On Fri, Jan 03, 2025 at 02:58:18PM +0200, Abel Vesa wrote: > Link Training Tunable PHY Repeaters (LTTPRs) are defined in DisplayPort > 1.4a specification. As the name suggests, these PHY repeaters are > capable of adjusting their output for link training purposes. > > According to the DisplayPort standard, LTTPRs have two operating > modes: > - non-transparent - it replies to DPCD LTTPR field specific AUX > requests, while passes through all other AUX requests > - transparent - it passes through all AUX requests. > > Switching between this two modes is done by the DPTX by issuing > an AUX write to the DPCD PHY_REPEATER_MODE register. > > The msm DP driver is currently lacking any handling of LTTPRs. > This means that if at least one LTTPR is found between DPTX and DPRX, > the link training would fail if that LTTPR was not already configured > in transparent mode. > > The section 3.6.6.1 from the DisplayPort v2.0 specification mandates > that before link training with the LTTPR is started, the DPTX may place > the LTTPR in non-transparent mode by first switching to transparent mode > and then to non-transparent mode. This operation seems to be needed only > on first link training and doesn't need to be done again until device is > unplugged. > > It has been observed on a few X Elite-based platforms which have > such LTTPRs in their board design that the DPTX needs to follow the > procedure described above in order for the link training to be successful. > > So add support for reading the LTTPR DPCD caps to figure out the number > of such LTTPRs first. Then, for platforms (or Type-C dongles) that have > at least one such an LTTPR, set its operation mode to transparent mode > first and then to non-transparent, just like the mentioned section of > the specification mandates. > > Tested-by: Johan Hovold <johan+linaro@xxxxxxxxxx> > Signed-off-by: Abel Vesa <abel.vesa@xxxxxxxxxx> > --- > drivers/gpu/drm/msm/dp/dp_display.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> -- With best wishes Dmitry
