Re: [PATCHv5] gpio: Remove VLA from gpiolib

[Geert Uytterhoeven]

Hi Laura,

On Tue, May 15, 2018 at 12:49 AM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> On 04/20/2018 02:02 AM, Geert Uytterhoeven wrote:
>> On Fri, Apr 13, 2018 at 11:24 PM, Laura Abbott <labbott@xxxxxxxxxx> 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@xxxxxxxxx>
>>> Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
>>> Signed-off-by: Laura Abbott <labbott@xxxxxxxxxx>
>>
>>
>>> --- 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.

You're right, sorry for missing that.

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

That's a good question.

As long as a VLA is used, it's probably fine, as chip->ngpio is quite small.
If you would change it to an array that can accommodate NR_GPIOS bits, you
have to start caring about recursion (e.g. gpio-74x164 driven from spi-gpio,
where I can extend the chain to increase the level of recursion arbitrarily).

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds