clk: divider: Fix divisor masking on 64 bit platforms

The clock framework handles clock rates as "unsigned long", so u32 on
32-bit architectures and u64 on 64-bit architectures.

The current code casts the dividend to u64 on 32-bit to avoid a
potential overflow. For example DIV_ROUND_UP(3000000000, 1500000000)
= (3.0G + 1.5G - 1) / 1.5G = = OVERFLOW / 1.5G, which has been
introduced in commit 9556f9da ("clk: divider: handle integer overflow
when dividing large clock rates").

On 64 bit platforms this masks the divisor, so that only the lower
32 bit are used. Thus requesting a frequency >= 4.3GHz results
in incorrect values. For example requesting 4300000000 (4.3 GHz) will
effectively request ca. 5 MHz. Requesting clk_round_rate(clk, ULONG_MAX)
is a bit of a special case, since that still returns correct values as
long as the parent clock is below 8.5 GHz.

Fix this by introducing a new helper, which avoids the overflow
by using a modulo operation instead of math tricks. This avoids
any requirements on the arguments (except that divisor should not
be 0 obviously).

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/clk/clk-divider.c

@@ -220,7 +220,7 @@ static int _div_round_up(const struct clk_div_table *table,
 			 unsigned long parent_rate, unsigned long rate,
 			 unsigned long flags)
 {
-	int div = DIV_ROUND_UP_ULL((u64)parent_rate, rate);
+	int div = DIV_ROUND_UP_NO_OVERFLOW(parent_rate, rate);
 
 	if (flags & CLK_DIVIDER_POWER_OF_TWO)
 		div = __roundup_pow_of_two(div);
@@ -237,7 +237,7 @@ static int _div_round_closest(const struct clk_div_table *table,
 	int up, down;
 	unsigned long up_rate, down_rate;
 
-	up = DIV_ROUND_UP_ULL((u64)parent_rate, rate);
+	up = DIV_ROUND_UP_NO_OVERFLOW(parent_rate, rate);
 	down = parent_rate / rate;
 
 	if (flags & CLK_DIVIDER_POWER_OF_TWO) {
@@ -473,7 +473,7 @@ int divider_get_val(unsigned long rate, unsigned long parent_rate,
 {
 	unsigned int div, value;
 
-	div = DIV_ROUND_UP_ULL((u64)parent_rate, rate);
+	div = DIV_ROUND_UP_NO_OVERFLOW(parent_rate, rate);
 
 	if (!_is_valid_div(table, div, flags))
 		return -EINVAL;

include/linux/math.h

@@ -36,6 +36,17 @@
 
 #define DIV_ROUND_UP __KERNEL_DIV_ROUND_UP
 
+/**
+ * DIV_ROUND_UP_NO_OVERFLOW - divide two numbers and always round up
+ * @n: numerator / dividend
+ * @d: denominator / divisor
+ *
+ * This functions does the same as DIV_ROUND_UP, but internally uses a
+ * division and a modulo operation instead of math tricks. This way it
+ * avoids overflowing when handling big numbers.
+ */
+#define DIV_ROUND_UP_NO_OVERFLOW(n, d) (((n) / (d)) + !!((n) % (d)))
+
 #define DIV_ROUND_DOWN_ULL(ll, d) \
 	({ unsigned long long _tmp = (ll); do_div(_tmp, d); _tmp; })