Re: [PATCH v3 1/1] clk: divider: Fix divisor masking on 64 bit platforms

[Stephen Boyd <sboyd@kernel.org>]

Quoting Sebastian Reichel (2023-06-30 11:38:35)
> 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>
> ---

Sorry this one fell off my review list :(

> 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)))

Can you get someone to review/ack this macro? Maybe Andy?

> +
>  #define DIV_ROUND_DOWN_ULL(ll, d) \
>         ({ unsigned long long _tmp = (ll); do_div(_tmp, d); _tmp; })
>