drm/dp_mst: Use full_pbn instead of available_pbn for bandwidth checks

DisplayPort specifications are fun. For a while, it's been really
unclear to us what available_pbn actually does. There's a somewhat vague
explanation in the DisplayPort spec (starting from 1.2) that partially
explains it:

  The minimum payload bandwidth number supported by the path. Each node
  updates this number with its available payload bandwidth number if its
  payload bandwidth number is less than that in the Message Transaction

So, it sounds like available_pbn represents the smallest link rate in
use between the source and the branch device. Cool, so full_pbn is just
the highest possible PBN that the branch device supports right?

Well, we assumed that for quite a while until Sean Paul noticed that on
some MST hubs, available_pbn will actually get set to 0 whenever there's
any active payloads on the respective branch device. This caused quite a
bit of confusion since clearing the payload ID table would end up fixing
the available_pbn value.

So, we just went with that until commit cd82d82cbc04 ("drm/dp_mst: Add
branch bandwidth validation to MST atomic check") started breaking
people's setups due to us getting erroneous available_pbn values. So, we
did some more digging and got confused until we finally looked at the
definition for full_pbn:

  The bandwidth of the link at the trained link rate and lane count
  between the DP Source device and the DP Sink device with no time slots
  allocated to VC Payloads, represented as a Payload Bandwidth Number. As
  with the Available_Payload_Bandwidth_Number, this number is determined
  by the link with the lowest lane count and link rate.

That's what we get for not reading specs closely enough, hehe. So, since
full_pbn is definitely what we want for doing bandwidth restriction
checks - let's start using that instead and ignore available_pbn

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: cd82d82cbc04 ("drm/dp_mst: Add branch bandwidth validation to MST atomic check")
Cc: Mikita Lipski <mikita.lipski@amd.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Reviewed-by: Mikita Lipski <mikita.lipski@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306234623.547525-3-lyude@redhat.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 9188c53..7df7676 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2309,7 +2309,7 @@
 		} else {
-			port->available_pbn = 0;
+			port->full_pbn = 0;
@@ -2404,7 +2404,7 @@
 		if (port->ddps) {
 			dowork = true;
 		} else {
-			port->available_pbn = 0;
+			port->full_pbn = 0;
@@ -2556,7 +2556,7 @@
 		if (port->input || !port->ddps)
-		if (!port->available_pbn) {
+		if (!port->full_pbn) {
 			drm_modeset_lock(&mgr->base.lock, NULL);
 			drm_dp_send_enum_path_resources(mgr, mstb, port);
@@ -3002,8 +3002,7 @@
-			port->available_pbn =
-				path_res->avail_payload_bw_number;
+			port->full_pbn = path_res->full_payload_bw_number;
 			port->fec_capable = path_res->fec_capable;
@@ -3598,7 +3597,7 @@
 	list_for_each_entry(port, &mstb->ports, next) {
 		/* The PBN for each port will also need to be re-probed */
-		port->available_pbn = 0;
+		port->full_pbn = 0;
 		if (port->mstb)
@@ -4842,8 +4841,8 @@
 			if (drm_dp_mst_atomic_check_bw_limit(port->mstb, mst_state))
 				return -ENOSPC;
-		if (port->available_pbn > 0)
-			pbn_limit = port->available_pbn;
+		if (port->full_pbn > 0)
+			pbn_limit = port->full_pbn;
 	DRM_DEBUG_ATOMIC("[MST BRANCH:%p] branch has %d PBN available\n",
 			 branch, pbn_limit);
diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
index bcb39da..41725d8 100644
--- a/include/drm/drm_dp_mst_helper.h
+++ b/include/drm/drm_dp_mst_helper.h
@@ -81,7 +81,7 @@
  * &drm_dp_mst_topology_mgr.base.lock.
  * @num_sdp_stream_sinks: Number of stream sinks. Protected by
  * &drm_dp_mst_topology_mgr.base.lock.
- * @available_pbn: Available bandwidth for this port. Protected by
+ * @full_pbn: Max possible bandwidth for this port. Protected by
  * &drm_dp_mst_topology_mgr.base.lock.
  * @next: link to next port on this branch device
  * @aux: i2c aux transport to talk to device connected to this port, protected
@@ -126,7 +126,7 @@
 	u8 dpcd_rev;
 	u8 num_sdp_streams;
 	u8 num_sdp_stream_sinks;
-	uint16_t available_pbn;
+	uint16_t full_pbn;
 	struct list_head next;
 	 * @mstb: the branch device connected to this port, if there is one.