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SubjectRe: [PATCHv5] gpio: Remove VLA from gpiolib
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On 15/05/2018 15:10, Geert Uytterhoeven wrote:
> Hi Laura,
>
> On Tue, May 15, 2018 at 12:49 AM, Laura Abbott <labbott@redhat.com> wrote:
>> On 04/20/2018 02:02 AM, Geert Uytterhoeven wrote:
>>> 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.
>
> 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).
>

I think a config option for FASTPATH_NGPIO is preferable.

As I've mentioned ARCH_NR_GPIOS is much greater than any chip->ngpio on
my platform.
It's at least one order of magnitude, almost 2.


--
Regards
Phil Reid

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