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

[Jani Nikula]

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.
>
> 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