Re: [Intel-gfx] [PATCH 1/5] drm/i915/skl: Work around incorrect BIOS WRPLL PDIV programming

[Imre Deak <imre.deak@intel.com>]

On Mon, Oct 05, 2020 at 11:08:19PM +0300, Ville Syrjälä wrote:
> On Sat, Oct 03, 2020 at 03:18:42AM +0300, Imre Deak wrote:
> > The BIOS of at least one ASUS-Z170M system with an SKL I have programs
> > the 101b WRPLL PDIV divider value, which is the encoding for PDIV=7 with
> > bit#0 incorrectly set.
> > 
> > This happens with the
> > 
> > "3840x2160": 30 262750 3840 3888 3920 4000 2160 2163 2168 2191 0x48 0x9
> > 
> > HDMI mode (scaled from a 1024x768 src fb) set by BIOS and the
> > 
> > ref_clock=24000, dco_integer=383, dco_fraction=5802, pdiv=7, qdiv=1, kdiv=1
> > 
> > WRPLL parameters (assuming PDIV=7 was the intended setting). This
> > corresponds to 262749 PLL frequency/port clock.
> > 
> > Later the driver sets the same mode for which it calculates the same
> > dco_int/dco_frac/div WRPLL parameters (with the correct PDIV=7 encoding).
> > 
> > Based on the above, let's assume that PDIV=7 was intended and the HW
> > just ignores bit#0 in the PDIV register field for this setting, treating
> > 100b and 101b encodings the same way.
> > 
> > While at it add the MISSING_CASE() for the p0,p2 divider decodings.
> > 
> > v2: (Ville)
> > - Add a define for the incorrect divider value.
> > - Emit only a debug message when detecting the incorrect divider value.
> > - Use fallthrough from the incorrect divider value case.
> > - Add the MISSING_CASE()s.
> > 
> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dpll_mgr.c | 14 ++++++++++++++
> >  drivers/gpu/drm/i915/i915_reg.h               |  1 +
> >  2 files changed, 15 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dpll_mgr.c b/drivers/gpu/drm/i915/display/intel_dpll_mgr.c
> > index e08684e34078..61cb558c60d1 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dpll_mgr.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dpll_mgr.c
> > @@ -1602,12 +1602,26 @@ static int skl_ddi_wrpll_get_freq(struct drm_i915_private *i915,
> >  	case DPLL_CFGCR2_PDIV_3:
> >  		p0 = 3;
> >  		break;
> > +	default:
> > +		if (p0 == DPLL_CFGCR2_PDIV_7_INVALID)
> 
> Why not just 'case DPLL_CFGCR2_PDIV_7_INVALID:' ?

So we can use fallthrough for both this one and the default case.

> 
> > +			/*
> > +			 * Incorrect ASUS-Z170M BIOS setting, the HW seems to ignore bit#0,
> > +			 * handling it the same way as PDIV_7.
> > +			 */
> > +			drm_dbg_kms(&i915->drm, "Invalid WRPLL PDIV divider value, fixing it.\n");
> > +		else
> > +			MISSING_CASE(p0);
> > +
> > +		fallthrough;
> >  	case DPLL_CFGCR2_PDIV_7:
> >  		p0 = 7;
> >  		break;
> >  	}
> >  
> >  	switch (p2) {
> > +	default:
> > +		MISSING_CASE(p2);
> > +		fallthrough;
> 
> Is there a specific reason we fall through to the 5 and 7 cases for
> bogus values?

Just to default to dividers that result in the minimum PLL freq.

> 
> >  	case DPLL_CFGCR2_KDIV_5:
> >  		p2 = 5;
> >  		break;
> > diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> > index 88c215cf97d4..d911583526db 100644
> > --- a/drivers/gpu/drm/i915/i915_reg.h
> > +++ b/drivers/gpu/drm/i915/i915_reg.h
> > @@ -10261,6 +10261,7 @@ enum skl_power_gate {
> >  #define  DPLL_CFGCR2_PDIV_2 (1 << 2)
> >  #define  DPLL_CFGCR2_PDIV_3 (2 << 2)
> >  #define  DPLL_CFGCR2_PDIV_7 (4 << 2)
> > +#define  DPLL_CFGCR2_PDIV_7_INVALID	(5 << 2)
> >  #define  DPLL_CFGCR2_CENTRAL_FREQ_MASK	(3)
> >  
> >  #define DPLL_CFGCR1(id)	_MMIO_PIPE((id) - SKL_DPLL1, _DPLL1_CFGCR1, _DPLL2_CFGCR1)
> > -- 
> > 2.25.1
> 
> -- 
> Ville Syrjälä
> Intel
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