Re: [Intel-gfx] [PATCH] drm/i915: Clamp linetime wm to <64usec

["Lisovskiy, Stanislav" <stanislav.lisovskiy@intel.com>]

On Fri, Jun 26, 2020 at 04:46:41PM +0300, Ville Syrjälä wrote:
> On Fri, Jun 26, 2020 at 12:16:06PM +0300, Lisovskiy, Stanislav wrote:
> > On Thu, Jun 25, 2020 at 11:00:03PM +0300, Ville Syrjala wrote:
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > 
> > > The linetime watermark is a 9 bit value, which gives us
> > > a maximum linetime of just below 64 usec. If the linetime
> > > exceeds that value we currently just discard the high bits
> > > and program the rest into the register, which angers the
> > > state checker.
> > > 
> > > To avoid that let's just clamp the value to the max. I believe
> > > it should be perfectly fine to program a smaller linetime wm
> > > than strictly required, just means the hardware may fetch data
> > > sooner than strictly needed. We are further reassured by the
> > > fact that with DRRS the spec tells us to program the smaller
> > > of the two linetimes corresponding to the two refresh rates.
> > > 
> > > Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_display.c | 18 ++++++++++++------
> > >  1 file changed, 12 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> > > index a11bb675f9b3..d486d675166f 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > > @@ -12581,12 +12581,15 @@ static u16 hsw_linetime_wm(const struct intel_crtc_state *crtc_state)
> > >  {
> > >  	const struct drm_display_mode *adjusted_mode =
> > >  		&crtc_state->hw.adjusted_mode;
> > > +	int linetime_wm;
> > >  
> > >  	if (!crtc_state->hw.enable)
> > >  		return 0;
> > >  
> > > -	return DIV_ROUND_CLOSEST(adjusted_mode->crtc_htotal * 1000 * 8,
> > > -				 adjusted_mode->crtc_clock);
> > > +	linetime_wm = DIV_ROUND_CLOSEST(adjusted_mode->crtc_htotal * 1000 * 8,
> > > +					adjusted_mode->crtc_clock);
> > > +
> > > +	return min(linetime_wm, 0x1ff);
> > 
> > Are we actually doing the right thing here? I just mean that we get value
> > 543 in the bug because pixel rate is 14874 which doesn't seem correct.
> 
> As explained in the commit msg programming this to lower than necessary
> value should be totally fine. It just won't be optimal.
> 
> The values in the jira (was there an actual gitlab bug for this btw?)
> look quite sensible to me. Some kind of low res 848xsomething mode with
> dotclock of 14.874 Mhz, which gives us that linetime of ~68 usec.

Htotal from modeline "848x480": 30 14874 848 896 928 1008 480 483 488 494 0x40 0x9
is 1008.

According to the formula above htotal(1008)*1000*8 / 14874(crtc_clock) = 542.154

So what's the catch? :)

Stan
> 
> > 
> > Stan
> > >  }
> > >  
> > >  static u16 hsw_ips_linetime_wm(const struct intel_crtc_state *crtc_state,
> > > @@ -12594,12 +12597,15 @@ static u16 hsw_ips_linetime_wm(const struct intel_crtc_state *crtc_state,
> > >  {
> > >  	const struct drm_display_mode *adjusted_mode =
> > >  		&crtc_state->hw.adjusted_mode;
> > > +	int linetime_wm;
> > >  
> > >  	if (!crtc_state->hw.enable)
> > >  		return 0;
> > >  
> > > -	return DIV_ROUND_CLOSEST(adjusted_mode->crtc_htotal * 1000 * 8,
> > > -				 cdclk_state->logical.cdclk);
> > > +	linetime_wm = DIV_ROUND_CLOSEST(adjusted_mode->crtc_htotal * 1000 * 8,
> > > +					cdclk_state->logical.cdclk);
> > > +
> > > +	return min(linetime_wm, 0x1ff);
> > >  }
> > >  
> > >  static u16 skl_linetime_wm(const struct intel_crtc_state *crtc_state)
> > > @@ -12608,7 +12614,7 @@ static u16 skl_linetime_wm(const struct intel_crtc_state *crtc_state)
> > >  	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
> > >  	const struct drm_display_mode *adjusted_mode =
> > >  		&crtc_state->hw.adjusted_mode;
> > > -	u16 linetime_wm;
> > > +	int linetime_wm;
> > >  
> > >  	if (!crtc_state->hw.enable)
> > >  		return 0;
> > > @@ -12620,7 +12626,7 @@ static u16 skl_linetime_wm(const struct intel_crtc_state *crtc_state)
> > >  	if (IS_GEN9_LP(dev_priv) && dev_priv->ipc_enabled)
> > >  		linetime_wm /= 2;
> > >  
> > > -	return linetime_wm;
> > > +	return min(linetime_wm, 0x1ff);
> > >  }
> > >  
> > >  static int hsw_compute_linetime_wm(struct intel_atomic_state *state,
> > > -- 
> > > 2.26.2
> > > 
> 
> -- 
> Ville Syrjälä
> Intel
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