Add a function to do signed 64-bit division with remainder. This is
implemented using div64_u64_rem in the same way that div_s64_rem is
implemented using div_u64_rem.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
---

 include/linux/math64.h | 18 ++++++++++++++++++
 lib/math/div64.c       | 20 ++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 6aaccc1626ab..0a414446af89 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -57,6 +57,20 @@ static inline u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
 	return dividend / divisor;
 }
 
+/**
+ * div64_s64_rem - signed 64bit divide with 64bit divisor and remainder
+ * @dividend: signed 64bit dividend
+ * @divisor: signed 64bit divisor
+ * @remainder: pointer to signed 64bit remainder
+ *
+ * Return: sets ``*remainder``, then returns dividend / divisor
+ */
+static inline s64 div64_s64_rem(s64 dividend, s64 divisor, s64 *remainder)
+{
+	*remainder = dividend % divisor;
+	return dividend / divisor;
+}
+
 /**
  * div64_u64 - unsigned 64bit divide with 64bit divisor
  * @dividend: unsigned 64bit dividend
@@ -102,6 +116,10 @@ extern s64 div_s64_rem(s64 dividend, s32 divisor, s32 *remainder);
 extern u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder);
 #endif
 
+#ifndef div64_s64_rem
+s64 div64_s64_rem(s64 dividend, s64 divisor, s64 *remainder);
+#endif
+
 #ifndef div64_u64
 extern u64 div64_u64(u64 dividend, u64 divisor);
 #endif
diff --git a/lib/math/div64.c b/lib/math/div64.c
index 5faa29208bdb..ccef0db85681 100644
--- a/lib/math/div64.c
+++ b/lib/math/div64.c
@@ -124,6 +124,26 @@ u64 div64_u64_rem(u64 dividend, u64 divisor, u64 *remainder)
 EXPORT_SYMBOL(div64_u64_rem);
 #endif
 
+#ifndef div_s64_rem
+s64 div64_s64_rem(s64 dividend, s64 divisor, s64 *remainder)
+{
+	u64 quotient;
+
+	if (dividend < 0) {
+		quotient = div64_u64_rem(-dividend, abs(divisor), (u64 *)remainder);
+		*remainder = -*remainder;
+		if (divisor > 0)
+			quotient = -quotient;
+	} else {
+		quotient = div64_u64_rem(dividend, abs(divisor), (u64 *)remainder);
+		if (divisor < 0)
+			quotient = -quotient;
+	}
+	return quotient;
+}
+EXPORT_SYMBOL(div64_s64_rem);
+#endif
+
 /*
  * div64_u64 - unsigned 64bit divide with 64bit divisor
  * @dividend:	64bit dividend
-- 
2.35.1.1320.gc452695387.dirty

On Mon, Jul 14, 2025 at 09:20:17PM -0400, Sean Anderson wrote:
> Add a function to do signed 64-bit division with remainder. This is
> implemented using div64_u64_rem in the same way that div_s64_rem is
> implemented using div_u64_rem.

LGTM, but one important Q. Can we (start to) add the test cases, please?

-- 
With Best Regards,
Andy Shevchenko

On 7/15/25 04:03, Andy Shevchenko wrote:
> On Mon, Jul 14, 2025 at 09:20:17PM -0400, Sean Anderson wrote:
>> Add a function to do signed 64-bit division with remainder. This is
>> implemented using div64_u64_rem in the same way that div_s64_rem is
>> implemented using div_u64_rem.
> 
> LGTM, but one important Q. Can we (start to) add the test cases, please?
> 

Well, this just calls div64_u64_rem. So I am inclined to make the test something
like

#define test(n, d, q, r) ({ \
	u64 _q, _r; \
	_q = div64_u64_rem(n, d, &r); \
	assert(_q == q); \
	assert(_r == r); \
})

test( 3,  2,  1,  1);
test( 3, -2, -1,  1);
test(-3,  2, -1, -1);
test(-3, -2,  1, -1);

--Sean

On Tue, Jul 15, 2025 at 01:36:33PM -0400, Sean Anderson wrote:
> On 7/15/25 04:03, Andy Shevchenko wrote:
> > On Mon, Jul 14, 2025 at 09:20:17PM -0400, Sean Anderson wrote:
> >> Add a function to do signed 64-bit division with remainder. This is
> >> implemented using div64_u64_rem in the same way that div_s64_rem is
> >> implemented using div_u64_rem.
> > 
> > LGTM, but one important Q. Can we (start to) add the test cases, please?
> 
> Well, this just calls div64_u64_rem. So I am inclined to make the test something
> like
> 
> #define test(n, d, q, r) ({ \
> 	u64 _q, _r; \
> 	_q = div64_u64_rem(n, d, &r); \
> 	assert(_q == q); \
> 	assert(_r == r); \
> })
> 
> test( 3,  2,  1,  1);
> test( 3, -2, -1,  1);
> test(-3,  2, -1, -1);
> test(-3, -2,  1, -1);

Perhaps, but it should be done somewhere in lib/tests/...

-- 
With Best Regards,
Andy Shevchenko