On 4/2/20 6:39 AM, Lukas Wunner wrote:
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On Fri, Mar 27, 2020 at 04:25:19PM -0500, Daniel Dadap wrote:
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.
As you may be aware, there's drivers/gpu/vga/vga_switcheroo.c (of which
I'm listed as a reviewer in MAINTAINERS) to support such hardware.
It also supports muxed configurations, including those that support
switching at runtime (and not only at boot) such as the MacBook Pro,
which uses drivers/platform/x86/apple-gmux.c to interface between
vga_switcheroo and the hardware mux.
However, so far switching only actually works on LVDS-based MacBook Pros,
i.e. all pre-retina machines introduced between Late 2008 and Mid 2012,
because that hardware is capable of switching the DDC pins separately
from the display, so we lock and switch them when probing the EDID.
I have observed that on at least some systems, the EDID for the internal
panel can be read via the ACPI _DDC method regardless of whether it's
actively muxed in. I don't know whether that's true for all systems
where the DDC line can't be switched independently, but maybe
vga_switcheroo could also export an interface for GPU drivers to cache
EDIDs so that a muxed-away GPU can read an EDID that was previously read
by another GPU? I guess the utility of that would depend on how
prevalent the combination of no DDC muxing + no ACPI EDID reads turns
out to be.
The retina machines introduced from Mid 2012 onward use eDP and run
into the issues you're describing: The AUX channel cannot be switched
separately from the display, so link training fails unless the entire
display is switched. Nevertheless macOS can switch the panel seamlessly.
So how are they doing it?
Well, I don't own a retina MacBook Pro, hence never got very far with
supporting them, but I did some research and experiments in the 2015/2016
time frame which a colleague, Bruno Bierbaumer, tested on his machine:
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.
I'll check one of the eDP-based systems I've been experimenting on to
see if setting the VGA_SWITCHER_NEEDS_EDP_CONFIG capability in the
handler is sufficient to make i915 avoid poking the AUX channel when
it's mux-switched away. (This would be in addition to hacking the
can_switch() callback in the GPU drivers to allow switching while there
are still active KMS clients for the purposes of this experiment, unless
somebody can point me to a tree with the WIP per-output switching Daniel
Vetter mentioned.
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.
At least i915 probes various capabilities in the DPCD without any
consideration that the AUX channel may currently not be available.
Back in the day I experimented with a read-only proxy mechanism
to make that work, whereby the inactive DRM driver uses the active
DRM driver to access the DPCD:
https://patchwork.kernel.org/patch/7000591/
An alternative would be to have the active DRM driver cache the
required portions of the DPCD for use by the inactive DRM driver.
Note that vga_switcheroo is currently controlled via debugfs.
That is a historic artefact. The kernel has since gained a
mux subsystem in drivers/mux/ which could be used to represent
the display mux in a standardized way in regular sysfs.
Thanks,
Lukas
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