1. 21 May, 2021 1 commit
    • Imre Deak's avatar
      drm/i915: Reenable LTTPR non-transparent LT mode for DPCD_REV<1.4 · cb4920cc
      Imre Deak authored
      The driver currently disables the LTTPR non-transparent link training
      mode for sinks with a DPCD_REV<1.4, based on the following description
      of the LTTPR DPCD register range in DP standard 2.0 (at the 0xF0000
      register description):
      
      ""
      LTTPR-related registers at DPCD Addresses F0000h through F02FFh are valid
      only for DPCD r1.4 (or higher).
      """
      
      The transparent link training mode should still work fine, however the
      implementation for this in some retimer FWs seems to be broken, see the
      References: link below.
      
      After discussions with DP standard authors the above "DPCD r1.4" does
      not refer to the DPCD revision (stored in the DPCD_REV reg at 0x00000),
      rather to the "LTTPR field data structure revision" stored in the
      0xF0000 reg. An update request has been filed at vesa.org (see
      wg/Link/documentComment/3746) for the upcoming v2.1 specification to
      clarify the above description along the following lines:
      
      """
      LTTPR-related registers at DPCD Addresses F0000h through F02FFh are
      valid only for LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV 1.4 (or
      higher)
      """
      
      Based on my tests Windows uses the non-transparent link training mode
      for DPCD_REV==1.2 sinks as well (so presumably for all DPCD_REVs), and
      forcing it to use transparent mode on ICL/TGL platforms leads to the
      same LT failure as reported at the References: link.
      
      Based on the above let's assume that the transparent link training mode
      is not well tested/supported and align the code to the correct
      interpretation of what the r1.4 version refers to.
      Reported-and-tested-by: default avatarCasey Harkins <caseyharkins@gmail.com>
      Tested-by: default avatarKhaled Almahallawy <khaled.almahallawy@intel.com>
      References: https://gitlab.freedesktop.org/drm/intel/-/issues/3415
      Fixes: 264613b4 ("drm/i915: Disable LTTPR support when the DPCD rev < 1.4")
      Cc: <stable@vger.kernel.org> # v5.11+
      Signed-off-by: default avatarImre Deak <imre.deak@intel.com>
      Reviewed-by: default avatarKhaled Almahallawy <khaled.almahallawy@intel.com>
      Link: https://patchwork.freedesktop.org/patch/msgid/20210512212809.1234701-1-imre.deak@intel.com
      cb4920cc
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