linux-kernel - Re: [PATCH] drm/tilcdc: fix LCD pixel clock setting

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Message-ID: <7df3a270-1cc4-7a71-5e55-49a0dfb2c21f@kernel.org>
Date:   Wed, 17 Mar 2021 10:19:46 +0200
From:   Tomi Valkeinen <tomba@...nel.org>
To:     Dario Binacchi <dariobin@...ero.it>, linux-kernel@...r.kernel.org,
        Jyri Sarha <jyri.sarha@....fi>
Cc:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/tilcdc: fix LCD pixel clock setting

On 14/03/2021 17:13, Dario Binacchi wrote:
> As reported by TI spruh73x RM, the LCD pixel clock (LCD_PCLK) frequency
> is obtained by dividing LCD_CLK, the LCD controller reference clock,
> for CLKDIV:
> 
> LCD_PCLK = LCD_CLK / CLKDIV
> 
> where CLKDIV must be greater than 1.
> 
> Therefore LCD_CLK must be set to 'req_rate * CLKDIV' instead of req_rate

The above doesn't make sense, the code already sets LCD_CLK to 'req_rate 
* clkdiv', not req_rate.

> and the real LCD_CLK rate must be compared with 'req_rate * CLKDIV' and
> not with req_rate.

This is true, the code looks at the wrong value.

> Passing req_rate instead of 'req_rate * CLKDIV' to the tilcdc_pclk_diff
> routine caused it to fail even if LCD_CLK was properly set.
> 
> Signed-off-by: Dario Binacchi <dariobin@...ero.it>
> 
> ---
> 
>   drivers/gpu/drm/tilcdc/tilcdc_crtc.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_crtc.c b/drivers/gpu/drm/tilcdc/tilcdc_crtc.c
> index 30213708fc99..02f56c9a5da5 100644
> --- a/drivers/gpu/drm/tilcdc/tilcdc_crtc.c
> +++ b/drivers/gpu/drm/tilcdc/tilcdc_crtc.c
> @@ -203,7 +203,7 @@ static void tilcdc_crtc_set_clk(struct drm_crtc *crtc)
>   	struct drm_device *dev = crtc->dev;
>   	struct tilcdc_drm_private *priv = dev->dev_private;
>   	struct tilcdc_crtc *tilcdc_crtc = to_tilcdc_crtc(crtc);
> -	unsigned long clk_rate, real_rate, req_rate;
> +	unsigned long clk_rate, real_rate, req_rate, clk_div_rate;
>   	unsigned int clkdiv;
>   	int ret;
>   
> @@ -211,10 +211,11 @@ static void tilcdc_crtc_set_clk(struct drm_crtc *crtc)
>   
>   	/* mode.clock is in KHz, set_rate wants parameter in Hz */
>   	req_rate = crtc->mode.clock * 1000;
> -
> -	ret = clk_set_rate(priv->clk, req_rate * clkdiv);
> +	/* LCD clock divisor input rate */
> +	clk_div_rate = req_rate * clkdiv;

"clk_div_rate" sounds a bit odd to me. Why not lcd_fck_rate, as that's 
the name used later? Or lcd_clk_rate. Or maybe lcd_clk_req_rate...

> +	ret = clk_set_rate(priv->clk, clk_div_rate);
>   	clk_rate = clk_get_rate(priv->clk);
> -	if (ret < 0 || tilcdc_pclk_diff(req_rate, clk_rate) > 5) {
> +	if (ret < 0 || tilcdc_pclk_diff(clk_div_rate, clk_rate) > 5) {
>   		/*
>   		 * If we fail to set the clock rate (some architectures don't
>   		 * use the common clock framework yet and may not implement
> 

I think this fix is fine, but looking at the current code, it's calling 
tilcdc_pclk_diff(), but doesn't actually provide pixel clocks to the 
function, but fclk.

  Tomi