Re: [PATCHv5] gpio: Remove VLA from gpiolib

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Laura,

Thanks for your patch!

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?

> @@ -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

-- 
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
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