linux-kernel - Re: [PATCH] clk: rockchip: re-add rational best approximation algorithm to the fractional divider

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Date:   Mon, 31 Jan 2022 20:22:58 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     quentin.schulz@...obroma-systems.com,
        Chris Morgan <macromorgan@...mail.com>,
        Douglas Anderson <dianders@...omium.org>
Cc:     mturquette@...libre.com, sboyd@...nel.org, heiko@...ech.de,
        linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Quentin Schulz <foss+kernel@...il.net>
Subject: Re: [PATCH] clk: rockchip: re-add rational best approximation
 algorithm to the fractional divider

On Mon, Jan 31, 2022 at 05:32:24PM +0100, quentin.schulz@...obroma-systems.com wrote:
> From: Quentin Schulz <quentin.schulz@...obroma-systems.com>

Thanks for your report.

> In commit 4e7cf74fa3b2 ("clk: fractional-divider: Export approximation
> algorithm to the CCF users"), the code handling the rational best
> approximation algorithm was replaced by a call to the core
> clk_fractional_divider_general_approximation function which did the same
> thing back then.
> 
> However, in commit 82f53f9ee577 ("clk: fractional-divider: Introduce
> POWER_OF_TWO_PS flag"), this common code was made conditional on
> CLK_FRAC_DIVIDER_POWER_OF_TWO_PS flag which was not added back to the
> rockchip clock driver.
> 
> This broke the ltk050h3146w-a2 MIPI DSI display present on a PX30-based
> downstream board.
> 
> Let's add the flag to the fractional divider flags so that the original
> and intended behavior is brought back to the rockchip clock drivers.

I believe this was the result of the discussion about 1000 in DRM code.

I Cc'ed this to the people from 64ec4912c51a ("drm/rockchip: Update crtc
fixup to account for fractional clk change").

> Fixes: 82f53f9ee577 ("clk: fractional-divider: Introduce POWER_OF_TWO_PS flag")
> Cc: stable@...r.kernel.org
> Cc: Quentin Schulz <foss+kernel@...il.net>
> Signed-off-by: Quentin Schulz <quentin.schulz@...obroma-systems.com>
> ---
>  drivers/clk/rockchip/clk.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/clk/rockchip/clk.c b/drivers/clk/rockchip/clk.c
> index b7be7e11b0df..bb8a844309bf 100644
> --- a/drivers/clk/rockchip/clk.c
> +++ b/drivers/clk/rockchip/clk.c
> @@ -180,6 +180,7 @@ static void rockchip_fractional_approximation(struct clk_hw *hw,
>  		unsigned long rate, unsigned long *parent_rate,
>  		unsigned long *m, unsigned long *n)
>  {
> +	struct clk_fractional_divider *fd = to_clk_fd(hw);
>  	unsigned long p_rate, p_parent_rate;
>  	struct clk_hw *p_parent;
>  
> @@ -190,6 +191,8 @@ static void rockchip_fractional_approximation(struct clk_hw *hw,
>  		*parent_rate = p_parent_rate;
>  	}
>  
> +	fd->flags |= CLK_FRAC_DIVIDER_POWER_OF_TWO_PS;
> +
>  	clk_fractional_divider_general_approximation(hw, rate, parent_rate, m, n);
>  }

-- 
With Best Regards,
Andy Shevchenko