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-by Geert Uytterhoeven <[email protected]>

Reviewed-by Phil Reid <[email protected]>

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

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

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

---
v9: switch to bitmap_zero

---
 drivers/gpio/Kconfig          | 12 ++++++
 drivers/gpio/gpiolib.c        | 76 +++++++++++++++++++++++++++--------
 drivers/gpio/gpiolib.h        |  2 +-
 include/linux/gpio/consumer.h | 10 +++--
 4 files changed, 78 insertions(+), 22 deletions(-)

drivers/gpio/Kconfig +12 -0

index b960f6f35abd..71c0ab46f216 100644
@@ -22,6 +22,18 @@ menuconfig GPIOLIB
 
 if GPIOLIB
 
+config GPIOLIB_FASTPATH_LIMIT
+       int "Maximum number of GPIOs for fast path"
+       range 32 512
+       default 512
+       help
+          This adjusts the point at which certain APIs will switch from
+          using a stack allocated buffer to a dynamically allocated buffer.
+
+          You shouldn't need to change this unless you really need to
+          optimize either stack space or performance. Change this carefully
+          since setting an incorrect value could cause stack corruption.
+
 config OF_GPIO
        def_bool y
        depends on OF

drivers/gpio/gpiolib.c +59 -17

index d8ccb500872f..982f19d62496 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.

@@ -450,12 +455,11 @@ 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;
 }

@@ -1244,6 +1248,10 @@ 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 %u is greater than fast path cnt %u\n",
+               chip->ngpio, FASTPATH_NGPIO);
+
        gdev->label = kstrdup_const(chip->label ?: "unknown", GFP_KERNEL);
        if (!gdev->label) {
                status = -ENOMEM;

@@ -2719,16 +2727,28 @@ 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)) {
+                       mask = fastpath;
+               } else {
+                       mask = kmalloc_array(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);
+               bitmap_zero(mask, 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);

@@ -2739,8 +2759,11 @@ 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];

@@ -2752,6 +2775,9 @@ 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;
 }

@@ -2935,7 +2961,7 @@ 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)

@@ -2944,14 +2970,26 @@ 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)) {
+                       mask = fastpath;
+               } else {
+                       mask = kmalloc_array(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);
+               bitmap_zero(mask, 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);

@@ -2982,7 +3020,11 @@ 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;
 }
 
 /**

@@ -3057,13 +3099,13 @@ 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);
 

@@ -3383,14 +3425,14 @@ 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 ad456b6f9d8b..1a8e20363861 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 +6 -5

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,
+static inline int gpiod_set_raw_array_value_cansleep(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_set_debounce(struct gpio_desc *desc, unsigned debounce)
--

2.17.0

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-by Geert Uytterhoeven <[email protected]>

Reviewed-by Phil Reid <[email protected]>

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

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

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

---
v9: switch to bitmap_zero