Re: [PATCH v2] drm/i915: Keep AUX block running when disabling DPMS for MST

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

On Mon, Apr 02, 2018 at 05:26:16PM -0400, Lyude Paul wrote:
> While enabling/disabling DPMS before link training with MST hubs is
> perfectly valid; unfortunately disabling DPMS results in some devices
> disabling their AUX CH block as well. For SST this isn't as much of a
> problem, but for MST we need to be able to continue handling aux
> transactions even when none of the sinks are turned on since it's
> possible for us to have a single atomic commit which results in
> disabling each downstream sink, followed by subsequently re-enabling
> each sink.
> 
> If we don't do this, we'll end up stalling any pending ESI interrupts
> from the sink for up to 1ms. Unfortunately, dropping ESIs during this
> timespan makes it so that link fallback retraining for MST (which I will
> be submitting to the ML shortly) fails due to the channel EQ failure
> interrupts potentially getting dropped. Additionally, when performing a
> modeset that brings the hub status's link status from bad -> good having
> ESIs disabled for that long causes us to miss the hub's response to us
> trying to start link training as well.
> 
> Since any sink with MST is going to support DisplayPort 1.2 anyway, save
> us the hassle of trying to wait until the sink comes back up and just
> never shut the aux block down.
> 
> Changes since v2:
>  - Fix patch name, no functional changes
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Cc: Laura Abbott <labbott@redhat.com>
> Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: stable@vger.kernel.org
> Fixes: ad260ab32a4d9 ("drm/i915/dp: Write to SET_POWER dpcd to enable MST hub.")
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 62f82c4298ac..0479c377981b 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -2589,11 +2589,13 @@ void intel_dp_sink_dpms(struct intel_dp *intel_dp, int mode)
>  		return;
>  
>  	if (mode != DRM_MODE_DPMS_ON) {
> +		unsigned char data = intel_dp->is_mst ?
> +			DP_SET_POWER_D3_AUX_ON : DP_SET_POWER_D3;

This smells like a workaround for an actual bug somewhere. Why exactly
is the slower wakeup or the AUX block a problem for MST but not for SST
when the link training is exactly the same for SST and MST?

> +
>  		if (downstream_hpd_needs_d0(intel_dp))
>  			return;
>  
> -		ret = drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER,
> -					 DP_SET_POWER_D3);
> +		ret = drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER, data);
>  	} else {
>  		struct intel_lspcon *lspcon = dp_to_lspcon(intel_dp);
>  
> -- 
> 2.14.3

-- 
Ville Syrjälä
Intel OTC