linux-kernel - Re: [PATCH 5/6] gpio: thunderx: Utilize for_each_set

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Date:   Fri, 24 Apr 2020 09:55:47 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     rrichter@...vell.com, linus.walleij@...aro.org,
        bgolaszewski@...libre.com
Cc:     akpm@...ux-foundation.org, andriy.shevchenko@...ux.intel.com,
        Syed Nayyar Waris <syednwaris@...il.com>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] gpio: thunderx: Utilize for_each_set_clump macro

On Fri, Apr 24, 2020 at 06:00:50PM +0530, Syed Nayyar Waris wrote:
> This patch reimplements the thunderx_gpio_set_multiple function in
> drivers/gpio/gpio-thunderx.c to use the new for_each_set_clump macro.
> Instead of looping for each bank in thunderx_gpio_set_multiple
> function, now we can skip bank which is not set and save cycles.
> 
> Cc: Robert Richter <rrichter@...vell.com>
> Cc: Linus Walleij <linus.walleij@...aro.org>
> Cc: Bartosz Golaszewski <bgolaszewski@...libre.com>
> Signed-off-by: Syed Nayyar Waris <syednwaris@...il.com>
> Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>
> ---
>  drivers/gpio/gpio-thunderx.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpio/gpio-thunderx.c b/drivers/gpio/gpio-thunderx.c
> index 9f66dea..74aea25 100644
> --- a/drivers/gpio/gpio-thunderx.c
> +++ b/drivers/gpio/gpio-thunderx.c
> @@ -275,12 +275,16 @@ static void thunderx_gpio_set_multiple(struct gpio_chip *chip,
>  				       unsigned long *bits)
>  {
>  	int bank;
> -	u64 set_bits, clear_bits;
> +	u64 set_bits, clear_bits, gpio_mask;
> +	const unsigned long bank_size = 64;
> +	unsigned long offset;
> +
>  	struct thunderx_gpio *txgpio = gpiochip_get_data(chip);
>  
> -	for (bank = 0; bank <= chip->ngpio / 64; bank++) {
> -		set_bits = bits[bank] & mask[bank];
> -		clear_bits = ~bits[bank] & mask[bank];
> +	for_each_set_clump(offset, gpio_mask, mask, chip->ngpio, bank_size) {
> +		bank = offset / bank_size;
> +		set_bits = bits[bank] & gpio_mask;
> +		clear_bits = ~bits[bank] & gpio_mask;
>  		writeq(set_bits, txgpio->register_base + (bank * GPIO_2ND_BANK) + GPIO_TX_SET);
>  		writeq(clear_bits, txgpio->register_base + (bank * GPIO_2ND_BANK) + GPIO_TX_CLR);
>  	}
> -- 
> 2.7.4

We noticed in the original code that this set_multiple callback does not
appear to work correctly on systems where BITS_PER_LONG == 32. On those
systems, the bits and mask values are 32-bit, but the for loop jumps 64
bits at a time -- that means the loop is skipping the upper 32 gpio
lines of every iteration.

Is the gpio-thunderx driver only intended for 64-bit systems? Or this
behavior a bug?

William Breathitt Gray

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