Re: [PATCH 1/3] drm/i915: Disable M2 frac division for integer case

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 03, 2015 at 08:41:54PM +0530, Vijay Purushothaman wrote:
> v2 : Handle M2 frac division for both M2 frac and int cases
> 
> v3 : Addressed Ville's review comments. Cleared the old bits for RMW
> 
> Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_reg.h      |    1 +
>  drivers/gpu/drm/i915/intel_display.c |   24 ++++++++++++++++++------
>  2 files changed, 19 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 55143cb..8200e98 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -1029,6 +1029,7 @@ enum skl_disp_power_wells {
>  #define  DPIO_CHV_FIRST_MOD		(0 << 8)
>  #define  DPIO_CHV_SECOND_MOD		(1 << 8)
>  #define  DPIO_CHV_FEEDFWD_GAIN_SHIFT	0
> +#define  DPIO_CHV_FEEDFWD_GAIN_MASK		(0xF << 0)
>  #define CHV_PLL_DW3(ch) _PIPE(ch, _CHV_PLL_DW3_CH0, _CHV_PLL_DW3_CH1)
>  
>  #define _CHV_PLL_DW6_CH0		0x8018
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 7298796..15904a8 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -6131,6 +6131,7 @@ static void chv_prepare_pll(struct intel_crtc *crtc,
>  	enum dpio_channel port = vlv_pipe_to_channel(pipe);
>  	u32 loopfilter, intcoeff;
>  	u32 bestn, bestm1, bestm2, bestp1, bestp2, bestm2_frac;
> +	u32 dpio_val;
>  	int refclk;
>  
>  	bestn = pipe_config->dpll.n;
> @@ -6139,6 +6140,7 @@ static void chv_prepare_pll(struct intel_crtc *crtc,
>  	bestm2 = pipe_config->dpll.m2 >> 22;
>  	bestp1 = pipe_config->dpll.p1;
>  	bestp2 = pipe_config->dpll.p2;
> +	dpio_val = 0;
>  
>  	/*
>  	 * Enable Refclk and SSC
> @@ -6163,13 +6165,23 @@ static void chv_prepare_pll(struct intel_crtc *crtc,
>  			DPIO_CHV_M1_DIV_BY_2 |
>  			1 << DPIO_CHV_N_DIV_SHIFT);
>  
> -	/* M2 fraction division */
> -	vlv_dpio_write(dev_priv, pipe, CHV_PLL_DW2(port), bestm2_frac);
> +	if (bestm2_frac) {
> +		/* M2 fraction division */
> +		vlv_dpio_write(dev_priv, pipe, CHV_PLL_DW2(port), bestm2_frac);
>  
> -	/* M2 fraction division enable */
> -	vlv_dpio_write(dev_priv, pipe, CHV_PLL_DW3(port),
> -		       DPIO_CHV_FRAC_DIV_EN |
> -		       (2 << DPIO_CHV_FEEDFWD_GAIN_SHIFT));
> +		/* M2 fraction division enable */
> +		dpio_val = vlv_dpio_read(dev_priv, pipe, CHV_PLL_DW3(port));
> +		dpio_val |= DPIO_CHV_FRAC_DIV_EN;
> +		dpio_val &= ~DPIO_CHV_FEEDFWD_GAIN_MASK;
> +		dpio_val |= (2 << DPIO_CHV_FEEDFWD_GAIN_SHIFT);
> +		vlv_dpio_write(dev_priv, pipe, CHV_PLL_DW3(port), dpio_val);
> +
> +	} else {
> +		/* M2 fraction division disable */
> +		dpio_val = vlv_dpio_read(dev_priv, pipe, CHV_PLL_DW3(port));
> +		dpio_val &= ~DPIO_CHV_FRAC_DIV_EN;
> +		vlv_dpio_write(dev_priv, pipe, CHV_PLL_DW3(port), dpio_val);

You're not writing the feedfwd gain here. The register docs list it as
'Feedforwad gain for fractional mode/SSC mode PLL'. The SSC part there
makes me think these bits might mean something even if the fractional
divider is not used.

At least I don't see any harm in setting it even if the fractional
divider is not used. So with that in mind I'd probably write this as
something like:

val = read(PLL_DW3)
val &= ~(FRAC_DIV_EN | FEEDFWD_GAIN_MASK)
val |= 2 << FEEDFWD_GAIN_SHIFT;
if (bestm2_trac)
	val |= FRAC_DIV_EN;
write(PLL_DW3, val);

This should also make it less likely we will accidentally update
only one of the branches in the future when both need changing.

> +	}
>  
>  	/* Loop filter */
>  	refclk = i9xx_get_refclk(crtc, 0);
> -- 
> 1.7.9.5
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Ville Syrjälä
Intel OTC
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
http://lists.freedesktop.org/mailman/listinfo/intel-gfx