Re: [PATCHv5] gpio: Remove VLA from gpiolib

[Laura Abbott]

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@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.

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