657586e474
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: Imre Deak <imre.deak@intel.com> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20220322143844.42616-1-imre.deak@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.