drm/i915/dp: Extract drm_dp_read_downstream_info()

We're going to be doing the same probing process in nouveau for
determining downstream DP port capabilities, so let's deduplicate the
work by moving i915's code for handling this into a shared helper:
drm_dp_read_downstream_info().

Note that when we do this, we also do make some functional changes while
we're at it:
* We always clear the downstream port info before trying to read it,
  just to make things easier for the caller
* We skip reading downstream port info if the DPCD indicates that we
  don't support downstream port info
* We only read as many bytes as needed for the reported number of
  downstream ports, no sense in reading the whole thing every time

v2:
* Fixup logic for calculating the downstream port length to account for
  the fact that downstream port caps can be either 1 byte or 4 bytes
  long. We can actually skip fixing the max_clock/max_bpc helpers here
  since they all check for DP_DETAILED_CAP_INFO_AVAILABLE anyway.
* Fix ret code check for drm_dp_dpcd_read
v5:
* Change name from drm_dp_downstream_read_info() to
  drm_dp_read_downstream_info()
* Also, add "See Also" sections for the various downstream info
  functions (drm_dp_read_downstream_info(), drm_dp_downstream_max_clock(),
  drm_dp_downstream_max_bpc())

Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826182456.322681-14-lyude@redhat.com

1 files changed, 2 insertions, 12 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 4c7314b7a84e..9c4b806af8c7 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -4685,18 +4685,8 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
 			return false;
 	}
 
-	if (!drm_dp_is_branch(intel_dp->dpcd))
-		return true; /* native DP sink */
-
-	if (intel_dp->dpcd[DP_DPCD_REV] == 0x10)
-		return true; /* no per-port downstream info */
-
-	if (drm_dp_dpcd_read(&intel_dp->aux, DP_DOWNSTREAM_PORT_0,
-			     intel_dp->downstream_ports,
-			     DP_MAX_DOWNSTREAM_PORTS) < 0)
-		return false; /* downstream port status fetch failed */
-
-	return true;
+	return drm_dp_read_downstream_info(&intel_dp->aux, intel_dp->dpcd,
+					   intel_dp->downstream_ports) == 0;
 }
 
 static bool