Messages in this thread Patch in this message | | | From | Sebastian Reichel <> | Subject | [PATCH v3 1/1] clk: divider: Fix divisor masking on 64 bit platforms | Date | Fri, 30 Jun 2023 20:38:35 +0200 |
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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 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> --- Changes since PATCHv2: * https://lore.kernel.org/all/20230526171057.66876-1-sebastian.reichel@collabora.com/ * Drop first patch (applied) * Update second patch to use newly introduced DIV_ROUND_UP_NO_OVERFLOW
Changes since PATCHv1: * https://lore.kernel.org/linux-clk/20230519190522.194729-1-sebastian.reichel@collabora.com/ * Add Christopher Obbard's Reviewed-by to the first patch * Update the second patch to use DIV_ROUND_UP instead of DIV64_U64_ROUND_UP --- drivers/clk/clk-divider.c | 6 +++--- include/linux/math.h | 11 +++++++++++ 2 files changed, 14 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; diff --git a/include/linux/math.h b/include/linux/math.h index 2d388650c556..cf14d436fc2e 100644 --- a/include/linux/math.h +++ b/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; }) -- 2.40.1
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