drm/dp: Add support for DP tunneling

Add support for Display Port tunneling. For now this includes the
support for Bandwidth Allocation Mode (BWA), leaving adding Panel Replay
support for later.

BWA allows using displays that share the same (Thunderbolt) link with
their maximum resolution. Atm, this may not be possible due to the
coarse granularity of partitioning the link BW among the displays on the
link: the BW allocation policy is in a SW/FW/HW component on the link
(on Thunderbolt it's the SW or FW Connection Manager), independent of
the driver. This policy will set the DPRX maximum rate and lane count
DPCD registers the GFX driver will see (0x00000, 0x00001, 0x02200,
0x02201) based on the available link BW.

The granularity of the current BW allocation policy is coarse, based on
the required link rate in the 1.62Gbs..8.1Gbps range and it may prevent
using higher resolutions all together: the display connected first will
get a share of the link BW which corresponds to its full DPRX capability
(regardless of the actual mode it uses). A subsequent display connected
will only get the remaining BW, which could be well below its full
capability.

BWA solves the above coarse granularity (reducing it to a 250Mbs..1Gps
range) and first-come/first-served issues by letting the driver request
the BW for each display on a link which reflects the actual modes the
displays use.

This patch adds the DRM core helper functions, while a follow-up change
in the patchset takes them into use in the i915 driver.

v2:
- Fix prepare_to_wait vs. wake-up cond check order in
  allocate_tunnel_bw(). (Ville)
- Move tunnel==NULL checks from callers in drivers to here. (Ville)
- Avoid var inits in declaration blocks that can fail or have
  side-effects. (Ville)
- Use u8 for driver and group IDs. (Ville)
- Simplify API removing drm_dp_tunnel_get/put_untracked(). (Ville)
- Reuse str_yes_no() instead of a local yes_no_chr(). (Ville)
- s/drm_dp_tunnel_atomic_clear_state()/free_tunnel_state() and unexport
  the function. (Ville)
- s/clear_tunnel_group_state()/free_group_state() and move kfree() to
  this function. (Ville)
- Add separate group_free_bw() helper and describe what the tunnel
  estimated BW includes. (Ville)
- Improve help text for CONFIG_DRM_DISPLAY_DP_TUNNEL. (Ville)
- Add code comment explaining the purpose of DPCD reg read helpers.
  (Ville)
- Add code comment describing the tunnel group name prefix format.
  (Ville)
- Report the allocated BW as undetermined until the first allocation
  request.
- Skip allocation requests matching the previous request.
- Clear any stale BW request status flags before a new request.
- Add missing error return check of drm_dp_tunnel_atomic_get_group_state()
  in drm_dp_tunnel_atomic_set_stream_bw().
- Add drm_dp_tunnel_get_allocated_bw().
- s/drm_dp_tunnel_atomic_get_tunnel_bw/drm_dp_tunnel_atomic_get_required_bw
- Fix return value description in function doc of drm_dp_tunnel_detect().
- Add function documentation to all exported functions.

v3:
- Improve grouping of fields in drm_dp_tunnel_group struct. (Uma)
- Fix validating the BW granularity DPCD reg value. (Uma)
- Document return value of check_and_clear_status_change(). (Uma)
- Fix resetting drm_dp_tunnel_ref::tunnel in drm_dp_tunnel_ref_put().
  (Ville)
- Allow for ALLOCATED_BW to change after a BWA enable/disable sequence.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240226185246.1276018-2-imre.deak@intel.com

drivers/gpu/drm/display/Kconfig

@@ -17,6 +17,27 @@ config DRM_DISPLAY_DP_HELPER
 	help
 	  DRM display helpers for DisplayPort.
 
+config DRM_DISPLAY_DP_TUNNEL
+	bool
+	select DRM_DISPLAY_DP_HELPER
+	help
+	  Enable support for DisplayPort tunnels. This allows drivers to use
+	  DP tunnel features like the Bandwidth Allocation mode to maximize the
+	  BW utilization for display streams on Thunderbolt links.
+
+config DRM_DISPLAY_DEBUG_DP_TUNNEL_STATE
+	bool "Enable debugging the DP tunnel state"
+	depends on REF_TRACKER
+	depends on DRM_DISPLAY_DP_TUNNEL
+	depends on DEBUG_KERNEL
+	depends on EXPERT
+	help
+	  Enables debugging the DP tunnel manager's state, including the
+	  consistency of all managed tunnels' reference counting and the state of
+	  streams contained in tunnels.
+
+	  If in doubt, say "N".
+
 config DRM_DISPLAY_HDCP_HELPER
 	bool
 	depends on DRM_DISPLAY_HELPER

drivers/gpu/drm/display/Makefile

@@ -8,6 +8,8 @@ drm_display_helper-$(CONFIG_DRM_DISPLAY_DP_HELPER) += \
 	drm_dp_helper.o \
 	drm_dp_mst_topology.o \
 	drm_dsc_helper.o
+drm_display_helper-$(CONFIG_DRM_DISPLAY_DP_TUNNEL) += \
+	drm_dp_tunnel.o
 drm_display_helper-$(CONFIG_DRM_DISPLAY_HDCP_HELPER) += drm_hdcp_helper.o
 drm_display_helper-$(CONFIG_DRM_DISPLAY_HDMI_HELPER) += \
 	drm_hdmi_helper.o \