Nouveau - Apr 2024 - [PATCH 1/2] drm/nouveau/kms/nv50-: Disable AUX bus for disconnected DP ports

Fixes for a few issues I've been seeing around regarding DP aux
transactions with nouveau and GSP support - mainly stemming from the
fact that GSP returns an error for aux transactions that are attempted
on disconnected ports. Some of these issues somehow manage to make
runtime PM fail on my Slimbook Executive 16!

Lyude Paul (2):
  drm/nouveau/kms/nv50-: Disable AUX bus for disconnected DP ports
  drm/nouveau/dp: Don't probe eDP ports twice harder

 drivers/gpu/drm/nouveau/nouveau_dp.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

-- 
2.44.0

GSP has its own state for keeping track of whether or not a given display
connector is plugged in or not, and enforces this state on the driver. In
particular, AUX transactions on a DisplayPort connector which GSP says is
disconnected can never succeed - and can in some cases even cause
unexpected timeouts, which can trickle up to cause other problems. A good
example of this is runtime power management: where we can actually get
stuck trying to resume the GPU if a userspace application like fwupd tries
accessing a drm_aux_dev for a disconnected port. This was an issue I hit a
few times with my Slimbook Executive 16 - where trying to offload something
to the discrete GPU would wake it up, and then potentially cause it to
timeout as fwupd tried to immediately access the dp_aux_dev nodes for
nouveau.

Likewise: we don't really have any cases I know of where we'd want to
ignore this state and try an aux transaction anyway - and failing pointless
aux transactions immediately can even speed things up. So - let's start
enabling/disabling the aux bus in nouveau_dp_detect() to fix this. We
enable the aux bus during connector probing, and leave it enabled if we
discover something is actually on the connector. Otherwise, we just shut it
off.

This should fix some people's runtime PM issues (like myself), and also get
rid of quite of a lot of GSP error spam in dmesg.

Signed-off-by: Lyude Paul <lyude at redhat.com>
---
 drivers/gpu/drm/nouveau/nouveau_dp.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/nouveau_dp.c
b/drivers/gpu/drm/nouveau/nouveau_dp.c
index fb06ee17d9e54..8b1be7dd64ebe 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dp.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dp.c
@@ -232,6 +232,9 @@ nouveau_dp_detect(struct nouveau_connector *nv_connector,
 	    dpcd[DP_DPCD_REV] != 0)
 		return NOUVEAU_DP_SST;
 
+	// Ensure that the aux bus is enabled for probing
+	drm_dp_dpcd_set_powered(&nv_connector->aux, true);
+
 	mutex_lock(&nv_encoder->dp.hpd_irq_lock);
 	if (mstm) {
 		/* If we're not ready to handle MST state changes yet, just
@@ -293,6 +296,13 @@ nouveau_dp_detect(struct nouveau_connector *nv_connector,
 	if (mstm && !mstm->suspended && ret != NOUVEAU_DP_MST)
 		nv50_mstm_remove(mstm);
 
+	/* GSP doesn't like when we try to do aux transactions on a port it
considers disconnected,
+	 * and since we don't really have a usecase for that anyway - just disable
the aux bus here
+	 * if we've decided the connector is disconnected
+	 */
+	if (ret == NOUVEAU_DP_NONE)
+		drm_dp_dpcd_set_powered(&nv_connector->aux, false);
+
 	mutex_unlock(&nv_encoder->dp.hpd_irq_lock);
 	return ret;
 }
-- 
2.44.0

I didn't pay close enough attention the last time I tried to fix this
problem - while we currently do correctly take care to make sure we don't
probe a connected eDP port more then once, we don't do the same thing for
eDP ports we found to be disconnected.

So, fix this and make sure we only ever probe eDP ports once and then leave
them at that connector state forever (since without HPD, it's not going to
change on its own anyway). This should get rid of the last few GSP errors
getting spit out during runtime suspend and resume on some machines, as we
tried to reprobe eDP ports in response to ACPI hotplug probe events.

Signed-off-by: Lyude Paul <lyude at redhat.com>
---
 drivers/gpu/drm/nouveau/nouveau_dp.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_dp.c
b/drivers/gpu/drm/nouveau/nouveau_dp.c
index 8b1be7dd64ebe..8b27d372e86da 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dp.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dp.c
@@ -225,12 +225,16 @@ nouveau_dp_detect(struct nouveau_connector *nv_connector,
 	u8 *dpcd = nv_encoder->dp.dpcd;
 	int ret = NOUVEAU_DP_NONE, hpd;
 
-	/* If we've already read the DPCD on an eDP device, we don't need to
-	 * reread it as it won't change
+	/* eDP ports don't support hotplugging - so there's no point in
probing eDP ports unless we
+	 * haven't probed them once before.
 	 */
-	if (connector->connector_type == DRM_MODE_CONNECTOR_eDP &&
-	    dpcd[DP_DPCD_REV] != 0)
-		return NOUVEAU_DP_SST;
+	if (connector->connector_type == DRM_MODE_CONNECTOR_eDP) {
+		if (connector->status == connector_status_connected) {
+			return NOUVEAU_DP_SST;
+		} else if (connector->status == connector_status_disconnected) {
+			return NOUVEAU_DP_NONE;
+		}
+	}
 
 	// Ensure that the aux bus is enabled for probing
 	drm_dp_dpcd_set_powered(&nv_connector->aux, true);
-- 
2.44.0