Re: [PATCH v3 3/3] drm/i915/dp_mst: Work around out-of-spec adapters filtering short pulses

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

On Thu, Jun 04, 2020 at 09:45:00PM +0300, Imre Deak wrote:
> Some TypeC -> native DP adapters, at least the Club 3D CAC-1557 adapter,
> incorrectly filter out HPD short pulses with a duration less than
> ~540 usec, leading to MST probe failures.
> 
> According to the DP Standard 2.0 section 5.1.4:
> - DP sinks should generate short pulses in the 500 usec -> 1 msec range
> - DP sources should detect short pulses in the 250 usec -> 2 msec range
> 
> According to the DP Alt Mode on TypeC Standard section 3.9.2, adapters
> should detect and forward short pulses according to how sources should
> detect them as specified in the DP Standard (250 usec -> 2 msec).
> 
> Based on the above filtering out short pulses with a duration less than
> 540 usec is incorrect.
> 
> To make such adapters work add support for a driver polling on MST
> inerrupt flags, and wire this up in the i915 driver. The sink can clear
> an interrupt it raised after 110 msec if the source doesn't respond, so
> use a 50 msec poll period to avoid missing an interrupt. Polling of the
> MST interrupt flags is explicitly allowed by the DP Standard.
> 
> This fixes MST probe failures I saw using this adapter and a DELL U2515H
> monitor.
> 
> v2:
> - Fix the wait event timeout for the no-poll case.
> v3 (Ville):
> - Fix the short pulse duration limits in the commit log prescribed by the
>   DP Standard.
> - Add code comment explaining why/how polling is used.
> - Factor out a helper to schedule the port's hpd irq handler and move it
>   to the rest of hotplug handlers.
> - Document the new MST callback.
> - s/update_hpd_irq_state/poll_hpd_irq/
> 
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Imre Deak <imre.deak@intel.com>

lgtm
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c        | 32 ++++++++++++++++++--
>  drivers/gpu/drm/i915/display/intel_dp_mst.c  | 10 ++++++
>  drivers/gpu/drm/i915/display/intel_hotplug.c | 18 +++++++++++
>  drivers/gpu/drm/i915/display/intel_hotplug.h |  2 ++
>  include/drm/drm_dp_mst_helper.h              |  9 ++++++
>  5 files changed, 68 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 5bc72e800b85..2a309fb2c4cc 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1178,11 +1178,37 @@ static int drm_dp_mst_wait_tx_reply(struct drm_dp_mst_branch *mstb,
>  				    struct drm_dp_sideband_msg_tx *txmsg)
>  {
>  	struct drm_dp_mst_topology_mgr *mgr = mstb->mgr;
> +	unsigned long wait_timeout = msecs_to_jiffies(4000);
> +	unsigned long wait_expires = jiffies + wait_timeout;
>  	int ret;
>  
> -	ret = wait_event_timeout(mgr->tx_waitq,
> -				 check_txmsg_state(mgr, txmsg),
> -				 (4 * HZ));
> +	for (;;) {
> +		/*
> +		 * If the driver provides a way for this, change to
> +		 * poll-waiting for the MST reply interrupt if we didn't receive
> +		 * it for 50 msec. This would cater for cases where the HPD
> +		 * pulse signal got lost somewhere, even though the sink raised
> +		 * the corresponding MST interrupt correctly. One example is the
> +		 * Club 3D CAC-1557 TypeC -> DP adapter which for some reason
> +		 * filters out short pulses with a duration less than ~540 usec.
> +		 *
> +		 * The poll period is 50 msec to avoid missing an interrupt
> +		 * after the sink has cleared it (after a 110msec timeout
> +		 * since it raised the interrupt).
> +		 */
> +		ret = wait_event_timeout(mgr->tx_waitq,
> +					 check_txmsg_state(mgr, txmsg),
> +					 mgr->cbs->poll_hpd_irq ?
> +						msecs_to_jiffies(50) :
> +						wait_timeout);
> +
> +		if (ret || !mgr->cbs->poll_hpd_irq ||
> +		    time_after(jiffies, wait_expires))
> +			break;
> +
> +		mgr->cbs->poll_hpd_irq(mgr);
> +	}
> +
>  	mutex_lock(&mgr->qlock);
>  	if (ret > 0) {
>  		if (txmsg->state == DRM_DP_SIDEBAND_TX_TIMEOUT) {
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index d18b406f2a7d..9be52643205d 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -33,6 +33,7 @@
>  #include "intel_connector.h"
>  #include "intel_ddi.h"
>  #include "intel_display_types.h"
> +#include "intel_hotplug.h"
>  #include "intel_dp.h"
>  #include "intel_dp_mst.h"
>  #include "intel_dpio_phy.h"
> @@ -765,8 +766,17 @@ static struct drm_connector *intel_dp_add_mst_connector(struct drm_dp_mst_topolo
>  	return NULL;
>  }
>  
> +static void
> +intel_dp_mst_poll_hpd_irq(struct drm_dp_mst_topology_mgr *mgr)
> +{
> +	struct intel_dp *intel_dp = container_of(mgr, struct intel_dp, mst_mgr);
> +
> +	intel_hpd_trigger_irq(dp_to_dig_port(intel_dp));
> +}
> +
>  static const struct drm_dp_mst_topology_cbs mst_cbs = {
>  	.add_connector = intel_dp_add_mst_connector,
> +	.poll_hpd_irq = intel_dp_mst_poll_hpd_irq,
>  };
>  
>  static struct intel_dp_mst_encoder *
> diff --git a/drivers/gpu/drm/i915/display/intel_hotplug.c b/drivers/gpu/drm/i915/display/intel_hotplug.c
> index 4f6f560e093e..664f88354101 100644
> --- a/drivers/gpu/drm/i915/display/intel_hotplug.c
> +++ b/drivers/gpu/drm/i915/display/intel_hotplug.c
> @@ -347,6 +347,24 @@ static void i915_digport_work_func(struct work_struct *work)
>  	}
>  }
>  
> +/**
> + * intel_hpd_trigger_irq - trigger an hpd irq event for a port
> + * @dig_port: digital port
> + *
> + * Trigger an HPD interrupt event for the given port, emulating a short pulse
> + * generated by the sink, and schedule the dig port work to handle it.
> + */
> +void intel_hpd_trigger_irq(struct intel_digital_port *dig_port)
> +{
> +	struct drm_i915_private *i915 = to_i915(dig_port->base.base.dev);
> +
> +	spin_lock_irq(&i915->irq_lock);
> +	i915->hotplug.short_port_mask |= BIT(dig_port->base.port);
> +	spin_unlock_irq(&i915->irq_lock);
> +
> +	queue_work(i915->hotplug.dp_wq, &i915->hotplug.dig_port_work);
> +}
> +
>  /*
>   * Handle hotplug events outside the interrupt handler proper.
>   */
> diff --git a/drivers/gpu/drm/i915/display/intel_hotplug.h b/drivers/gpu/drm/i915/display/intel_hotplug.h
> index 777b0743257e..a704d7c94d16 100644
> --- a/drivers/gpu/drm/i915/display/intel_hotplug.h
> +++ b/drivers/gpu/drm/i915/display/intel_hotplug.h
> @@ -10,6 +10,7 @@
>  
>  struct drm_i915_private;
>  struct intel_connector;
> +struct intel_digital_port;
>  struct intel_encoder;
>  enum port;
>  
> @@ -18,6 +19,7 @@ enum intel_hotplug_state intel_encoder_hotplug(struct intel_encoder *encoder,
>  					       struct intel_connector *connector);
>  void intel_hpd_irq_handler(struct drm_i915_private *dev_priv,
>  			   u32 pin_mask, u32 long_mask);
> +void intel_hpd_trigger_irq(struct intel_digital_port *dig_port);
>  void intel_hpd_init(struct drm_i915_private *dev_priv);
>  void intel_hpd_init_work(struct drm_i915_private *dev_priv);
>  void intel_hpd_cancel_work(struct drm_i915_private *dev_priv);
> diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
> index 9e1ffcd7cb68..b230ff6f7081 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -475,6 +475,15 @@ struct drm_dp_mst_topology_mgr;
>  struct drm_dp_mst_topology_cbs {
>  	/* create a connector for a port */
>  	struct drm_connector *(*add_connector)(struct drm_dp_mst_topology_mgr *mgr, struct drm_dp_mst_port *port, const char *path);
> +	/*
> +	 * Checks for any pending MST interrupts, passing them to MST core for
> +	 * processing, the same way an HPD IRQ pulse handler would do this.
> +	 * If provided MST core calls this callback from a poll-waiting loop
> +	 * when waiting for MST down message replies. The driver is expected
> +	 * to guard against a race between this callback and the driver's HPD
> +	 * IRQ pulse handler.
> +	 */
> +	void (*poll_hpd_irq)(struct drm_dp_mst_topology_mgr *mgr);
>  };
>  
>  #define DP_MAX_PAYLOAD (sizeof(unsigned long) * 8)
> -- 
> 2.23.1

-- 
Ville Syrjälä
Intel
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