linux-kernel - [PATCH v4 2/3] clk: divider: Fix divisor masking on 64 bit platforms

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Date:   Tue, 24 Oct 2023 18:18:16 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        David Laight <David.Laight@...LAB.COM>,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sebastian Reichel <sebastian.reichel@...labora.com>,
        kernel@...labora.com
Subject: [PATCH v4 2/3] 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 9556f9dad8f5 ("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 switching to DIV_ROUND_UP_NO_OVERFLOW, which cannot
overflow. This avoids any requirements on the arguments (except
that divisor should not be 0 obviously).

Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
---
 drivers/clk/clk-divider.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index a2c2b5203b0a..94b4fb66a60f 100644
--- a/drivers/clk/clk-divider.c
+++ b/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;
-- 
2.42.0