How to address individual vga-switcheroo muxes on systems with multiple muxes

[Daniel Dadap]

Hi all,


Some time ago, I asked some questions about how to handle issues with 
DRM drivers attempting to touch eDP while muxed away, due to implicit 
assumptions about eDP being permanently connected. I've proposed some 
changes which prevent eDP access on switched-away eDP outputs for at 
least i915, but now I'd like to follow up on one of the topics that was 
brought up during that previous conversation: namely, how to deal with 
systems with multiple mux devices, where it may be desirable to address 
the muxes independently of each other. Currently, the apple-gmux driver 
seems to support multiple muxes by just switching them in sync with each 
other, exposing only one logical mux device to the vga-switcheroo 
subsystem to control both physical muxes.


Since then, I've proposed a driver for a mux that is present on some 
upcoming notebook designs, some of which have distinct muxes for 
internal and external display connections [1]. I don't believe any of 
these systems are available publicly just yet, but it's only a matter of 
time before they are. During the previous discussion, I believe Daniel 
Vetter stated that the ability to address multiple muxes is something 
that could be addressed when designing a "proper" userspace API for 
vga-switcheroo (i.e., not debugfs), so I'd like to get the conversation 
started on how we could go about designing this API.


[1] https://patchwork.kernel.org/patch/11751453/