Re: [PATCH v2] drm/i915/tgl: Add memory type decoding for bandwidth checking

[James Ausmus <james.ausmus@intel.com>]

On Wed, Sep 25, 2019 at 07:33:38AM -0700, Summers, Stuart wrote:
> On Tue, 2019-09-24 at 15:28 -0700, James Ausmus wrote:
> > The memory type values have changed in TGL, so we need to translate
> > them
> > differently than ICL. While we're moving it, fix up the ICL
> > translation
> > for LPDDR4.
> > 
> > BSpec: 53998
> > 
> > v2: Fix up ICL LPDDR4 entry (Ville); Drop unused values from TGL
> > (Ville)
> > 
> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> > Signed-off-by: James Ausmus <james.ausmus@intel.com>
> > Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_bw.c | 55 ++++++++++++++++++-----
> > --
> >  1 file changed, 39 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_bw.c
> > b/drivers/gpu/drm/i915/display/intel_bw.c
> > index cd58e47ab7b2..22e83f857de8 100644
> > --- a/drivers/gpu/drm/i915/display/intel_bw.c
> > +++ b/drivers/gpu/drm/i915/display/intel_bw.c
> > @@ -35,22 +35,45 @@ static int icl_pcode_read_mem_global_info(struct
> > drm_i915_private *dev_priv,
> >  	if (ret)
> >  		return ret;
> >  
> > -	switch (val & 0xf) {
> > -	case 0:
> > -		qi->dram_type = INTEL_DRAM_DDR4;
> > -		break;
> > -	case 1:
> > -		qi->dram_type = INTEL_DRAM_DDR3;
> > -		break;
> > -	case 2:
> > -		qi->dram_type = INTEL_DRAM_LPDDR3;
> > -		break;
> > -	case 3:
> > -		qi->dram_type = INTEL_DRAM_LPDDR3;
> > -		break;
> > -	default:
> > -		MISSING_CASE(val & 0xf);
> > -		break;
> > +	if (IS_GEN(dev_priv, 12)) {
> > +		switch (val & 0xf) {
> > +		case 0:
> > +			qi->dram_type = INTEL_DRAM_DDR4;
> > +			break;
> > +		case 3:
> > +			qi->dram_type = INTEL_DRAM_LPDDR4;
> > +			break;
> > +		case 4:
> > +			qi->dram_type = INTEL_DRAM_DDR3;
> > +			break;
> > +		case 5:
> > +			qi->dram_type = INTEL_DRAM_LPDDR3;
> > +			break;
> > +		default:
> > +			MISSING_CASE(val & 0xf);
> > +			break;
> > +		}
> > +	} else if (IS_GEN(dev_priv, 11)) {
> > +		switch (val & 0xf) {
> > +		case 0:
> > +			qi->dram_type = INTEL_DRAM_DDR4;
> > +			break;
> > +		case 1:
> > +			qi->dram_type = INTEL_DRAM_DDR3;
> > +			break;
> > +		case 2:
> > +			qi->dram_type = INTEL_DRAM_LPDDR3;
> > +			break;
> > +		case 3:
> > +			qi->dram_type = INTEL_DRAM_LPDDR4;
> > +			break;
> > +		default:
> > +			MISSING_CASE(val & 0xf);
> > +			break;
> 
> James, is there a reason we can't just combine these two conditions
> into one switch statement? At initial glance it looks like the cases
> are the same for the common ones and the only real difference is the
> supported bits.

The info I got from the HW guys indicates that the same values are very
likely to have different meanings for different gens, and likely to even
have different values for variants of a single gen, so as more platforms
are added in the future, a single switch would get very messy. Even now,
I think it would be fairly ugly, as it would look something like:

switch (val) {
	case 0:
		DDR4;
	case 1:
		if (GEN == 11)
			DDR3;
		else
			MISSING_CASE(val);
	case 2:
		if (GEN == 11)
			LPDDR3;
		else
			MISSING_CASE(val);
	case 3:
		LPDDR4;
	case 4:
		if (GEN == 12)
			DDR3;
		else
			MISSING_CASE(val);
	case 5:
		if (GEN == 12)
			LPDDR3;
		else
			MISSING_CASE(val);
}

And then start adding special cases for variants within a gen, as well
as additional gen checks, and I think it starts looking fairly
spaghetti. :)

-James

> 
> Thanks,
> Stuart
> 
> > +		}
> > +	} else {
> > +		MISSING_CASE(INTEL_GEN(dev_priv));
> > +		qi->dram_type = INTEL_DRAM_LPDDR3; /* Conservative
> > default */
> >  	}
> >  
> >  	qi->num_channels = (val & 0xf0) >> 4;


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