Re: [PATCH 1/3] drm/i915: Protect MST retraining with connection_mutex

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

On Thu, Aug 27, 2015 at 06:36:48PM +0300, ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> Grab the connection_mutex around MSR link retraining to protect it
> against a concurrent modeset. We already do the same for SST.
> 
> DP hpd_pulse can still otherwise race against modeset and ->detect(), so
> it's not clear what will happen when both want to scribble into eg.
> intel_dp->dpcd[] at the same time. But sorting it all out requires way
> more thought than I'm willing to expend now.

Actually I suppose this might not work out so well after all. I suppose
MST depends on short HPDs during modeset due to the sideband stuff.

So if we want to grab modeset locks for retraining, I suppose we'd
need to move the retraining to happen from .hot_plug() which gets run
from the other hotplug work, and so wouldn't interfere with sideband.

> 
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 9e90a2b..1259c9b 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -4391,6 +4391,7 @@ update_status:
>  static int
>  intel_dp_check_mst_status(struct intel_dp *intel_dp)
>  {
> +	struct drm_device *dev = dp_to_dig_port(intel_dp)->base.base.dev;
>  	bool bret;
>  
>  	if (intel_dp->is_mst) {
> @@ -4402,6 +4403,8 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
>  go_again:
>  		if (bret == true) {
>  
> +			drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> +
>  			/* check link status - esi[10] = 0x200c */
>  			if (intel_dp->active_mst_links &&
>  			    !drm_dp_channel_eq_ok(&esi[10], intel_dp->lane_count)) {
> @@ -4411,6 +4414,8 @@ go_again:
>  				intel_dp_stop_link_train(intel_dp);
>  			}
>  
> +			drm_modeset_unlock(&dev->mode_config.connection_mutex);
> +
>  			DRM_DEBUG_KMS("got esi %3ph\n", esi);
>  			ret = drm_dp_mst_hpd_irq(&intel_dp->mst_mgr, esi, &handled);
>  
> -- 
> 2.4.6

-- 
Ville Syrjälä
Intel OTC
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