[PATCH 0/4] log2: make is_power_of_2() more generic

[PATCH 0/4] log2: make is_power_of_2() more generic

[Jani Nikula]

is_power_of_2() only works for types <= sizeof(unsigned long) and it's
also not a constant expression. There are a number of places in kernel
where is_power_of_2() is called on u64, which fails on 32-bit
builds. Try to remedy that. While at it, make it a constant expression
when possible.

I admit I've only lightly tested this, and I haven't tried it with
allmodconfig.


Jani Nikula (4):
  log2: add helper __IS_POWER_OF_2()
  log2: have is_power_of_2() support bigger types than unsigned long
  log2: allow use of is_power_of_2() in constant expressions
  drm/i915/reg: use is_power_of_2() from log2.h

 drivers/gpu/drm/i915/i915_reg_defs.h |  7 +------
 include/linux/log2.h                 | 25 ++++++++++++++++++++-----
 2 files changed, 21 insertions(+), 11 deletions(-)

-- 
2.39.2

[PATCH 1/4] log2: add helper __IS_POWER_OF_2()

[Jani Nikula]

Add a helper to avoid duplication in the subsequent changes.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Gow <davidgow@google.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 include/linux/log2.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index 9f30d087a128..19e773116ae3 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -33,6 +33,8 @@ int __ilog2_u64(u64 n)
 }
 #endif
 
+#define __IS_POWER_OF_2(n) ((n) != 0 && (((n) & ((n) - 1)) == 0))
+
 /**
  * is_power_of_2() - check if a value is a power of two
  * @n: the value to check
@@ -44,7 +46,7 @@ int __ilog2_u64(u64 n)
 static inline __attribute__((const))
 bool is_power_of_2(unsigned long n)
 {
-	return (n != 0 && ((n & (n - 1)) == 0));
+	return __IS_POWER_OF_2(n);
 }
 
 /**
-- 
2.39.2

[PATCH 2/4] log2: have is_power_of_2() support bigger types than unsigned long

[Jani Nikula]

is_power_of_2() does not work properly for e.g. u64 in 32-bit
builds. Choose an unsigned long long version if the argument is bigger
than unsigned long.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20230329065532.2122295-2-davidgow@google.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 include/linux/log2.h | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index 19e773116ae3..4027d1eecd7f 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -35,6 +35,18 @@ int __ilog2_u64(u64 n)
 
 #define __IS_POWER_OF_2(n) ((n) != 0 && (((n) & ((n) - 1)) == 0))
 
+static inline __attribute__((const))
+bool __is_power_of_2_ull(unsigned long long n)
+{
+	return __IS_POWER_OF_2(n);
+}
+
+static inline __attribute__((const))
+bool __is_power_of_2(unsigned long n)
+{
+	return __IS_POWER_OF_2(n);
+}
+
 /**
  * is_power_of_2() - check if a value is a power of two
  * @n: the value to check
@@ -43,11 +55,10 @@ int __ilog2_u64(u64 n)
  * *not* considered a power of two.
  * Return: true if @n is a power of 2, otherwise false.
  */
-static inline __attribute__((const))
-bool is_power_of_2(unsigned long n)
-{
-	return __IS_POWER_OF_2(n);
-}
+#define is_power_of_2(n)						\
+	__builtin_choose_expr(sizeof(n) > sizeof(unsigned long),	\
+			      __is_power_of_2_ull(n),			\
+			      __is_power_of_2(n))
 
 /**
  * __roundup_pow_of_two() - round up to nearest power of two
-- 
2.39.2

[PATCH 3/4] log2: allow use of is_power_of_2() in constant expressions

[Jani Nikula]

Even though the static inline functions are __attribute__((const)) you
can't use them in constant expressions. Make is_power_of_2() a constant
expression if possible.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: David Gow <davidgow@google.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 include/linux/log2.h | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index 4027d1eecd7f..447a85102de0 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -56,9 +56,11 @@ bool __is_power_of_2(unsigned long n)
  * Return: true if @n is a power of 2, otherwise false.
  */
 #define is_power_of_2(n)						\
-	__builtin_choose_expr(sizeof(n) > sizeof(unsigned long),	\
-			      __is_power_of_2_ull(n),			\
-			      __is_power_of_2(n))
+	__builtin_choose_expr(__is_constexpr(n),			\
+			      __IS_POWER_OF_2(n),			\
+			      __builtin_choose_expr(sizeof(n) > sizeof(unsigned long), \
+						    __is_power_of_2_ull(n), \
+						    __is_power_of_2(n)))
 
 /**
  * __roundup_pow_of_two() - round up to nearest power of two
-- 
2.39.2

[PATCH 4/4] drm/i915/reg: use is_power_of_2() from log2.h

[Jani Nikula]

Now that log2.h is_power_of_2() supports constant expressions, use it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/i915/i915_reg_defs.h | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_reg_defs.h b/drivers/gpu/drm/i915/i915_reg_defs.h
index be43580a6979..44e99a9a381f 100644
--- a/drivers/gpu/drm/i915/i915_reg_defs.h
+++ b/drivers/gpu/drm/i915/i915_reg_defs.h
@@ -52,11 +52,6 @@
 				 __is_constexpr(__low) &&		\
 				 ((__low) < 0 || (__high) > 63 || (__low) > (__high)))))
 
-/*
- * Local integer constant expression version of is_power_of_2().
- */
-#define IS_POWER_OF_2(__x)		((__x) && (((__x) & ((__x) - 1)) == 0))
-
 /**
  * REG_FIELD_PREP() - Prepare a u32 bitfield value
  * @__mask: shifted mask defining the field's length and position
@@ -71,7 +66,7 @@
 	((u32)((((typeof(__mask))(__val) << __bf_shf(__mask)) & (__mask)) +	\
 	       BUILD_BUG_ON_ZERO(!__is_constexpr(__mask)) +		\
 	       BUILD_BUG_ON_ZERO((__mask) == 0 || (__mask) > U32_MAX) +		\
-	       BUILD_BUG_ON_ZERO(!IS_POWER_OF_2((__mask) + (1ULL << __bf_shf(__mask)))) + \
+	       BUILD_BUG_ON_ZERO(!is_power_of_2((__mask) + (1ULL << __bf_shf(__mask)))) + \
 	       BUILD_BUG_ON_ZERO(__builtin_choose_expr(__is_constexpr(__val), (~((__mask) >> __bf_shf(__mask)) & (__val)), 0))))
 
 /**
-- 
2.39.2

Re: [PATCH 0/4] log2: make is_power_of_2() more generic

[Christian König]

Am 30.03.23 um 12:42 schrieb Jani Nikula:
> is_power_of_2() only works for types <= sizeof(unsigned long) and it's
> also not a constant expression. There are a number of places in kernel
> where is_power_of_2() is called on u64, which fails on 32-bit
> builds. Try to remedy that. While at it, make it a constant expression
> when possible.
>
> I admit I've only lightly tested this, and I haven't tried it with
> allmodconfig.

Looks good to me from a one mile high level pov. But I'm really not an 
expert on that type of compiler magic, so only:

Acked-by: Christian König <christian.koenig@amd.com>

for the series.

Regards,
Christian.

>
>
> Jani Nikula (4):
>    log2: add helper __IS_POWER_OF_2()
>    log2: have is_power_of_2() support bigger types than unsigned long
>    log2: allow use of is_power_of_2() in constant expressions
>    drm/i915/reg: use is_power_of_2() from log2.h
>
>   drivers/gpu/drm/i915/i915_reg_defs.h |  7 +------
>   include/linux/log2.h                 | 25 ++++++++++++++++++++-----
>   2 files changed, 21 insertions(+), 11 deletions(-)
>

Re: [PATCH 0/4] log2: make is_power_of_2() more generic

[Andrew Morton]

On Thu, 30 Mar 2023 13:42:39 +0300 Jani Nikula <jani.nikula@intel.com> wrote:

> is_power_of_2() only works for types <= sizeof(unsigned long) and it's
> also not a constant expression. There are a number of places in kernel
> where is_power_of_2() is called on u64, which fails on 32-bit
> builds. Try to remedy that. While at it, make it a constant expression
> when possible.

Yes, the current `is_power_of_2(unsigned long n)' isn't very general.

But wouldn't all these issues be addressed by simply doing

#define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))

?

(With suitable tweaks to avoid evaluating `n' more than once)

RE: [PATCH 0/4] log2: make is_power_of_2() more generic

[David Laight]

From: Andrew Morton
> Sent: 30 March 2023 20:51
> 
> On Thu, 30 Mar 2023 13:42:39 +0300 Jani Nikula <jani.nikula@intel.com> wrote:
> 
> > is_power_of_2() only works for types <= sizeof(unsigned long) and it's
> > also not a constant expression. There are a number of places in kernel
> > where is_power_of_2() is called on u64, which fails on 32-bit
> > builds. Try to remedy that. While at it, make it a constant expression
> > when possible.
> 
> Yes, the current `is_power_of_2(unsigned long n)' isn't very general.
> 
> But wouldn't all these issues be addressed by simply doing
> 
> #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
> 
> ?
> 
> (With suitable tweaks to avoid evaluating `n' more than once)

I think you need to use the 'horrid tricks' from min() to get
a constant expression from constant inputs.

For non-constants this looks ok (see https://godbolt.org/z/G73MTr9jn)

	David

static inline int lisp2(unsigned long n)
{
    return n && !(n & (n - 1));
}

static inline int llisp2(unsigned long long lln)
{
#if 0  // I think this looks worse, esp. for gcc on x86
    return lln && !(lln & (lln - 1));
#else
    unsigned long n = lln;
    if (lln >= 1ull << 32) {
        if (n)
            return 0;
        n = lln >> 32; 
    }
    return lisp2(n);
#endif
}

#define isp2(n) (sizeof ((n)+0) == sizeof (long) ? lisp2(n) : llisp2(n))

int is12(unsigned long long  i)
{
    return isp2(i);
}

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH 0/4] log2: make is_power_of_2() more generic

[Andrew Morton]

On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@ACULAB.COM> wrote:

> > But wouldn't all these issues be addressed by simply doing
> > 
> > #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
> > 
> > ?
> > 
> > (With suitable tweaks to avoid evaluating `n' more than once)
> 
> I think you need to use the 'horrid tricks' from min() to get
> a constant expression from constant inputs.

This

--- a/include/linux/log2.h~a
+++ a/include/linux/log2.h
@@ -41,11 +41,11 @@ int __ilog2_u64(u64 n)
  * *not* considered a power of two.
  * Return: true if @n is a power of 2, otherwise false.
  */
-static inline __attribute__((const))
-bool is_power_of_2(unsigned long n)
-{
-	return (n != 0 && ((n & (n - 1)) == 0));
-}
+#define is_power_of_2(_n)				\
+	({						\
+		typeof(_n) n = (_n);			\
+		n != 0 && ((n & (n - 1)) == 0);		\
+	})
 
 /**
  * __roundup_pow_of_two() - round up to nearest power of two
_

worked for me in a simple test.

--- a/fs/open.c~b
+++ a/fs/open.c
@@ -1564,3 +1564,10 @@ int stream_open(struct inode *inode, str
 }
 
 EXPORT_SYMBOL(stream_open);
+
+#include <linux/log2.h>
+
+int foo(void)
+{
+	return is_power_of_2(43);
+}
_


foo:
# fs/open.c:1573: }
	xorl	%eax, %eax	#
	ret	


Is there some more tricky situation where it breaks?

RE: [PATCH 0/4] log2: make is_power_of_2() more generic

[David Laight]

From: Andrew Morton
> Sent: 30 March 2023 23:19
> 
> On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@ACULAB.COM> wrote:
> 
> > > But wouldn't all these issues be addressed by simply doing
> > >
> > > #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
> > >
> > > ?
> > >
> > > (With suitable tweaks to avoid evaluating `n' more than once)
> >
> > I think you need to use the 'horrid tricks' from min() to get
> > a constant expression from constant inputs.
> 
> This
> 
> --- a/include/linux/log2.h~a
> +++ a/include/linux/log2.h
> @@ -41,11 +41,11 @@ int __ilog2_u64(u64 n)
>   * *not* considered a power of two.
>   * Return: true if @n is a power of 2, otherwise false.
>   */
> -static inline __attribute__((const))
> -bool is_power_of_2(unsigned long n)
> -{
> -	return (n != 0 && ((n & (n - 1)) == 0));
> -}
> +#define is_power_of_2(_n)				\
> +	({						\
> +		typeof(_n) n = (_n);			\
> +		n != 0 && ((n & (n - 1)) == 0);		\
> +	})
> 
>  /**
>   * __roundup_pow_of_two() - round up to nearest power of two
> _
> 
> worked for me in a simple test.
> 
> --- a/fs/open.c~b
> +++ a/fs/open.c
> @@ -1564,3 +1564,10 @@ int stream_open(struct inode *inode, str
>  }
> 
>  EXPORT_SYMBOL(stream_open);
> +
> +#include <linux/log2.h>
> +
> +int foo(void)
> +{
> +	return is_power_of_2(43);
> +}
> _
> 
> 
> foo:
> # fs/open.c:1573: }
> 	xorl	%eax, %eax	#
> 	ret
> 
> 
> Is there some more tricky situation where it breaks?

Try:
static int x = is_power_of_2(43);

I suspect that some (all?) of the compile-time assert checks won't
like ({...}) either.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH 0/4] log2: make is_power_of_2() more generic

[Jani Nikula]

On Thu, 30 Mar 2023, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@ACULAB.COM> wrote:
>
>> > But wouldn't all these issues be addressed by simply doing
>> > 
>> > #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
>> > 
>> > ?
>> > 
>> > (With suitable tweaks to avoid evaluating `n' more than once)
>> 
>> I think you need to use the 'horrid tricks' from min() to get
>> a constant expression from constant inputs.
>
> This
>
> --- a/include/linux/log2.h~a
> +++ a/include/linux/log2.h
> @@ -41,11 +41,11 @@ int __ilog2_u64(u64 n)
>   * *not* considered a power of two.
>   * Return: true if @n is a power of 2, otherwise false.
>   */
> -static inline __attribute__((const))
> -bool is_power_of_2(unsigned long n)
> -{
> -	return (n != 0 && ((n & (n - 1)) == 0));
> -}
> +#define is_power_of_2(_n)				\
> +	({						\
> +		typeof(_n) n = (_n);			\
> +		n != 0 && ((n & (n - 1)) == 0);		\
> +	})
>  
>  /**
>   * __roundup_pow_of_two() - round up to nearest power of two
> _
>
> worked for me in a simple test.
>
> --- a/fs/open.c~b
> +++ a/fs/open.c
> @@ -1564,3 +1564,10 @@ int stream_open(struct inode *inode, str
>  }
>  
>  EXPORT_SYMBOL(stream_open);
> +
> +#include <linux/log2.h>
> +
> +int foo(void)
> +{
> +	return is_power_of_2(43);
> +}
> _
>
>
> foo:
> # fs/open.c:1573: }
> 	xorl	%eax, %eax	#
> 	ret	
>
>
> Is there some more tricky situation where it breaks?

It doesn't work with BUILD_BUG_ON_ZERO().

test.c:
#define IS_POWER_OF_2(_n)				\
	({						\
		typeof(_n) n = (_n);			\
		n != 0 && ((n & (n - 1)) == 0);		\
	})

#define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))

#define FOO(n) ((n) + BUILD_BUG_ON_ZERO(!IS_POWER_OF_2(n)))

int main(void)
{
	return FOO(2);
}

$ gcc test.c
test.c: In function ‘main’:
test.c:16:51: error: bit-field ‘<anonymous>’ width not an integer constant
   16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
      |                                                   ^
test.c:18:23: note: in expansion of macro ‘BUILD_BUG_ON_ZERO’
   18 | #define FOO(n) ((n) + BUILD_BUG_ON_ZERO(!IS_POWER_OF_2(n)))
      |                       ^~~~~~~~~~~~~~~~~
test.c:22:9: note: in expansion of macro ‘FOO’
   22 |  return FOO(2);
      |         ^~~


BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center

Re: [PATCH 0/4] log2: make is_power_of_2() more generic

[Steven Price]

On 31/03/2023 09:31, Jani Nikula wrote:
> On Thu, 30 Mar 2023, Andrew Morton <akpm@linux-foundation.org> wrote:
>> On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@ACULAB.COM> wrote:
>>
>>>> But wouldn't all these issues be addressed by simply doing
>>>>
>>>> #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
>>>>
>>>> ?
>>>>
>>>> (With suitable tweaks to avoid evaluating `n' more than once)
>>>
>>> I think you need to use the 'horrid tricks' from min() to get
>>> a constant expression from constant inputs.
>>
>> This
>>
>> --- a/include/linux/log2.h~a
>> +++ a/include/linux/log2.h
>> @@ -41,11 +41,11 @@ int __ilog2_u64(u64 n)
>>   * *not* considered a power of two.
>>   * Return: true if @n is a power of 2, otherwise false.
>>   */
>> -static inline __attribute__((const))
>> -bool is_power_of_2(unsigned long n)
>> -{
>> -	return (n != 0 && ((n & (n - 1)) == 0));
>> -}
>> +#define is_power_of_2(_n)				\
>> +	({						\
>> +		typeof(_n) n = (_n);			\
>> +		n != 0 && ((n & (n - 1)) == 0);		\
>> +	})
>>  
>>  /**
>>   * __roundup_pow_of_two() - round up to nearest power of two
>> _
>>
>> worked for me in a simple test.
>>
>> --- a/fs/open.c~b
>> +++ a/fs/open.c
>> @@ -1564,3 +1564,10 @@ int stream_open(struct inode *inode, str
>>  }
>>  
>>  EXPORT_SYMBOL(stream_open);
>> +
>> +#include <linux/log2.h>
>> +
>> +int foo(void)
>> +{
>> +	return is_power_of_2(43);
>> +}
>> _
>>
>>
>> foo:
>> # fs/open.c:1573: }
>> 	xorl	%eax, %eax	#
>> 	ret	
>>
>>
>> Is there some more tricky situation where it breaks?
> 
> It doesn't work with BUILD_BUG_ON_ZERO().

Like most programming problems, you just need another layer of
indirection! The below works for me in all the cases I could think of
(including __uint128_t).


#define __IS_POWER_OF_2(n) (n != 0 && ((n & (n - 1)) == 0))

#define _IS_POWER_OF_2(n, unique_n)				\
	({							\
		typeof(n) unique_n = (n);			\
		__IS_POWER_OF_2(unique_n);			\
	})

#define is_power_of_2(n)					\
	__builtin_choose_expr(__is_constexpr((n)),		\
			      __IS_POWER_OF_2((n)),		\
			      _IS_POWER_OF_2(n, __UNIQUE_ID(_n)))


Although Jani's original might be easier to understand.

Steve

Re: [PATCH 0/4] log2: make is_power_of_2() more generic

[Jani Nikula]

On Wed, 05 Apr 2023, Steven Price <steven.price@arm.com> wrote:
> On 31/03/2023 09:31, Jani Nikula wrote:
>> On Thu, 30 Mar 2023, Andrew Morton <akpm@linux-foundation.org> wrote:
>>> On Thu, 30 Mar 2023 21:53:03 +0000 David Laight <David.Laight@ACULAB.COM> wrote:
>>>
>>>>> But wouldn't all these issues be addressed by simply doing
>>>>>
>>>>> #define is_power_of_2(n) (n != 0 && ((n & (n - 1)) == 0))
>>>>>
>>>>> ?
>>>>>
>>>>> (With suitable tweaks to avoid evaluating `n' more than once)
>>>>
>>>> I think you need to use the 'horrid tricks' from min() to get
>>>> a constant expression from constant inputs.
>>>
>>> This
>>>
>>> --- a/include/linux/log2.h~a
>>> +++ a/include/linux/log2.h
>>> @@ -41,11 +41,11 @@ int __ilog2_u64(u64 n)
>>>   * *not* considered a power of two.
>>>   * Return: true if @n is a power of 2, otherwise false.
>>>   */
>>> -static inline __attribute__((const))
>>> -bool is_power_of_2(unsigned long n)
>>> -{
>>> -	return (n != 0 && ((n & (n - 1)) == 0));
>>> -}
>>> +#define is_power_of_2(_n)				\
>>> +	({						\
>>> +		typeof(_n) n = (_n);			\
>>> +		n != 0 && ((n & (n - 1)) == 0);		\
>>> +	})
>>>  
>>>  /**
>>>   * __roundup_pow_of_two() - round up to nearest power of two
>>> _
>>>
>>> worked for me in a simple test.
>>>
>>> --- a/fs/open.c~b
>>> +++ a/fs/open.c
>>> @@ -1564,3 +1564,10 @@ int stream_open(struct inode *inode, str
>>>  }
>>>  
>>>  EXPORT_SYMBOL(stream_open);
>>> +
>>> +#include <linux/log2.h>
>>> +
>>> +int foo(void)
>>> +{
>>> +	return is_power_of_2(43);
>>> +}
>>> _
>>>
>>>
>>> foo:
>>> # fs/open.c:1573: }
>>> 	xorl	%eax, %eax	#
>>> 	ret	
>>>
>>>
>>> Is there some more tricky situation where it breaks?
>> 
>> It doesn't work with BUILD_BUG_ON_ZERO().
>
> Like most programming problems, you just need another layer of
> indirection! The below works for me in all the cases I could think of
> (including __uint128_t).
>
>
> #define __IS_POWER_OF_2(n) (n != 0 && ((n & (n - 1)) == 0))
>
> #define _IS_POWER_OF_2(n, unique_n)				\
> 	({							\
> 		typeof(n) unique_n = (n);			\
> 		__IS_POWER_OF_2(unique_n);			\
> 	})
>
> #define is_power_of_2(n)					\
> 	__builtin_choose_expr(__is_constexpr((n)),		\
> 			      __IS_POWER_OF_2((n)),		\
> 			      _IS_POWER_OF_2(n, __UNIQUE_ID(_n)))
>
>
> Although Jani's original might be easier to understand.

I dropped the ball since I couldn't make heads or tails what I should be
doing. And a year has passed. I'll note that the kernel has a number of
helpers for "is power of 2" for u64 and for constant expressions,
outside of log2.h.

I tried to make is_power_of_2() work for all the cases. Would it be more
palatable if I just added all the variants separately to log2.h?

- Leave is_power_of_2() as is
- Add is_power_of_2_u64() for 32-bit build compatible 64-bit checks
- Add IS_POWER_OF_2() macro for constant expressions

Please just tell me what to do and I'll do it.

BR,
Jani.


-- 
Jani Nikula, Intel