Currently, nouveau will re-write the DP_MSTM_CTRL register for an MST hub every time it receives a long HPD pulse on DP. This isn't actually necessary and additionally, has some unintended side effects.

With the P50 I've got here, rewriting DP_MSTM_CTRL constantly seems to make it rather likely (1 out of 5 times usually) that bringing up MST with it's ThinkPad dock will fail and result in sideband messages timing out in the middle. Afterwards, successive probes don't manage to get the dock to communicate properly over MST sideband properly.

Many times sideband message timeouts from MST hubs are indicative of either the source or the sink dropping an ESI event, which can cause DRM's perspective of the topology's current state to go out of sync with reality. While it's tough to really know for sure what's happening to the dock, using userspace tools to write to DP_MSTM_CTRL in the middle of the MST link probing process does appear to make things flaky. It's possible that when we write to DP_MSTM_CTRL, the function that gets triggered to respond in the dock's firmware temporarily puts it in a state where it might end up not reporting an ESI to the source, or ends up dropping a sideband message we sent it.

So, to fix this we make it so that when probing an MST topology, we respect it's current state. If the dock's already enabled, we simply read DP_MSTM_CTRL and disable the topology if it's value is not what we expected. Otherwise, we perform the normal MST probing dance. We avoid taking any action except if the state of the MST topology actually changes.

This fixes MST sideband message timeouts and detection failures on my P50 with its ThinkPad dock.

Signed-off-by: Lyude Paul lyude@redhat.com Cc: stable@vger.kernel.org Cc: Karol Herbst karolherbst@gmail.com --- drivers/gpu/drm/nouveau/dispnv50/disp.c | 45 ++++++++++++++++++++----- 1 file changed, 36 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/dispnv50/disp.c b/drivers/gpu/drm/nouveau/dispnv50/disp.c index f6c0b2aa9857..cdb3ef7c5d86 100644 --- a/drivers/gpu/drm/nouveau/dispnv50/disp.c +++ b/drivers/gpu/drm/nouveau/dispnv50/disp.c @@ -1117,31 +1117,58 @@ nv50_mstm_enable(struct nv50_mstm *mstm, u8 dpcd, int state) int nv50_mstm_detect(struct nv50_mstm *mstm, u8 dpcd[8], int allow) { - int ret, state = 0; + struct drm_dp_aux *aux; + int ret; + bool old_state, new_state; + u8 mstm_ctrl;

if (!mstm) return 0;

- if (dpcd[0] >= 0x12) { - ret = drm_dp_dpcd_readb(mstm->mgr.aux, DP_MSTM_CAP, &dpcd[1]); + mutex_lock(&mstm->mgr.lock); + + old_state = mstm->mgr.mst_state; + new_state = old_state; + aux = mstm->mgr.aux; + + if (old_state) { + /* Just check that the MST hub is still as we expect it */ + ret = drm_dp_dpcd_readb(aux, DP_MSTM_CTRL, &mstm_ctrl); + if (ret < 0 || !(mstm_ctrl & DP_MST_EN)) { + DRM_DEBUG_KMS("Hub gone, disabling MST topology\n"); + new_state = false; + } + } else if (dpcd[0] >= 0x12) { + ret = drm_dp_dpcd_readb(aux, DP_MSTM_CAP, &dpcd[1]); if (ret < 0) - return ret; + goto probe_error;

if (!(dpcd[1] & DP_MST_CAP)) dpcd[0] = 0x11; else - state = allow; + new_state = allow; }

- ret = nv50_mstm_enable(mstm, dpcd[0], state); + if (new_state == old_state) { + mutex_unlock(&mstm->mgr.lock); + return new_state; + } + + ret = nv50_mstm_enable(mstm, dpcd[0], new_state); if (ret) - return ret; + goto probe_error; + + mutex_unlock(&mstm->mgr.lock);

- ret = drm_dp_mst_topology_mgr_set_mst(&mstm->mgr, state); + ret = drm_dp_mst_topology_mgr_set_mst(&mstm->mgr, new_state); if (ret) return nv50_mstm_enable(mstm, dpcd[0], 0);

- return mstm->mgr.mst_state; + return new_state; + +probe_error: + mutex_unlock(&mstm->mgr.lock); + return ret; }

static void