Re: [PATCH 2/3] drm/i915/psr: Improve fast and IO wake lines calculation

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

On Wed, Feb 21, 2024 at 09:16:22PM +0200, Ville Syrjälä wrote:
> On Wed, Feb 21, 2024 at 09:05:43PM +0200, Ville Syrjälä wrote:
> > On Wed, Feb 21, 2024 at 09:53:21AM +0200, Jouni Högander wrote:
> > > Current fast and IO wake lines calculation is assuming fast wake sync
> > > length is 18 pulses. Let's improve this by checking the actual length.
> > > 
> > > Also 10 us IO buffer wake time is currently assumed. This is not the case
> > > with LunarLake and beyond. Fix this by adding getter for IO wake time and
> > > return values there according to Bspec.
> > > 
> > > Bspec: 65450
> > > 
> > > Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_psr.c | 40 +++++++++++++++++++-----
> > >  1 file changed, 33 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/display/intel_psr.c b/drivers/gpu/drm/i915/display/intel_psr.c
> > > index 72cadad09db5..4a1e07411716 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_psr.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_psr.c
> > > @@ -1150,6 +1150,28 @@ static bool _lnl_compute_alpm_params(struct intel_dp *intel_dp,
> > >  	return true;
> > >  }
> > >  
> > > +/*
> > > + * From Bspec:
> > > + *
> > > + * For Xe2 and beyond
> > > + * RBR 15us, HBR1 11us, higher rates 10us
> > > + *
> > > + * For pre-Xe2
> > > + * 10 us
> > > + */
> > > +static int get_io_wake_time(struct intel_dp *intel_dp,
> > 
> > No point in passing that. You can dig out the i915 from the crtc state.
> > 
> > > +			struct intel_crtc_state *crtc_state)
> > 
> > const
> > 
> > > +{
> > > +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > > +
> > > +	if (DISPLAY_VER(i915) < 20 || crtc_state->port_clock > 270000)
> > > +		return 10;
> > > +	else if (crtc_state->port_clock > 162000)
> > > +		return 11;
> > > +	else
> > > +		return 15;
> > 
> > The new rate dependent stuff should be a separate patch.
> > 
> > And looks like the 10 usec will give us 44 usec io wake time, so
> > that should probably be a separate patch as well, to avoid
> > any functional changes when we introduce the formula.
> > 
> > > +}
> > > +
> > >  static bool _compute_alpm_params(struct intel_dp *intel_dp,
> > >  				 struct intel_crtc_state *crtc_state)
> > >  {
> > > @@ -1157,13 +1179,17 @@ static bool _compute_alpm_params(struct intel_dp *intel_dp,
> > >  	int io_wake_lines, io_wake_time, fast_wake_lines, fast_wake_time;
> > >  	u8 max_wake_lines;
> > >  
> > > -	if (DISPLAY_VER(i915) >= 12) {
> > > -		io_wake_time = 42;
> > > -		/*
> > > -		 * According to Bspec it's 42us, but based on testing
> > > -		 * it is not enough -> use 45 us.
> > > -		 */
> > > -		fast_wake_time = 45;
> > > +	if (intel_dp->get_aux_fw_sync_len) {
> > > +		int io_wake_time = get_io_wake_time(intel_dp, crtc_state);
> > 
> > Looks like this will shadow the variable you're trying to change.
> > Does the compiler not complain about this?
> > 
> > > +		int tfw_exit_latency = 20; /* eDP spec */
> > > +		int phy_wake = 4;	   /* eDP spec */
> > > +		int preamble = 8;	   /* eDP spec */
> > > +		int precharge = intel_dp->get_aux_fw_sync_len() - preamble;
> > > +
> > > +		io_wake_time = max(precharge, io_wake_time) + preamble +
> > > +			phy_wake + tfw_exit_latency;
> > > +		fast_wake_time = precharge + preamble + phy_wake +
> > > +			tfw_exit_latency;
> > >  
> > >  		/* TODO: Check how we can use ALPM_CTL fast wake extended field */
> > >  		max_wake_lines = 12;
> > 
> > I would also convert the older platforms to use the formula.
> > We do need to reverse calculate the io buffer on latency since
> > AFAICS it's not directly specified in bspec. But I think
> > that's better than not converting it since with the formula we
> > can't totally screw things up when eg. changing the precharge
> > length.
> 
> Hmm. The older platforms are apparently using fast_wake=32
> which implies zero precharge pulses. That definitely does
> not match what we program into the AUX control register...

Looks like Windows just uses:
pre-tgl:
 fast_wake=50
 io_fast_wake=50
tgl-mtl:
 fast_wake=42
 io_fast_wake=42

Also for pre-tgl they clamp these to 5-8 instead of using
the min=7 we have. For tgl+ they do clamp to 7-12.
And if the values exceed those limits they just proceed
blindly with the clamped values, which is pretty dodgy.

-- 
Ville Syrjälä
Intel