Re: [PATCH] drm/i915: Add documentation to gen9_set_dc_state()

[Jani Nikula <jani.nikula@intel.com>]

Cc: Rodrigo, DK, Ville

On Tue, 17 Apr 2018, Imre Deak <imre.deak@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@intel.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Signed-off-by: Imre Deak <imre.deak@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.

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
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