How to handle disconnection of eDP panels due to dynamic display mux switches

[Lukas Wunner]

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