[PATCH 0/2] drm/atmel-hlcdc: revise selection of pixel-clock frequency divider

[PATCH 0/2] drm/atmel-hlcdc: revise selection of pixel-clock frequency divider

[Peter Rosin]

Hi!

Some background can be found here:
https://lists.freedesktop.org/archives/dri-devel/2018-August/187182.html

The "10 times" discriminator in patch 2/2 can certainly be discussed...

Cheers,
Peter

Peter Rosin (2):
  drm/atmel-hlcdc: prefer a higher rate clock as pixel-clock base
  drm/atmel-hlcdc: allow selecting a higher pixel-clock that requested

 drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c | 29 ++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

-- 
2.11.0

[PATCH 1/2] drm/atmel-hlcdc: prefer a higher rate clock as pixel-clock base

[Peter Rosin]

If the divider used to get the pixel-clock is small, the granularity
of the frequencies possible for the pixel-clock is quite coarse. E.g.
requesting a pixel-clock of 65MHz with a sys_clk of 132MHz results
in the divider being set to 3 ending up with 44MHz.

By preferring the doubled sys_clk as base, the divider instead ends
up as 5 yielding a pixel-clock of 52.8Mhz, which is a definite
improvement.

While at it, clamp the divider so that it does not overflow in case
it gets big.

Signed-off-by: Peter Rosin <peda@axentia.se>
---
 drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
index c38a479ada98..71c9cd90d2ae 100644
--- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
+++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
@@ -101,18 +101,22 @@ static void atmel_hlcdc_crtc_mode_set_nofb(struct drm_crtc *c)
 		     (adj->crtc_hdisplay - 1) |
 		     ((adj->crtc_vdisplay - 1) << 16));
 
-	cfg = 0;
+	cfg = ATMEL_HLCDC_CLKSEL;
 
-	prate = clk_get_rate(crtc->dc->hlcdc->sys_clk);
+	prate = 2 * clk_get_rate(crtc->dc->hlcdc->sys_clk);
 	mode_rate = adj->crtc_clock * 1000;
-	if ((prate / 2) < mode_rate) {
-		prate *= 2;
-		cfg |= ATMEL_HLCDC_CLKSEL;
-	}
 
 	div = DIV_ROUND_UP(prate, mode_rate);
 	if (div < 2)
 		div = 2;
+	else if (ATMEL_HLCDC_CLKDIV(div) & ~ATMEL_HLCDC_CLKDIV_MASK) {
+		/* the divider ended up too big, try a lower base rate */
+		cfg &= ~ATMEL_HLCDC_CLKSEL;
+		prate /= 2;
+		div = DIV_ROUND_UP(prate, mode_rate);
+		if (ATMEL_HLCDC_CLKDIV(div) & ~ATMEL_HLCDC_CLKDIV_MASK)
+			div = ATMEL_HLCDC_CLKDIV_MASK;
+	}
 
 	cfg |= ATMEL_HLCDC_CLKDIV(div);
 
-- 
2.11.0

[PATCH 1/2] drm/atmel-hlcdc: prefer a higher rate clock as pixel-clock base

[Boris Brezillon]

On Fri, 24 Aug 2018 10:55:00 +0200
Peter Rosin <peda@axentia.se> wrote:

> If the divider used to get the pixel-clock is small, the granularity
> of the frequencies possible for the pixel-clock is quite coarse. E.g.
> requesting a pixel-clock of 65MHz with a sys_clk of 132MHz results
> in the divider being set to 3 ending up with 44MHz.
> 
> By preferring the doubled sys_clk as base, the divider instead ends
> up as 5 yielding a pixel-clock of 52.8Mhz, which is a definite
> improvement.
> 
> While at it, clamp the divider so that it does not overflow in case
> it gets big.
> 
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
> index c38a479ada98..71c9cd90d2ae 100644
> --- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
> +++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
> @@ -101,18 +101,22 @@ static void atmel_hlcdc_crtc_mode_set_nofb(struct drm_crtc *c)
>  		     (adj->crtc_hdisplay - 1) |
>  		     ((adj->crtc_vdisplay - 1) << 16));
>  
> -	cfg = 0;
> +	cfg = ATMEL_HLCDC_CLKSEL;
>  
> -	prate = clk_get_rate(crtc->dc->hlcdc->sys_clk);
> +	prate = 2 * clk_get_rate(crtc->dc->hlcdc->sys_clk);
>  	mode_rate = adj->crtc_clock * 1000;
> -	if ((prate / 2) < mode_rate) {
> -		prate *= 2;
> -		cfg |= ATMEL_HLCDC_CLKSEL;
> -	}
>  
>  	div = DIV_ROUND_UP(prate, mode_rate);
>  	if (div < 2)
>  		div = 2;

I'm nitpicking, but can you add braces around the if() block?

Looks good otherwise.

> +	else if (ATMEL_HLCDC_CLKDIV(div) & ~ATMEL_HLCDC_CLKDIV_MASK) {
> +		/* the divider ended up too big, try a lower base rate */
> +		cfg &= ~ATMEL_HLCDC_CLKSEL;
> +		prate /= 2;
> +		div = DIV_ROUND_UP(prate, mode_rate);
> +		if (ATMEL_HLCDC_CLKDIV(div) & ~ATMEL_HLCDC_CLKDIV_MASK)
> +			div = ATMEL_HLCDC_CLKDIV_MASK;
> +	}
>  
>  	cfg |= ATMEL_HLCDC_CLKDIV(div);
>  

[PATCH 2/2] drm/atmel-hlcdc: allow selecting a higher pixel-clock that requested

[Peter Rosin]

But only if the highest pixel-clock frequency lower than requested
is significantly much less accurate that the lowest frequency higher
than requested.

I pulled "10 times" as the discriminator out of the hat, and went with
that.

This is useful, if e.g. the target pixel-clock is 65MHz and the sys_clk
is 132MHz. In this case the highest possible pixel-clock lower than the
requested 65MHz is 52.8MHz, which is almost 20% off (and outside the
spec for the panel). The lowest possible pixel-clock higher than 65MHz
is 66MHz, which is a *much* better match, and only 1.5% off.

Signed-off-by: Peter Rosin <peda@axentia.se>
---
 drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
index 71c9cd90d2ae..0c2717ed4ac6 100644
--- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
+++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
@@ -116,6 +116,19 @@ static void atmel_hlcdc_crtc_mode_set_nofb(struct drm_crtc *c)
 		div = DIV_ROUND_UP(prate, mode_rate);
 		if (ATMEL_HLCDC_CLKDIV(div) & ~ATMEL_HLCDC_CLKDIV_MASK)
 			div = ATMEL_HLCDC_CLKDIV_MASK;
+	} else {
+		int div_low = prate / mode_rate;
+
+		if (div_low >= 2 &&
+		    ((prate / div_low - mode_rate) <
+		     10 * (mode_rate - prate / div)))
+			/*
+			 * At least 10 times better when
+			 * using a higher frequency than
+			 * requested, instead of a lower.
+			 * So, go with that.
+			 */
+			div = div_low;
 	}
 
 	cfg |= ATMEL_HLCDC_CLKDIV(div);
-- 
2.11.0

[PATCH 2/2] drm/atmel-hlcdc: allow selecting a higher pixel-clock that requested

[Boris Brezillon]

On Fri, 24 Aug 2018 10:55:01 +0200
Peter Rosin <peda@axentia.se> wrote:

> But only if the highest pixel-clock frequency lower than requested
> is significantly much less accurate that the lowest frequency higher
> than requested.
> 
> I pulled "10 times" as the discriminator out of the hat, and went with
> that.

Okay, let's go with that until we have a way to properly expose display
tolerance.

> 
> This is useful, if e.g. the target pixel-clock is 65MHz and the sys_clk
> is 132MHz. In this case the highest possible pixel-clock lower than the
> requested 65MHz is 52.8MHz, which is almost 20% off (and outside the
> spec for the panel). The lowest possible pixel-clock higher than 65MHz
> is 66MHz, which is a *much* better match, and only 1.5% off.
> 
> Signed-off-by: Peter Rosin <peda@axentia.se>
> ---
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
> index 71c9cd90d2ae..0c2717ed4ac6 100644
> --- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
> +++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_crtc.c
> @@ -116,6 +116,19 @@ static void atmel_hlcdc_crtc_mode_set_nofb(struct drm_crtc *c)
>  		div = DIV_ROUND_UP(prate, mode_rate);
>  		if (ATMEL_HLCDC_CLKDIV(div) & ~ATMEL_HLCDC_CLKDIV_MASK)
>  			div = ATMEL_HLCDC_CLKDIV_MASK;
> +	} else {
> +		int div_low = prate / mode_rate;
> +
> +		if (div_low >= 2 &&
> +		    ((prate / div_low - mode_rate) <
> +		     10 * (mode_rate - prate / div)))
> +			/*
> +			 * At least 10 times better when
> +			 * using a higher frequency than
> +			 * requested, instead of a lower.
> +			 * So, go with that.
> +			 */
> +			div = div_low;
>  	}
>  
>  	cfg |= ATMEL_HLCDC_CLKDIV(div);