linux-kernel - Re: [PATCH] clk: fractional-divider: check parent rate only for general approximation

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Message-ID: <154879892760.136743.9274469856102314518@swboyd.mtv.corp.google.com>
Date:   Tue, 29 Jan 2019 13:55:27 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Katsuhiro Suzuki <katsuhiro@...suster.net>,
        Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Katsuhiro Suzuki <katsuhiro@...suster.net>
Subject: Re: [PATCH] clk: fractional-divider: check parent rate only for general approximation

Quoting Katsuhiro Suzuki (2019-01-07 05:21:24)
> Custom approximation of fractional-divider may not need parent clock
> rate checking. For example Rockchip SoCs work fine using grand parent
> clock rate evne if target rate is greater than parent.
> 
> This patch removes parent clock rate check from custom approximation.
> 
> Signed-off-by: Katsuhiro Suzuki <katsuhiro@...suster.net>
> ---
>  drivers/clk/clk-fractional-divider.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
> index 545dceec0bbf..b0fc5509e0ff 100644
> --- a/drivers/clk/clk-fractional-divider.c
> +++ b/drivers/clk/clk-fractional-divider.c
> @@ -79,13 +79,17 @@ static long clk_fd_round_rate(struct clk_hw *hw, unsigned long rate,
>         unsigned long m, n;
>         u64 ret;
>  
> -       if (!rate || rate >= *parent_rate)
> +       if (!rate)

Is your fraction multiplying the rate up? If it's a divider it shouldn't
be increasing the rate more than whatever the parent is supplying so
this looks odd. How does the grandparent matter here? Maybe you can show
the example in the commit text so we can all understand what's going on
here.

>                 return *parent_rate;
>