cansleep(unsigned int array_size,
  					struct gpio_desc **desc_array,
  					int *value_array)
  {
  	might_sleep_if(extra_checks);
  	if (!desc_array)
-		return;
-	gpiod_set_array_value_complex(true, true, array_size, desc_array,
+		return -EINVAL;
+	return gpiod_set_array_value_complex(true, true, array_size, desc_array,
  				      value_array);
  }
  EXPORT_SYMBOL_GPL(gpiod_set_raw_array_value_cansleep);

drivers/gpio/gpiolib.h +1 -1

index b17ec6795c81..b64813e3876e 100644
@@ -188,7 +188,7 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
  				  unsigned int array_size,
  				  struct gpio_desc **desc_array,
  				  int *value_array);
-void gpiod_set_array_value_complex(bool raw, bool can_sleep,
+int gpiod_set_array_value_complex(bool raw, bool can_sleep,
  				   unsigned int array_size,
  				   struct gpio_desc **desc_array,
  				   int *value_array);

include/linux/gpio/consumer.h +4 -4

index dbd065963296..243112c7fa7d 100644
@@ -116,7 +116,7 @@ int gpiod_get_raw_array_value(unsigned int array_size,
  			      struct gpio_desc **desc_array,
  			      int *value_array);
  void gpiod_set_raw_value(struct gpio_desc *desc, int value);
-void gpiod_set_raw_array_value(unsigned int array_size,
+int gpiod_set_raw_array_value(unsigned int array_size,
  			       struct gpio_desc **desc_array,
  			       int *value_array);
 

@@ -134,7 +134,7 @@ int gpiod_get_raw_array_value_cansleep(unsigned int array_size,
  				       struct gpio_desc **desc_array,
  				       int *value_array);
  void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value);
-void gpiod_set_raw_array_value_cansleep(unsigned int array_size,
+int gpiod_set_raw_array_value_cansleep(unsigned int array_size,
  					struct gpio_desc **desc_array,
  					int *value_array);
 

@@ -369,12 +369,13 @@ static inline void gpiod_set_raw_value(struct gpio_desc *desc, int value)
  	/* GPIO can never have been requested */
  	WARN_ON(1);
  }
-static inline void gpiod_set_raw_array_value(unsigned int array_size,
+static inline int gpiod_set_raw_array_value(unsigned int array_size,
  					     struct gpio_desc **desc_array,
  					     int *value_array)
  {
  	/* GPIO can never have been requested */
  	WARN_ON(1);
+	return 0;
  }
 
  static inline int gpiod_get_value_cansleep(const struct gpio_desc *desc)

@@ -423,12 +424,13 @@ static inline void gpiod_set_raw_value_cansleep(struct gpio_desc *desc,
  	/* GPIO can never have been requested */
  	WARN_ON(1);
  }
-static inline void gpiod_set_raw_array_value_cansleep(unsigned int array_size,

The new challenge is to remove VLAs from the kernel (see https://lkml.org/lkml/2018/3/7/621) to eventually turn on -Wvla.

Using a kmalloc array is the easy way to fix this but kmalloc is still more expensive than stack allocation. Introduce a fast path with a fixed size stack array to cover most chip with gpios below some fixed amount. The slow path dynamically allocates an array to cover those chips with a large number of gpios.

Reviewed-and-tested-by Lukas Wunner <[email protected]>

Signed-off-by Lukas Wunner <[email protected]>

Signed-off-by Laura Abbott <[email protected]>

Also to other points: I don't think the warning should be triggerable from userspace, it should only happen on probe. I also think only memsetting half the array is more likely to be error prone. We can change it if there is significant overhead.

drivers/gpio/gpiolib.c +6 -2

+               chip->ngpio, FASTPATH_NGPIO);
+
        gdev->label = kstrdup_const(chip->label ?: "unknown", GFP_KERNEL);
        if (!gdev->label) {
                status = -ENOMEM;

@@ -2662,16 +2670,28 @@ int gpiod_get_array_value_complex(bool raw, bool can_sleep,
 
        while (i < array_size) {
                struct gpio_chip *chip = desc_array[i]->gdev->chip;
-               unsigned long mask[BITS_TO_LONGS(chip->ngpio)];
-               unsigned long bits[BITS_TO_LONGS(chip->ngpio)];
+               unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)];
+               unsigned long *mask, *bits;
                int first, j, ret;
 
+               if (likely(chip->ngpio <= FASTPATH_NGPIO)) {
+                       memset(fastpath, 0, sizeof(fastpath));