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

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Date:   Thu, 18 Mar 2021 22:47:09 +0100 (CET)
From:   Dario Binacchi <dariobin@...ero.it>
To:     Tomi Valkeinen <tomba@...nel.org>, 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


> Il 17/03/2021 09:19 Tomi Valkeinen <tomba@...nel.org> ha scritto:
> 
>  
> 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...

I prefer lcd_clk_rate.

How about adding an additional patch that changes the variable names to make 
the code more readable?

req_rate -> lcd_pclk_rate
clk_rate -> real_lcd_clk_rate

And add a comment to the function which highlights the relationship 
LCD_CLK = LCD_PCLK * CLDIV ?

> 
> > +	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.

Yes, I agree.

Thanks and regards,
Dario

> 
>   Tomi