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

[Jyri Sarha]

On 2021-03-18 23:47, Dario Binacchi wrote:
> > Il 17/03/2021 09:19 Tomi Valkeinen <tomba@xxxxxxxxxx> 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@xxxxxxxxx>
> > >
> > > ---
> > >
> > >   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 ?

What about renaming current req_rate to pclk_rate (for pixel clock 
rate), and calling pclk_rate * clkdiv = req_rate, as that is the rate we 
need to request from the input clock? Adding lcd to local variable names 
here is quite redundant after all. In any case req_rate is bit 
misleading name here and probably part of the reason why the bug exists 
in the first place.

Best regards,
Jyri



> > > +	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