linux-kernel - Re: [PATCH 03/11] drm/bridge: tc358768: Cleanup PLL calculations

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Message-ID: <37cf3e0d-5a30-466f-86d3-cda077c3921f@gmail.com>
Date:   Fri, 11 Aug 2023 19:25:42 +0300
From:   Péter Ujfalusi <peter.ujfalusi@...il.com>
To:     Tomi Valkeinen <tomi.valkeinen@...asonboard.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>,
        Francesco Dolcini <francesco@...cini.it>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Aradhya Bhatia <a-bhatia1@...com>
Subject: Re: [PATCH 03/11] drm/bridge: tc358768: Cleanup PLL calculations



On 04/08/2023 13:44, Tomi Valkeinen wrote:
> As is quite common, some of TC358768's PLL register fields are to be
> programmed with (value - 1). Specifically, the FBD and PRD, multiplier
> and divider, are such fields.
> 
> However, what the driver currently does is that it considers that the
> formula used for PLL rate calculation is:
> 
> RefClk * [(FBD + 1)/ (PRD + 1)] * [1 / (2^FRS)]
> 
> where FBD and PRD are values directly from the registers, while a more
> sensible way to look at it is:
> 
> RefClk * FBD / PRD * (1 / (2^FRS))
> 
> and when the FBD and PRD values are written to the registers, they will
> be subtracted by one.
> 
> Change the driver accordingly, as it simplifies the PLL code.

Reviewed-by: Peter Ujfalusi <peter.ujfalusi@...il.com>

> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
> ---
>  drivers/gpu/drm/bridge/tc358768.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
> index b668f77673c3..d5831a1236e9 100644
> --- a/drivers/gpu/drm/bridge/tc358768.c
> +++ b/drivers/gpu/drm/bridge/tc358768.c
> @@ -316,7 +316,7 @@ static int tc358768_calc_pll(struct tc358768_priv *priv,
>  
>  	target_pll = tc358768_pclk_to_pll(priv, mode->clock * 1000);
>  
> -	/* pll_clk = RefClk * [(FBD + 1)/ (PRD + 1)] * [1 / (2^FRS)] */
> +	/* pll_clk = RefClk * FBD / PRD * (1 / (2^FRS)) */
>  
>  	for (i = 0; i < ARRAY_SIZE(frs_limits); i++)
>  		if (target_pll >= frs_limits[i])
> @@ -336,19 +336,19 @@ static int tc358768_calc_pll(struct tc358768_priv *priv,
>  	best_prd = 0;
>  	best_fbd = 0;
>  
> -	for (prd = 0; prd < 16; ++prd) {
> -		u32 divisor = (prd + 1) * (1 << frs);
> +	for (prd = 1; prd <= 16; ++prd) {
> +		u32 divisor = prd * (1 << frs);
>  		u32 fbd;
>  
> -		for (fbd = 0; fbd < 512; ++fbd) {
> +		for (fbd = 1; fbd <= 512; ++fbd) {
>  			u32 pll, diff, pll_in;
>  
> -			pll = (u32)div_u64((u64)refclk * (fbd + 1), divisor);
> +			pll = (u32)div_u64((u64)refclk * fbd, divisor);
>  
>  			if (pll >= max_pll || pll < min_pll)
>  				continue;
>  
> -			pll_in = (u32)div_u64((u64)refclk, prd + 1);
> +			pll_in = (u32)div_u64((u64)refclk, prd);
>  			if (pll_in < 4000000)
>  				continue;
>  
> @@ -611,7 +611,7 @@ static int tc358768_setup_pll(struct tc358768_priv *priv,
>  		mode->clock * 1000);
>  
>  	/* PRD[15:12] FBD[8:0] */
> -	tc358768_write(priv, TC358768_PLLCTL0, (prd << 12) | fbd);
> +	tc358768_write(priv, TC358768_PLLCTL0, ((prd - 1) << 12) | (fbd - 1));
>  
>  	/* FRS[11:10] LBWS[9:8] CKEN[4] RESETB[1] EN[0] */
>  	tc358768_write(priv, TC358768_PLLCTL1,
> 

-- 
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