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

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Message-ID: <f4f5d85d24519367ec5e80a5d97f936a806dfcbb.camel@collabora.com>
Date:   Sat, 20 May 2023 08:57:38 +0100
From:   Christopher Obbard <chris.obbard@...labora.com>
To:     Sebastian Reichel <sebastian.reichel@...labora.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     kernel@...labora.com
Subject: Re: [PATCH v1 2/2] clk: divider: Properly handle rates exceeding
 UINT_MAX

Hi Sebastian,

On Fri, 2023-05-19 at 21:05 +0200, Sebastian Reichel wrote:
> 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>

This patch series fixes the error on Rockchip RK3588 on ROCK 5 Model B.

Tested-by: Christopher Obbard <chris.obbard@...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
> 
>