linux-kernel - [PATCH v1 2/2] clk: divider: Properly handle rates exceeding UINT

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Date:   Fri, 19 May 2023 21:05:22 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.com>
To:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Sebastian Reichel <sebastian.reichel@...labora.com>,
        kernel@...labora.com
Subject: [PATCH v1 2/2] clk: divider: Properly handle rates exceeding UINT_MAX

Requesting rates exceeding UINT_MAX (so roughly 4.3 GHz) results
in very small rate being chosen instead of very high ones, since
DIV_ROUND_UP_ULL takes a 32 bit integer as second argument.

Correct this by using DIV64_U64_ROUND_UP instead, which takes proper
64 bit values for dividend and divisor.

Note, that this is usually not an issue. ULONG_MAX sets the lower
32 bits and thus effectively requests UINT_MAX. On most platforms
that is good enough. To trigger a real bug one of the following
conditions must be met:

 * A parent clock with more than 8.5 GHz is available
 * Instead of ULONG_MAX a specific frequency like 4.3 GHz is
   requested. That would end up becoming 5 MHz instead :)

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..dddaaf0f9d25 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 = DIV64_U64_ROUND_UP(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 = DIV64_U64_ROUND_UP(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 = DIV64_U64_ROUND_UP(parent_rate, rate);
 
 	if (!_is_valid_div(table, div, flags))
 		return -EINVAL;
-- 
2.39.2