• Imre Deak's avatar
    drm/i915: Add a DP1.2 compatible way to read LTTPR capabilities · 657586e4
    Imre Deak authored
    At least some DELL monitors (P2715Q) with DPCD_REV 1.2 return corrupted
    DPCD register values when reading from the 0xF0000- LTTPR range with an
    AUX transaction block size bigger than 1. The DP standard requires 0 to
    be returned - as for any other reserved/invalid addresses - but these
    monitors return the DPCD_REV register value repeated in each byte of the
    read buffer. This will in turn corrupt the values returned by the LTTPRs
    between the source and the monitor: LTTPRs must adjust the values they
    read from the downstream DPRX, for instance right-shift/init the
    downstream DP_PHY_REPEATER_CNT value. Since the value returned by the
    monitor's DPRX is non-zero the adjusted values will be corrupt.
    
    Reading the LTTPR registers one-by-one instead of reading all of them
    with a single AUX transfer works around the issue.
    
    According to the DP standard's 0xF0000 register description:
    "LTTPR-related registers at DPCD Addresses F0000h through F02FFh are
    valid only for DPCD r1.4 (or higher)." While it's unclear if DPCD r1.4
    refers to the DPCD_REV or to the
    LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV register (tickets filed
    at the VESA site to clarify this haven't been addressed), one
    possibility is that it's a restriction due to non-compliant monitors
    described above. Disabling the non-transparent LTTPR mode for all such
    monitors is not a viable solution: the transparent LTTPR mode has its
    own issue causing link training failures and this would affect a lot of
    monitors in use with DPCD_REV < 1.4. Instead this patch works around
    the problem by reading the LTTPR common and PHY cap registers one-by-one
    for any monitor with a DPCD_REV < 1.4.
    
    The standard requires the DPCD capabilities to be read after the LTTPR
    common capabilities are read, so re-read the DPCD capabilities after
    the LTTPR common and PHY caps were read out.
    
    v2:
    - Use for instead of a while loop. (Ville)
    - Add to code comment the monitor model with the problem.
    
    Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/4531
    Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
    Signed-off-by: default avatarImre Deak <imre.deak@intel.com>
    Reviewed-by: default avatarVille Syrjälä <ville.syrjala@linux.intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20220322143844.42616-1-imre.deak@intel.com
    657586e4
drm_dp.c 107 KB