Subject: Re: [PATCH 09/14] bitmap: extend bitmap_{get,set}_value8() to bitmap_{get,set}_bits()

From: Yury Norov <[email-masked]>
Date: Mon, 9 Oct 2023 09:31:15 -0700

+ Alexander Potapenko <[email-masked]>

On Mon, Oct 09, 2023 at 05:10:21PM +0200, Alexander Lobakin wrote:
> Sometimes there's need to get a 8/16/...-bit piece of a bitmap at a
> particular offset. Currently, there are only bitmap_{get,set}_value8()
> to do that for 8 bits and that's it.

And also a series from Alexander Potapenko, which I really hope will
get into the -next really soon. It introduces bitmap_read/write which
can set up to BITS_PER_LONG at once, with no limitations on alignment
of position and length:

https://lore.kernel.org/linux-arm-kernel/ZRXbOoKHHafCWQCW@yury-ThinkPad/T/#mc311037494229647088b3a84b9f0d9b50bf227cb

Can you consider building your series on top of it?

> Instead of introducing a separate pair for u16 and so on, which doesn't
> scale well, extend the existing functions to be able to pass the wanted
> value width. Make both offset and width arbitrary, but in order to not
> over complicate the current logic and keep the helpers as optimized as
> the current ones, require the width to be a pow-2 value and the offset
> to be a multiple of the width, while the target piece should not cross
> a %BITS_PER_LONG boundary and stay within one long.
> Avoid adjusting all the already existing callsites by defining oneliner
> wrapper macros named after the former functions. bloat-o-meter shows
> almost no difference (+1-2 bytes in a couple of places), meaning the
> new helpers get optimized just nicely.
> 
> Reviewed-by: Przemek Kitszel <[email-masked]>
> Signed-off-by: Alexander Lobakin <[email-masked]>
> ---
>  include/linux/bitmap.h | 51 ++++++++++++++++++++++++++++++------------
>  1 file changed, 37 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 63e422f8ba3d..9c010a7fa331 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -6,8 +6,10 @@
>  
>  #include <linux/align.h>
>  #include <linux/bitops.h>
> +#include <linux/bug.h>
>  #include <linux/find.h>
>  #include <linux/limits.h>
> +#include <linux/log2.h>
>  #include <linux/string.h>
>  #include <linux/types.h>
>  
> @@ -569,38 +571,59 @@ static inline void bitmap_from_u64(unsigned long *dst, u64 mask)
>  }
>  
>  /**
> - * bitmap_get_value8 - get an 8-bit value within a memory region
> + * bitmap_get_bits - get a 8/16/32/64-bit value within a memory region
>   * @map: address to the bitmap memory region
> - * @start: bit offset of the 8-bit value; must be a multiple of 8
> + * @start: bit offset of the value; must be a multiple of @len
> + * @len: bit width of the value; must be a power of two
>   *
> - * Returns the 8-bit value located at the @start bit offset within the @src
> - * memory region.
> + * Return: the 8/16/32/64-bit value located at the @start bit offset within
> + * the @src memory region. Its position (@start + @len) can't cross
> + * a ``BITS_PER_LONG`` boundary.
>   */
> -static inline unsigned long bitmap_get_value8(const unsigned long *map,
> -					      unsigned long start)
> +static inline unsigned long bitmap_get_bits(const unsigned long *map,
> +					    unsigned long start, size_t len)
>  {
>  	const size_t index = BIT_WORD(start);
>  	const unsigned long offset = start % BITS_PER_LONG;
>  
> -	return (map[index] >> offset) & 0xFF;
> +	if (WARN_ON_ONCE(!is_power_of_2(len) || offset % len ||
> +			 offset + len > BITS_PER_LONG))
> +		return 0;
> +
> +	return (map[index] >> offset) & GENMASK(len - 1, 0);
>  }
>  
>  /**
> - * bitmap_set_value8 - set an 8-bit value within a memory region
> + * bitmap_set_bits - set a 8/16/32/64-bit value within a memory region
>   * @map: address to the bitmap memory region
> - * @value: the 8-bit value; values wider than 8 bits may clobber bitmap
> - * @start: bit offset of the 8-bit value; must be a multiple of 8
> + * @start: bit offset of the value; must be a multiple of @len
> + * @value: new value to set
> + * @len: bit width of the value; must be a power of two
> + *
> + * Replaces the 8/16/32/64-bit value located at the @start bit offset within
> + * the @src memory region with the new @value. Its position (@start + @len)
> + * can't cross a ``BITS_PER_LONG`` boundary.
>   */
> -static inline void bitmap_set_value8(unsigned long *map, unsigned long value,
> -				     unsigned long start)
> +static inline void bitmap_set_bits(unsigned long *map, unsigned long start,
> +				   unsigned long value, size_t len)
>  {
>  	const size_t index = BIT_WORD(start);
>  	const unsigned long offset = start % BITS_PER_LONG;
> +	unsigned long mask = GENMASK(len - 1, 0);
>  
> -	map[index] &= ~(0xFFUL << offset);
> -	map[index] |= value << offset;
> +	if (WARN_ON_ONCE(!is_power_of_2(len) || offset % len ||
> +			 offset + len > BITS_PER_LONG))
> +		return;
> +
> +	map[index] &= ~(mask << offset);
> +	map[index] |= (value & mask) << offset;
>  }
>  
> +#define bitmap_get_value8(map, start)				\
> +	bitmap_get_bits(map, start, BITS_PER_BYTE)
> +#define bitmap_set_value8(map, value, start)			\
> +	bitmap_set_bits(map, start, value, BITS_PER_BYTE)
> +
>  #endif /* __ASSEMBLY__ */
>  
>  #endif /* __LINUX_BITMAP_H */
> -- 
> 2.41.0