linux-kernel - Re: [PATCH 3/5] drm/bridge: samsung-dsim: update PLL reference clock

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Date:   Mon, 28 Aug 2023 18:41:48 +0200
From:   Marco Felsch <m.felsch@...gutronix.de>
To:     Michael Tretter <m.tretter@...gutronix.de>
Cc:     Inki Dae <inki.dae@...sung.com>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, kernel@...gutronix.de,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 3/5] drm/bridge: samsung-dsim: update PLL reference clock

On 23-08-28, Michael Tretter wrote:
> The PLL requires a clock between 2 MHz and 30 MHz after the pre-divider.
> The reference clock for the PLL may change due to changes to it's parent
> clock. Thus, the frequency may be out of range or unsuited for
> generating the high speed clock for MIPI DSI.
> 
> Try to keep the pre-devider small, and set the reference clock close to
> 30 MHz before recalculating the PLL configuration. Use a divider with a
> power of two for the reference clock as this seems to work best in
> my tests.
> 
> Signed-off-by: Michael Tretter <m.tretter@...gutronix.de>
> ---
>  drivers/gpu/drm/bridge/samsung-dsim.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index da90c2038042..4de6e4f116db 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -611,10 +611,21 @@ static unsigned long samsung_dsim_set_pll(struct samsung_dsim *dsi,
>  	u16 m;
>  	u32 reg;
>  
> -	if (dsi->pll_clk)
> +	if (dsi->pll_clk) {
> +		/*
> +		 * Ensure that the reference clock is generated with a power of
> +		 * two divider from its parent, but close to the PLLs upper
> +		 * limit of the valid range of 2 MHz to 30 MHz.
> +		 */
> +		fin = clk_get_rate(clk_get_parent(dsi->pll_clk));
> +		while (fin > 30 * MHZ)
> +			fin = fin / 2;

Really just a cosmetic nit: fin /= 2;

Reviewed-by: Marco Felsch <m.felsch@...gutronix.de>

> +		clk_set_rate(dsi->pll_clk, fin);
> +
>  		fin = clk_get_rate(dsi->pll_clk);
> -	else
> +	} else {
>  		fin = dsi->pll_clk_rate;
> +	}
>  	dev_dbg(dsi->dev, "PLL ref clock freq %lu\n", fin);
>  
>  	fout = samsung_dsim_pll_find_pms(dsi, fin, freq, &p, &m, &s);
> 
> -- 
> 2.39.2
> 
> 
>