On Thu, Apr 02, 2020 at 01:39:25PM +0200, Lukas Wunner wrote:
First of all, there's DPCD byte 3 bit 6 (NO_AUX_HANDSHAKE_LINK_TRAINING) which is documented as follows:
Does not require AUX CH handshake when the link configuration is already known. [...] The known-good drive current and pre-emphasis level (or those used in the last "full" link training with AUX CH handshake) must be used when the link training is performed without AUX CH handshake.That bit is set on the MacBook Pros in question.
So I think what we should be doing here is that the DRM driver which happens to be muxed to the panel on boot performs link training and informs vga_switcheroo of the drive current, pre-emph level, etc. The other DRM driver is notified when that information is available and uses it to set up its eDP output, skipping an actual AUX CH handshake.
Actually I now remember that I already prepared for this by adding a vga_switcheroo handler capability:
* @VGA_SWITCHEROO_NEEDS_EDP_CONFIG: whether the handler is unable to switch * the AUX channel separately. This signals to clients that the active * GPU needs to train the link and communicate the link parameters to the * inactive GPU (mediated by vga_switcheroo). The inactive GPU may then * skip the AUX handshake and set up its output with these pre-calibrated * values (DisplayPort specification v1.1a, section 2.5.3.3)
And the referenced DP spec section says:
For a closed, embedded connection, the DisplayPort transmitter and receiver may be set to pre-calibrated parameters without going through the full link training sequence. In this mode, the DisplayPort transmitter may start a normal operation following the transmission of the Clock Recovery Pattern with pre-calibrated drive current and pre-emphasis level, as shown with a dotted arrow in Figure 2-36.
Thanks,
Lukas