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

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

[Dario Binacchi]

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
and the real LCD_CLK rate must be compared with 'req_rate * CLKDIV' and
not with req_rate.
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@libero.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;
+	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
-- 
2.17.1

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

[Tomi Valkeinen]

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@libero.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

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

[Dario Binacchi]

> Il 17/03/2021 09:19 Tomi Valkeinen <tomba@kernel.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@libero.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

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@kernel.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@libero.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 ?
> 

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