linux-kernel - Re: [PATCH v2] clk: fractional-divider: check parent rate only if flag is set

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Message-ID: <155082319829.77512.7935951458664373330@swboyd.mtv.corp.google.com>
Date:   Fri, 22 Feb 2019 00:13:18 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Katsuhiro Suzuki <katsuhiro@...suster.net>,
        Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.org
Cc:     Heiko Stuebner <heiko@...ech.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Katsuhiro Suzuki <katsuhiro@...suster.net>
Subject: Re: [PATCH v2] clk: fractional-divider: check parent rate only if flag is set

Quoting Katsuhiro Suzuki (2019-02-10 07:38:06)
> Custom approximation of fractional-divider may not need parent clock
> rate checking. For example Rockchip SoCs work fine using grand parent
> clock rate even if target rate is greater than parent.
> 
> This patch checks parent clock rate only if CLK_SET_RATE_PARENT flag
> is set.
> 
> For detailed example, clock tree of Rockchip I2S audio hardware.
>   - Clock rate of CPLL is 1.2GHz, GPLL is 491.52MHz.
>   - i2s1_div is integer divider can divide N (N is 1~128).
>     Input clock is CPLL or GPLL. Initial divider value is N = 1.
>     Ex) PLL = CPLL, N = 10, i2s1_div output rate is
>       CPLL / 10 = 1.2GHz / 10 = 120MHz
>   - i2s1_frac is fractional divider can divide input to x/y, x and
>     y are 16bit integer.
> 
> CPLL --> | selector | ---> i2s1_div -+--> | selector | --> I2S1 MCLK

Applied to clk-next but I made the function into a macro because I don't
see the benefit to exporting such a simple thing.