A number of hybrid GPU notebook computer designs with dual (integrated
plus discrete) GPUs are equipped with multiplexers (muxes) that allow
display panels to be driven by either the integrated GPU or the discrete
GPU. Typically, this is a selection that can be made at boot time as a
menu option in the system firmware's setup screen, and the mux selection
stays fixed for as long as the system is running and persists across
reboots until it is explicitly changed. However, some muxed hybrid GPU
systems have dynamically switchable muxes which can be switched while
the system is running.
NVIDIA is exploring the possibility of taking advantage of dynamically
switchable muxes to enhance the experience of using a hybrid GPU system.
For example, on a system configured for PRIME render offloading, it may
be possible to keep the discrete GPU powered down and use the integrated
GPU for rendering and displaying the desktop when no applications are
using the discrete GPU, and dynamically switch the panel to be driven
directly by the discrete GPU when render-offloading a fullscreen
application.
We have been conducting some experiments on systems with dynamic muxes,
and have found some limitations that would need to be addressed in order
to support use cases like the one suggested above:
* In at least the i915 DRM-KMS driver, and likely in other DRM-KMS
drivers as well, eDP panels are assumed to be always connected. This
assumption is broken when the panel is muxed away, which can cause
problems. A typical symptom is i915 repeatedly attempting to retrain the
link, severely impacting system performance and printing messages like
the following every five seconds or so:
[drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link
training
[drm] Reducing the compressed framebuffer size. This may lead to less
power savings than a non-reduced-size. Try to increase stolen memory
size if available in BIOS.
This symptom might occur if something causes the DRM-KMS driver to probe
the display while it's muxed away, for example a modeset or DPMS state
change.
* When switching the mux back to a GPU that was previously driving a
mode, it is necessary to at the very least retrain DP links to restore
the previously displayed image. In a proof of concept I have been
experimenting with, I am able to accomplish this from userspace by
triggering DPMS off and then back on again; however, it would be good to
have an in-kernel API to request that an output owned by a DRM-KMS
driver be refreshed to resume driving a mode on a disconnected and
reconnected display. This API would need to be accessible from outside
of the DRM-KMS driver handling the output. One reason it would be good
to do this within the kernel, rather than rely on e.g. DPMS operations
in the xf86-video-modesetting driver, is that it would be useful for
restoring the console if X crashes or is forcefully killed while the mux
is switched to a GPU other than the one which drives the console.
Basically, we'd like to be able to do the following:
1) Communicate to a DRM-KMS driver that an output is disconnected and
can't be used. Ideally, DRI clients such as X should still see the
output as being connected, so user applications don't need to keep track
of the change.
2) Request that a mode that was previously driven on a disconnected
output be driven again upon reconnection.
If APIs to do the above are already available, I wasn't able to find
information about them. These could be handled as separate APIs, e.g.,
one to set connected/disconnected state and another to restore an
output, or as a single API, e.g., signal a disconnect or reconnect,
leaving it up to the driver receiving the signal to set the appropriate
internal state and restore the reconnected output. Another possibility
would be an API to disable and enable individual outputs from outside of
the DRM-KMS driver that owns them. I'm curious to hear the thoughts of
the DRM subsystem maintainers and contributors on what the best approach
to this would be.
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