[PATCHv3] gpio: Remove VLA from gpiolib

[PATCHv3] gpio: Remove VLA from gpiolib

[Laura Abbott]

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.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Laura Abbott <labbott@redhat.com>
---
v3: Split out from the series since patches have been picked up
independently. Fold in patch from Lukas Wunner to introduce slow/fast
paths. I took his suggestions to go with 384 as the maximum number of
gpios. Also fixed one 0-day bot issue where I forgot to change the
return type.
---
 drivers/gpio/gpiolib.c        | 74 +++++++++++++++++++++++++++++++++----------
 drivers/gpio/gpiolib.h        |  2 +-
 include/linux/gpio/consumer.h | 10 +++---
 3 files changed, 65 insertions(+), 21 deletions(-)

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index d66de67ef307..e3ca9e31bcbc 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -390,6 +390,7 @@ static long linehandle_ioctl(struct file *filep, unsigned int cmd,
 
 		return 0;
 	} else if (cmd == GPIOHANDLE_SET_LINE_VALUES_IOCTL) {
+		int ret;
 		/* TODO: check if descriptors are really output */
 		if (copy_from_user(&ghd, ip, sizeof(ghd)))
 			return -EFAULT;
@@ -399,11 +400,14 @@ 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,
+		ret = gpiod_set_array_value_complex(false,
 					      true,
 					      lh->numdescs,
 					      lh->descs,
 					      vals);
+		if (ret)
+			return ret;
+
 		return 0;
 	}
 	return -EINVAL;
@@ -2653,6 +2657,8 @@ static int gpio_chip_get_multiple(struct gpio_chip *chip,
 	return -EIO;
 }
 
+#define FASTPATH_NGPIO 384
+
 int gpiod_get_array_value_complex(bool raw, bool can_sleep,
 				  unsigned int array_size,
 				  struct gpio_desc **desc_array,
@@ -2662,16 +2668,29 @@ 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 *slowpath = NULL, *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 {
+			slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
+					   sizeof(*slowpath),
+					   can_sleep ? GFP_KERNEL : GFP_ATOMIC);
+			if (!slowpath)
+				return -ENOMEM;
+			mask = slowpath;
+			bits = slowpath + 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 +2701,11 @@ 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 (slowpath)
+				kfree(slowpath);
 			return ret;
+		}
 
 		for (j = first; j < i; j++) {
 			const struct gpio_desc *desc = desc_array[j];
@@ -2695,6 +2717,9 @@ 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 (slowpath)
+			kfree(slowpath);
 	}
 	return 0;
 }
@@ -2878,7 +2903,7 @@ 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 +2912,27 @@ 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 *slowpath = NULL, *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 {
+			slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
+					   sizeof(*slowpath),
+					   can_sleep ? GFP_KERNEL : GFP_ATOMIC);
+			if (!slowpath)
+				return -ENOMEM;
+			mask = slowpath;
+			bits = slowpath + 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 (slowpath)
+			kfree(slowpath);
 	}
+	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);
diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
index b17ec6795c81..b64813e3876e 100644
--- a/drivers/gpio/gpiolib.h
+++ b/drivers/gpio/gpiolib.h
@@ -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);
diff --git a/include/linux/gpio/consumer.h b/include/linux/gpio/consumer.h
index dbd065963296..243112c7fa7d 100644
--- a/include/linux/gpio/consumer.h
+++ b/include/linux/gpio/consumer.h
@@ -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)
-- 
2.14.3

Re: [PATCHv3] gpio: Remove VLA from gpiolib

[Lukas Wunner]

On Wed, Mar 28, 2018 at 11:18:09AM -0700, Laura Abbott wrote:
> 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.
> 
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
> v3: Split out from the series since patches have been picked up
> independently. Fold in patch from Lukas Wunner to introduce slow/fast
> paths. I took his suggestions to go with 384 as the maximum number of
> gpios. Also fixed one 0-day bot issue where I forgot to change the
> return type.

I've just given this a whirl with gpio-hammer and it works nicely, so FWIW:

Reviewed-and-tested-by: Lukas Wunner <lukas@wunner.de>

Thanks a lot for doing this Laura, and most of all thanks for the
proverbial "dogged persistence and patience"!
(https://lwn.net/Articles/697029/)

Lukas

Re: [PATCHv3] gpio: Remove VLA from gpiolib

[Andy Shevchenko]

On Wed, Mar 28, 2018 at 9:18 PM, Laura Abbott <labbott@redhat.com> wrote:
> 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.

> +               ret = gpiod_set_array_value_complex(false,
>                                               true,
>                                               lh->numdescs,
>                                               lh->descs,
>                                               vals);
> +               if (ret)
> +                       return ret;
> +
>                 return 0;

Can't we

return gpiod_set_array_value_complex(); ?


> +                       slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
> +                                          sizeof(*slowpath),
> +                                          can_sleep ? GFP_KERNEL : GFP_ATOMIC);


> +                       if (slowpath)
> +                               kfree(slowpath);

> +               if (slowpath)
> +                       kfree(slowpath);

Since slowpath is a pointer, conditionals above are redundant.

> +                       slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
> +                                          sizeof(*slowpath),
> +                                          can_sleep ? GFP_KERNEL : GFP_ATOMIC);

> +               if (slowpath)
> +                       kfree(slowpath);

Ditto.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCHv3] gpio: Remove VLA from gpiolib

[Laura Abbott]

On 03/30/2018 07:33 AM, Andy Shevchenko wrote:
> On Wed, Mar 28, 2018 at 9:18 PM, Laura Abbott <labbott@redhat.com> wrote:
>> 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.
> 
>> +               ret = gpiod_set_array_value_complex(false,
>>                                                true,
>>                                                lh->numdescs,
>>                                                lh->descs,
>>                                                vals);
>> +               if (ret)
>> +                       return ret;
>> +
>>                  return 0;
> 
> Can't we
> 
> return gpiod_set_array_value_complex(); ?
> 
> 

Yeah I'll clean that up for v4.

>> +                       slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
>> +                                          sizeof(*slowpath),
>> +                                          can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> 
> 
>> +                       if (slowpath)
>> +                               kfree(slowpath);
> 
>> +               if (slowpath)
>> +                       kfree(slowpath);
> 
> Since slowpath is a pointer, conditionals above are redundant.
> 
>> +                       slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
>> +                                          sizeof(*slowpath),
>> +                                          can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> 
>> +               if (slowpath)
>> +                       kfree(slowpath);
> 
> Ditto.
> 

This was caught by a coccinelle script via 0-day but I think the request
was to not do it. I'll add a comment explaining why we are going against
style.

Thanks,
Laura

Re: [PATCHv3] gpio: Remove VLA from gpiolib

[Rasmus Villemoes]

On 2018-03-28 20:18, Laura Abbott wrote:
> 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.
> 
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
> v3: Split out from the series since patches have been picked up
> independently. Fold in patch from Lukas Wunner to introduce slow/fast
> paths. I took his suggestions to go with 384 as the maximum number of
> gpios. Also fixed one 0-day bot issue where I forgot to change the
> return type.
>  
>  	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 *slowpath = NULL, *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 {
> +			slowpath = kcalloc(2 * BITS_TO_LONGS(chip->ngpio),
> +					   sizeof(*slowpath),
> +					   can_sleep ? GFP_KERNEL : GFP_ATOMIC);
> +			if (!slowpath)
> +				return -ENOMEM;
> +			mask = slowpath;
> +			bits = slowpath + BITS_TO_LONGS(chip->ngpio);
> +		}
> +

To avoid the static analysis complaints about the "if (slowpath)
kfree(slowpath)" pattern, I'd suggest getting rid of the slowpath
variable and simply assign the kcalloc directly to mask. Then the
condition becomes "if (mask != fastpath) kfree(mask)". A comment won't
silence subsequenct coccicheck runs, and unfortunately probably won't
prevent certain individuals from sending auto-generated patches.

Maybe also pull out the "bits = mask + BITS_TO_LONGS(chip->ngpio)" to
the common path.

Another thing: Maybe a pr_warn or at least a pr_info would be in order
when a gpio chip with > FASTPATH_NGPIO lines is registered? And if that
triggers for a lot of different SoCs, one could consider changing it to
a CONFIG_ thing, or just bumping it to 512 if that would seem to cover
all the reports.

Rasmus