Re: [PATCHv5] gpio: Remove VLA from gpiolib

[Laura Abbott <labbott@redhat.com>]

On 04/20/2018 02:02 AM, Geert Uytterhoeven wrote:
> Hi Laura,
> 
> Thanks for your patch!
> 
> On Fri, Apr 13, 2018 at 11:24 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.
> 
> Blindly replacing VLAs by allocated arrays is IMHO not such a good solution.
> On the largest systems, NR_GPIOS is 2048, so that makes 2 arrays of 256
> bytes. That's an uppper limit, and assumes they are all on the same gpiochip,
> which they probably aren't.
> 
> Still, 2 x 256 bytes is a lot, so I agree it should be fixed.
> 
> So, wouldn't it make more sense to not allocate memory, but just process
> the request in chunks (say, at most 128 gpios per chunk, i.e. 2 x
> 16 bytes)? The code already caters for handling chunks due to not all gpios
> belonging to the same gpiochip. That will probably also be faster than
> allocating memory, which is the main idea behind this API.
> 
>> Reviewed-and-tested-by: Lukas Wunner <lukas@wunner.de>
>> Signed-off-by: Lukas Wunner <lukas@wunner.de>
>> Signed-off-by: Laura Abbott <labbott@redhat.com>
> 
>> --- a/drivers/gpio/gpiolib.c
>> +++ b/drivers/gpio/gpiolib.c
> 
>> @@ -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);
> 
> FWIW, can this warning be triggered from userspace?
> 
>> @@ -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)];
> 
> Hence just use a fixed size here...
> 
>> +               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);
> 
> Out-of-context, the code does:
> 
> |                       __set_bit(hwgpio, mask);
> |                       i++;
> |                 } while ((i < array_size) &&
> 
> ... and change this limit to "(i < min(array_size, first +
> ARRAY_SIZE(mask) * BITS_PER_BYTE))"
> 
> |                         (desc_array[i]->gdev->chip == chip));
> 
> ... and you're done?
> 
I don't think this approach will work since gpio_chip_{get,set}_multiple
expect to be working on arrays for the entire chip. There doesn't seem
to be a nice way to work on a subset of GPIOs without defeating the point
of the multiple API.

is 2*256 = 512 bytes really too much stack space? I guess we could
switch to a Kconfig to allow for better bounds.

Thanks,
Laura

>> @@ -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,
> 
> Same here.
> 
> Gr{oetje,eeting}s,
> 
>                          Geert
>