From: Alexander Kaplan Date: Wed, 10 Jun 2026 19:38:25 +0000 (+0200) Subject: drm/dp: Read the PCON max FRL bandwidth only for HDMI DFPs X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e40e20ac089e32f1d910636155dc82e61e61dcf3;p=thirdparty%2Flinux.git drm/dp: Read the PCON max FRL bandwidth only for HDMI DFPs The PCON max FRL bandwidth field lives in byte 2 of the DFP Detailed Capability Info (DPCD 0x82 for the first DFP). The DP standard defines the meaning of descriptor bytes 1-3 strictly per DFP type, and for a DisplayPort type DFP all of them are reserved, with "read all 0s" semantics (DP v2.0, section 2.12.3, Table 2-183). The FRL bandwidth field is an HDMI DFP extension added by the VESA DP-to-HDMI PCON specification. drm_dp_get_pcon_max_frl_bw() however parses the byte without checking the DFP type, the branch presence or DETAILED_CAP_INFO_AVAILABLE. Without the latter the port descriptors are one byte wide and port_cap[2] is not even the right register. All neighbouring helpers parsing the same descriptor are scoped by the DFP type already, see for instance drm_dp_downstream_max_bpc() reading the same byte and returning 0 for a DP type DFP. amdgpu's DC parses the field only for HDMI(/DP++) detailed types as well. This is not theoretical. A Synaptics VMM7100 based USB-C to HDMI adapter with a macOS targeted firmware advertises a DisplayPort type DFP with the type byte replicated across the whole descriptor (08 08 08 08). i915 decodes that as "PCON limited to 18 Gbps FRL" and prunes every mode above ~750 MHz dotclock, including all the 4k@100/120 modes the sink EDID offers, while macOS drives 4k@120 through the same adapter just fine via DP DSC (and amdgpu's type-scoped parser would ignore the bogus field as well). Only parse the field for an HDMI DFP behind a DPCD 1.1+ branch device that reports detailed cap info, matching the type-scoped field layout of the spec and the rest of the helpers. Fixes: ce32a6239de6 ("drm/dp_helper: Add Helpers for FRL Link Training support for DP-HDMI2.1 PCON") Cc: Ankit Nautiyal Cc: Uma Shankar (v2) Cc: Jani Nikula Cc: Maarten Lankhorst Cc: dri-devel@lists.freedesktop.org Cc: # v5.12+ Signed-off-by: Alexander Kaplan Reviewed-by: Ankit Nautiyal Signed-off-by: Ankit Nautiyal Link: https://patch.msgid.link/20260610193825.2933-1-alexander.kaplan@sms-medipool.de --- diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c index 9c31e14cc413..e623ccb4c1d8 100644 --- a/drivers/gpu/drm/display/drm_dp_helper.c +++ b/drivers/gpu/drm/display/drm_dp_helper.c @@ -3686,6 +3686,18 @@ int drm_dp_get_pcon_max_frl_bw(const u8 dpcd[DP_RECEIVER_CAP_SIZE], int bw; u8 buf; + if (!drm_dp_is_branch(dpcd)) + return 0; + + if (dpcd[DP_DPCD_REV] < 0x11) + return 0; + + if ((dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE) == 0) + return 0; + + if ((port_cap[0] & DP_DS_PORT_TYPE_MASK) != DP_DS_PORT_TYPE_HDMI) + return 0; + buf = port_cap[2]; bw = buf & DP_PCON_MAX_FRL_BW;