6.12-stable review patch. If anyone has any objections, please let me know.
------------------ From: Alexander Kaplan <[email protected]> commit e40e20ac089e32f1d910636155dc82e61e61dcf3 upstream. 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 <[email protected]> Cc: Uma Shankar <[email protected]> (v2) Cc: Jani Nikula <[email protected]> Cc: Maarten Lankhorst <[email protected]> Cc: [email protected] Cc: <[email protected]> # v5.12+ Signed-off-by: Alexander Kaplan <[email protected]> Reviewed-by: Ankit Nautiyal <[email protected]> Signed-off-by: Ankit Nautiyal <[email protected]> Link: https://patch.msgid.link/[email protected] Signed-off-by: Greg Kroah-Hartman <[email protected]> --- drivers/gpu/drm/display/drm_dp_helper.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) --- a/drivers/gpu/drm/display/drm_dp_helper.c +++ b/drivers/gpu/drm/display/drm_dp_helper.c @@ -3145,6 +3145,18 @@ int drm_dp_get_pcon_max_frl_bw(const u8 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;
