lib: add mul_u64_add_u64_div_u64() and mul_u64_u64_div_u64_roundup()

The existing mul_u64_u64_div_u64() rounds down, a 'rounding up' variant
needs 'divisor - 1' adding in between the multiply and divide so cannot
easily be done by a caller.

Add mul_u64_add_u64_div_u64(a, b, c, d) that calculates (a * b + c)/d and
implement the 'round down' and 'round up' using it.

Update the x86-64 asm to optimise for 'c' being a constant zero.

Add kerndoc definitions for all three functions.

Link: https://lkml.kernel.org/r/20251105201035.64043-5-david.laight.linux@gmail.com
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-by: Nicolas Pitre <npitre@baylibre.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 47 insertions, 1 deletions

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 6aaccc1626ab..e889d850b7f1 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -282,7 +282,53 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
 }
 #endif /* mul_u64_u32_div */
 
-u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div);
+/**
+ * mul_u64_add_u64_div_u64 - unsigned 64bit multiply, add, and divide
+ * @a: first unsigned 64bit multiplicand
+ * @b: second unsigned 64bit multiplicand
+ * @c: unsigned 64bit addend
+ * @d: unsigned 64bit divisor
+ *
+ * Multiply two 64bit values together to generate a 128bit product
+ * add a third value and then divide by a fourth.
+ * The Generic code divides by 0 if @d is zero and returns ~0 on overflow.
+ * Architecture specific code may trap on zero or overflow.
+ *
+ * Return: (@a * @b + @c) / @d
+ */
+u64 mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d);
+
+/**
+ * mul_u64_u64_div_u64 - unsigned 64bit multiply and divide
+ * @a: first unsigned 64bit multiplicand
+ * @b: second unsigned 64bit multiplicand
+ * @d: unsigned 64bit divisor
+ *
+ * Multiply two 64bit values together to generate a 128bit product
+ * and then divide by a third value.
+ * The Generic code divides by 0 if @d is zero and returns ~0 on overflow.
+ * Architecture specific code may trap on zero or overflow.
+ *
+ * Return: @a * @b / @d
+ */
+#define mul_u64_u64_div_u64(a, b, d) mul_u64_add_u64_div_u64(a, b, 0, d)
+
+/**
+ * mul_u64_u64_div_u64_roundup - unsigned 64bit multiply and divide rounded up
+ * @a: first unsigned 64bit multiplicand
+ * @b: second unsigned 64bit multiplicand
+ * @d: unsigned 64bit divisor
+ *
+ * Multiply two 64bit values together to generate a 128bit product
+ * and then divide and round up.
+ * The Generic code divides by 0 if @d is zero and returns ~0 on overflow.
+ * Architecture specific code may trap on zero or overflow.
+ *
+ * Return: (@a * @b + @d - 1) / @d
+ */
+#define mul_u64_u64_div_u64_roundup(a, b, d) \
+	({ u64 _tmp = (d); mul_u64_add_u64_div_u64(a, b, _tmp - 1, _tmp); })
+
 
 /**
  * DIV64_U64_ROUND_UP - unsigned 64bit divide with 64bit divisor rounded up