bitops: Introduce the the for_each_set_clump macro

include/linux/bitmap.h +11 -0

+static inline void bitmap_set_value(unsigned long *map,
+				    unsigned long value,
+				    unsigned long start, unsigned long nbits)
+{
+	const size_t index = BIT_WORD(start);
+	const unsigned long offset = start % BITS_PER_LONG;
+	const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
+	const unsigned long space = ceiling - start;
+
+	value &= GENMASK(nbits - 1, 0);
+

Quoted 11 lines · Syed Nayyar Waris

include/linux/bitmap.h +11 -0

+static inline void bitmap_set_value(unsigned long *map,
+                                 unsigned long value,
+                                 unsigned long start, unsigned long nbits)
+{
+     const size_t index = BIT_WORD(start);
+     const unsigned long offset = start % BITS_PER_LONG;
+     const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
+     const unsigned long space = ceiling - start;
+
+     value &= GENMASK(nbits - 1, 0);
+

include/linux/bitmap.h static inline void bitmap_set_value8(unsigned long *map, unsigned long value, · line 641

Quoted 10 lines

include/linux/bitmap.h +10 -0

+     if (space >= nbits) {
+             map[index] &= ~(GENMASK(nbits + offset - 1, offset));
+             map[index] |= value << offset;
+     } else {
+             map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
+             map[index] |= value << offset;
+             map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
+             map[index + 1] |= (value >> space);
+     }
+}

Sorry but what's the advantage of using this complicated function as a replacement for the much simpler bitmap_set_value8()?

The drivers calling bitmap_set_value8() *know* that 8-bit accesses are possible and take advantage of that knowledge by using a small, speed-optimized function. Replacing that with a more complicated (potentially less performant) function doesn't seem to be a step forward.

Thanks,

Lukas

Quoted 27 lines · Lukas Wunner

Quoted 11 lines · Syed Nayyar Waris

include/linux/bitmap.h +11 -0

+static inline void bitmap_set_value(unsigned long *map,
+                                 unsigned long value,
+                                 unsigned long start, unsigned long nbits)
+{
+     const size_t index = BIT_WORD(start);
+     const unsigned long offset = start % BITS_PER_LONG;
+     const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
+     const unsigned long space = ceiling - start;
+
+     value &= GENMASK(nbits - 1, 0);
+

include/linux/bitmap.h static inline void bitmap_set_value8(unsigned long *map, unsigned long value, · line 641

Quoted 10 lines

include/linux/bitmap.h +10 -0

+     if (space >= nbits) {
+             map[index] &= ~(GENMASK(nbits + offset - 1, offset));
+             map[index] |= value << offset;
+     } else {
+             map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
+             map[index] |= value << offset;
+             map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
+             map[index + 1] |= (value >> space);
+     }
+}

Sorry but what's the advantage of using this complicated function as a replacement for the much simpler bitmap_set_value8()?

Quoted 5 lines

The drivers calling bitmap_set_value8() *know* that 8-bit accesses are possible and take advantage of that knowledge by using a small, speed-optimized function. Replacing that with a more complicated (potentially less performant) function doesn't seem to be a step forward.

Actually this generic function can work with n-bits of any size (less than equal to BITS_PER_LONG), while the earlier bitmap_set_value8 worked with n-bits having size of 8 bits only.

In the case when n-bits is 8-bits, this new bitmap_set_value() function would behave very similar to the earlier bitmap_set_value8() function. For example, in case of n-bits being 8-bits it will always execute the 'if' condition and not the 'else' condition, hence offering the same performance (because of encountering similar code statements) as earlier bitmap_set_value8() function, most probably.

There is an additional advantage (this can happen when n-bits is not 8 bits): during setting value of n-bit in bitmap, if a situation arise that the width of next n-bit is exceeding the word boundary, then it will divide itself such that some portion of it is stored in that word, while the remaining portion is stored in the next higher word.

So, this function preserves the behaviour of earlier bitmap_set_value8() function and also adds extra functionality to that.

Quoted 51 lines · Syed Nayyar Waris

Quoted 27 lines · Lukas Wunner

Quoted 11 lines · Syed Nayyar Waris

include/linux/bitmap.h +11 -0

+static inline void bitmap_set_value(unsigned long *map,
+                                 unsigned long value,
+                                 unsigned long start, unsigned long nbits)
+{
+     const size_t index = BIT_WORD(start);
+     const unsigned long offset = start % BITS_PER_LONG;
+     const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
+     const unsigned long space = ceiling - start;
+
+     value &= GENMASK(nbits - 1, 0);
+

include/linux/bitmap.h static inline void bitmap_set_value8(unsigned long *map, unsigned long value, · line 641

Quoted 10 lines

include/linux/bitmap.h +10 -0

+     if (space >= nbits) {
+             map[index] &= ~(GENMASK(nbits + offset - 1, offset));
+             map[index] |= value << offset;
+     } else {
+             map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
+             map[index] |= value << offset;
+             map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
+             map[index + 1] |= (value >> space);
+     }
+}

Sorry but what's the advantage of using this complicated function as a replacement for the much simpler bitmap_set_value8()?

Quoted 5 lines

The drivers calling bitmap_set_value8() *know* that 8-bit accesses are possible and take advantage of that knowledge by using a small, speed-optimized function. Replacing that with a more complicated (potentially less performant) function doesn't seem to be a step forward.

Actually this generic function can work with n-bits of any size (less than equal to BITS_PER_LONG), while the earlier bitmap_set_value8 worked with n-bits having size of 8 bits only.

In the case when n-bits is 8-bits, this new bitmap_set_value() function would behave very similar to the earlier bitmap_set_value8() function. For example, in case of n-bits being 8-bits it will always execute the 'if' condition and not the 'else' condition, hence offering the same performance (because of encountering similar code statements) as earlier bitmap_set_value8() function, most probably.

There is an additional advantage (this can happen when n-bits is not 8 bits): during setting value of n-bit in bitmap, if a situation arise that the width of next n-bit is exceeding the word boundary, then it will divide itself such that some portion of it is stored in that word, while the remaining portion is stored in the next higher word.

Quoted 3 lines

So, this function preserves the behaviour of earlier bitmap_set_value8() function and also adds extra functionality to that.

Please leave drivers as is which use exclusively 8-bit accesses, e.g. gpio-max3191x.c and gpio-74x164.c. I'm fearing a performance regression if your new generic variant is used. They work perfectly fine the way they are and I don't see any benefit this series may have for them.

If there are other drivers which benefit from the flexibility of your generic variant then I'm not opposed to changing those.

Thanks,

Lukas

Quoted 3 lines · Lukas Wunner

Quoted 2 lines · Syed Nayyar Waris

Quoted 1 line · Lukas Wunner

On Fri, Apr 24, 2020 at 05:55:21PM +0530, Syed Nayyar Waris wrote:

Quoted 4 lines

Quoted 3 lines

So, this function preserves the behaviour of earlier bitmap_set_value8() function and also adds extra functionality to that.

Quoted 8 lines

Please leave drivers as is which use exclusively 8-bit accesses, e.g. gpio-max3191x.c and gpio-74x164.c. I'm fearing a performance regression if your new generic variant is used. They work perfectly fine the way they are and I don't see any benefit this series may have for them.

If there are other drivers which benefit from the flexibility of your generic variant then I'm not opposed to changing those.

We can leave of course bitmap_set_value8 alone, but for 8-bit values the difference in latency I suspect is primarily due to the conditional test for the word boundaries. This latency is surely overshadowed by the I/O latency of the GPIO drivers, so I don't think there's much harm in changing those to use the generic function when the bottleneck will not be due to the bitmap_set_value/bitmap_get_value operations.

Quoted 4 lines · William Breathitt Gray

Quoted 3 lines · Lukas Wunner

Quoted 2 lines · Syed Nayyar Waris

Quoted 1 line · Lukas Wunner

On Fri, Apr 24, 2020 at 05:55:21PM +0530, Syed Nayyar Waris wrote:

...

Quoted 12 lines

Quoted 12 lines

Quoted 3 lines

So, this function preserves the behaviour of earlier bitmap_set_value8() function and also adds extra functionality to that.

Please leave drivers as is which use exclusively 8-bit accesses, e.g. gpio-max3191x.c and gpio-74x164.c. I'm fearing a performance regression if your new generic variant is used. They work perfectly fine the way they are and I don't see any benefit this series may have for them.

If there are other drivers which benefit from the flexibility of your generic variant then I'm not opposed to changing those.

Quoted 6 lines

We can leave of course bitmap_set_value8 alone, but for 8-bit values the difference in latency I suspect is primarily due to the conditional test for the word boundaries. This latency is surely overshadowed by the I/O latency of the GPIO drivers, so I don't think there's much harm in changing those to use the generic function when the bottleneck will not be due to the bitmap_set_value/bitmap_get_value operations.

Okay, how many new (non-8-bit) users this will target?

Signature · 2 lines

With Best Regards,
Andy Shevchenko

Within this patchset the only non-8-bit users are gpio-thunderx and gpio-xilinix. The gpio-xilinx has configurable port widths so in some instances it can behave like the 8-bit users, but not always.

If you want to keep the existing for_each_set_clump8 and related functions, ignore [PATCH 3/6] and [PATCH 4/6]. That should allow this patchset to be just an introduction of the new generic functions without affecting the existing 8-bit users.