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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: Casey Harkins <caseyharkins@gmail.com> Tested-by: Khaled Almahallawy <khaled.almahallawy@intel.com> References: https://gitlab.freedesktop.org/drm/intel/-/issues/3415 Fixes: 264613b406eb ("drm/i915: Disable LTTPR support when the DPCD rev < 1.4") Cc: <stable@vger.kernel.org> # v5.11+ Signed-off-by: Imre Deak <imre.deak@intel.com> Reviewed-by: Khaled Almahallawy <khaled.almahallawy@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20210512212809.1234701-1-imre.deak@intel.com (cherry picked from commit cb4920cc40f630b5a247f4ed7d3dea66749df588) Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.