Re: [PATCH v1 5/5] gpiolib: cdev: Utilize more bitmap APIs

[Kent Gibson <warthog618@gmail.com>]

On Tue, Sep 26, 2023 at 08:20:07AM +0300, Andy Shevchenko wrote:
> Currently we have a few bitmap calls that are open coded in the library
> module. Let's convert them to use generic bitmap APIs instead.
> 

Firstly, I didn't consider using the bitmap module here as, in my mind at
least, that is intended for bitmaps wider than 64 bits, or with variable
width. In this case the bitmap is fixed at 64 bits, so bitops seemed more
appropriate.

And I would argue that they aren't "open coded" - they are parallelized
to reduce the number of passes over the bitmap.
This change serialises them, e.g. the get used to require 2 passes over
the bitmap, it now requires 3 or 4.  The set used to require 1 and now
requires 2.
And there are additional copies that the original doesn't require.
So your change looks less efficient to me - unless there is direct
hardware support for bitmap ops??

Wrt the argument that the serialized form is clearer and more
maintainable, optimised code is frequently more cryptic - as noted in
bitmap.c itself, and this code has remained unchanged since it was merged
3 years ago, so the only maintenance it has required is to be more
maintainable??  Ok then.

Your patch is functionally equivalent and pass my uAPI tests, so 

Tested-by: Kent Gibson <warthog618@gmail.com>

but my preference is to leave it as is.

Cheers,
Kent.

> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
>  drivers/gpio/gpiolib-cdev.c | 79 +++++++++++++++++--------------------
>  1 file changed, 36 insertions(+), 43 deletions(-)
> 
> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index e39d344feb28..a5bbbd44531f 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -1263,35 +1263,32 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
>  {
>  	struct gpio_v2_line_values lv;
>  	DECLARE_BITMAP(vals, GPIO_V2_LINES_MAX);
> +	DECLARE_BITMAP(mask, GPIO_V2_LINES_MAX);
> +	DECLARE_BITMAP(bits, GPIO_V2_LINES_MAX);
>  	struct gpio_desc **descs;
>  	unsigned int i, didx, num_get;
> -	bool val;
>  	int ret;
>  
>  	/* NOTE: It's ok to read values of output lines. */
>  	if (copy_from_user(&lv, ip, sizeof(lv)))
>  		return -EFAULT;
>  
> -	for (num_get = 0, i = 0; i < lr->num_lines; i++) {
> -		if (lv.mask & BIT_ULL(i)) {
> -			num_get++;
> -			descs = &lr->lines[i].desc;
> -		}
> -	}
> +	bitmap_from_arr64(mask, &lv.mask, GPIO_V2_LINES_MAX);
>  
> +	num_get = bitmap_weight(mask, lr->num_lines);
>  	if (num_get == 0)
>  		return -EINVAL;
>  
> -	if (num_get != 1) {
> +	if (num_get == 1) {
> +		descs = &lr->lines[find_first_bit(mask, lr->num_lines)].desc;
> +	} else {
>  		descs = kmalloc_array(num_get, sizeof(*descs), GFP_KERNEL);
>  		if (!descs)
>  			return -ENOMEM;
> -		for (didx = 0, i = 0; i < lr->num_lines; i++) {
> -			if (lv.mask & BIT_ULL(i)) {
> -				descs[didx] = lr->lines[i].desc;
> -				didx++;
> -			}
> -		}
> +
> +		didx = 0;
> +		for_each_set_bit(i, mask, lr->num_lines)
> +			descs[didx++] = lr->lines[i].desc;
>  	}
>  	ret = gpiod_get_array_value_complex(false, true, num_get,
>  					    descs, NULL, vals);
> @@ -1301,19 +1298,15 @@ static long linereq_get_values(struct linereq *lr, void __user *ip)
>  	if (ret)
>  		return ret;
>  
> -	lv.bits = 0;
> -	for (didx = 0, i = 0; i < lr->num_lines; i++) {
> -		if (lv.mask & BIT_ULL(i)) {
> -			if (lr->lines[i].sw_debounced)
> -				val = debounced_value(&lr->lines[i]);
> -			else
> -				val = test_bit(didx, vals);
> -			if (val)
> -				lv.bits |= BIT_ULL(i);
> -			didx++;
> -		}
> +	bitmap_scatter(bits, vals, mask, lr->num_lines);
> +
> +	for_each_set_bit(i, mask, lr->num_lines) {
> +		if (lr->lines[i].sw_debounced)
> +			__assign_bit(i, bits, debounced_value(&lr->lines[i]));
>  	}
>  
> +	bitmap_to_arr64(&lv.bits, bits, GPIO_V2_LINES_MAX);
> +
>  	if (copy_to_user(ip, &lv, sizeof(lv)))
>  		return -EFAULT;
>  
> @@ -1324,35 +1317,35 @@ static long linereq_set_values_unlocked(struct linereq *lr,
>  					struct gpio_v2_line_values *lv)
>  {
>  	DECLARE_BITMAP(vals, GPIO_V2_LINES_MAX);
> +	DECLARE_BITMAP(mask, GPIO_V2_LINES_MAX);
> +	DECLARE_BITMAP(bits, GPIO_V2_LINES_MAX);
>  	struct gpio_desc **descs;
>  	unsigned int i, didx, num_set;
>  	int ret;
>  
> -	bitmap_zero(vals, GPIO_V2_LINES_MAX);
> -	for (num_set = 0, i = 0; i < lr->num_lines; i++) {
> -		if (lv->mask & BIT_ULL(i)) {
> -			if (!test_bit(FLAG_IS_OUT, &lr->lines[i].desc->flags))
> -				return -EPERM;
> -			if (lv->bits & BIT_ULL(i))
> -				__set_bit(num_set, vals);
> -			num_set++;
> -			descs = &lr->lines[i].desc;
> -		}
> -	}
> +	bitmap_from_arr64(mask, &lv->mask, GPIO_V2_LINES_MAX);
> +	bitmap_from_arr64(bits, &lv->bits, GPIO_V2_LINES_MAX);
> +
> +	num_set = bitmap_gather(vals, bits, mask, lr->num_lines);
>  	if (num_set == 0)
>  		return -EINVAL;
>  
> -	if (num_set != 1) {
> +	for_each_set_bit(i, mask, lr->num_lines) {
> +		if (!test_bit(FLAG_IS_OUT, &lr->lines[i].desc->flags))
> +			return -EPERM;
> +	}
> +
> +	if (num_set == 1) {
> +		descs = &lr->lines[find_first_bit(mask, lr->num_lines)].desc;
> +	} else {
>  		/* build compacted desc array and values */
>  		descs = kmalloc_array(num_set, sizeof(*descs), GFP_KERNEL);
>  		if (!descs)
>  			return -ENOMEM;
> -		for (didx = 0, i = 0; i < lr->num_lines; i++) {
> -			if (lv->mask & BIT_ULL(i)) {
> -				descs[didx] = lr->lines[i].desc;
> -				didx++;
> -			}
> -		}
> +
> +		didx = 0;
> +		for_each_set_bit(i, mask, lr->num_lines)
> +			descs[didx++] = lr->lines[i].desc;
>  	}
>  	ret = gpiod_set_array_value_complex(false, true, num_set,
>  					    descs, NULL, vals);
> -- 
> 2.40.0.1.gaa8946217a0b
>