[Intel-gfx] [PATCH] drm/i915: Add documentation to gen9_set_dc_state()

[Imre Deak]

On Wed, Apr 25, 2018 at 12:50:06PM +0300, Jani Nikula wrote:
> 
> Argh, now with Ville's correct address.
> 
> On Wed, 25 Apr 2018, Jani Nikula <jani.nikula at intel.com> wrote:
> > Cc: Rodrigo, DK, Ville
> >
> > On Tue, 17 Apr 2018, Imre Deak <imre.deak at intel.com> wrote:
> >> Add documentation to gen9_set_dc_state() on what enabling a given DC
> >> state means and at what point HW/DMC actually enters/exits these states.
> >>
> >> Cc: Jani Nikula <jani.nikula at intel.com>
> >> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> >> Signed-off-by: Imre Deak <imre.deak at intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_runtime_pm.c | 23 +++++++++++++++++++++++
> >>  1 file changed, 23 insertions(+)
> >>
> >> [ On IRC I stated that PSR entry would be prevented in a given DC state,
> >>   but looking more at it I haven't found any proof for this. So as I
> >>   understand the only connection between PSR and DC states is that if
> >>   DC5/6 is disabled power saving will be blocked, which would otherwise
> >>   be possible when PSR is active and the display pipe is off. ]
> >
> > I think I'm still missing a definitive answer to the question, who
> > disables PSR before DP AUX transactions?
> >
> > Does intel_display_power_get(dev_priv, intel_dp->aux_power_domain) in
> > pps_lock() cause PSR exit, through the DC state change? If yes, this
> > needs to be properly documented in code. Maybe with a WARN_ON(psr
> > active) on top.

No, that was only my misunderstanding earlier on IRC. Disabling DC
states doesn't prevent PSR entry. So the PSR active state is independent
of DC states; if PSR is enabled with DC5/6 disabled it just means there
will be less power saving during PSR active periods than it would be
possible with DC5/6 enabled.

Disabling PSR then has to happen by some other means while the driver
performs AUX transfers, either disabling it manually from the driver, or
by using the AUX HW mutex.

> >
> > Quoting bspec 7530, "DDI A AUX channel transactions must not be sent
> > while SRD is enabled. SRD must be completely disabled before a DDI A AUX
> > channel transaction can be sent."
> >
> > I'm also confused how sink CRC would ever work with PSR enabled.
> >
> > BR,
> > Jani.
> >
> >
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_runtime_pm.c b/drivers/gpu/drm/i915/intel_runtime_pm.c
> >> index 53ea564f971e..40a7955886d4 100644
> >> --- a/drivers/gpu/drm/i915/intel_runtime_pm.c
> >> +++ b/drivers/gpu/drm/i915/intel_runtime_pm.c
> >> @@ -542,6 +542,29 @@ void gen9_sanitize_dc_state(struct drm_i915_private *dev_priv)
> >>  	dev_priv->csr.dc_state = val;
> >>  }
> >>  
> >> +/**
> >> + * gen9_set_dc_state - set target display C power state
> >> + * @dev_priv: i915 device instance
> >> + * @state: target DC power state
> >> + * - DC_STATE_DISABLE
> >> + * - DC_STATE_EN_UPTO_DC5
> >> + * - DC_STATE_EN_UPTO_DC6
> >> + * - DC_STATE_EN_DC9
> >> + *
> >> + * Signal to DMC firmware/HW the target DC power state passed in @state.
> >> + * DMC/HW can turn off individual display clocks and power rails when entering
> >> + * a deeper DC power state (higher in number) and turns these back when exiting
> >> + * that state to a shallower power state (lower in number). The HW will decide
> >> + * when to actually enter a given state on an on-demand basis, for instance
> >> + * depending on the active state of display pipes. The state of display
> >> + * registers backed by affected power rails are saved/restored as needed.
> >> + *
> >> + * Based on the above enabling a deeper DC power state is asynchronous wrt.
> >> + * enabling it. Disabling a deeper power state is synchronous: for instance
> >> + * setting %DC_STATE_DISABLE won't complete until all HW resources are turned
> >> + * back on and register state is restored. This is guaranteed by the MMIO write
> >> + * to DC_STATE_EN blocking until the state is restored.
> >> + */
> >>  static void gen9_set_dc_state(struct drm_i915_private *dev_priv, uint32_t state)
> >>  {
> >>  	uint32_t val;
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center