On Wed, 10 Jun 2026, [email protected] wrote: > If detailed capability info is not available, downstream_ports contains 1-byte > descriptors per port. Doesn't this mean downstream_ports[2] would represent > the third downstream port rather than byte 2 of Port 0?
The DPCD addressing concern is valid. Without DP_DETAILED_CAP_INFO_AVAILABLE the registers at DPCD 0x80 hold one byte per port, so downstream_ports[2] is the descriptor of port 2 and not byte 2 of the port 0 descriptor. Bit 5 can even be legitimately set in that format, where it is part of the non-EDID format bits (DP_DS_NON_EDID_720x480i_50). So intel_dp_check_frl_training() evaluating DP_PCON_SOURCE_CTL_MODE on that byte is indeed unguarded. The practical exposure is small, though. The same condition also requires intel_dp_is_hdmi_2_1_sink(), so the EDID of the attached sink must additionally advertise HDMI FRL rates in the HF-VSDB. drm_dp_read_downstream_info() zeroes descriptor bytes beyond the advertised port count, so hitting the bogus bit takes a branch device that exposes three or more downstream ports without detailed capability info while driving an HDMI 2.1 sink, which should be a rare combination. > Should intel_dp_check_frl_training() be updated with similar checks to ensure > we only evaluate DP_PCON_SOURCE_CTL_MODE when appropriate? This patch also narrows the remaining window. With it, dfp.pcon_max_frl_bw is only non-zero for a branch device with detailed capability info and an HDMI downstream port, so in the scenario above intel_dp_pcon_start_frl_training() now fails the max_frl_bw check before any FRL DPCD access happens. What remains is the TMDS fallback write to DP_PCON_HDMI_LINK_CONFIG_1 in the error path of intel_dp_check_frl_training(). Gating the function on intel_dp->dfp.pcon_max_frl_bw, which after this patch encodes exactly the checks added here, would close that. That is an i915 change and out of scope for this helper patch. If the maintainers think it is worth closing, I am happy to send a separate follow-up patch for that. Thanks, Alexander
