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

Fri Apr 3 18:07:14 UTC 2020

On 4/3/20 2:16 AM, Daniel Vetter wrote:
> On Fri, Apr 3, 2020 at 8:54 AM Daniel Dadap <ddadap at nvidia.com> wrote:
>>
>> On 4/2/20 6:39 AM, Lukas Wunner wrote:
>>
>>
>> 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.
> Two things: I thought (but not sure) that for the output switching
> muxes we'd run vgaswitcheroo in a different mode, where it doesn't
> check whether whether the driver can be killed. Because it wont. On a
> quick search only thing I've found is the ddc-only switching done by
> vga_switcheroo_lock/unlock_ddc. Maybe misremembering, but I thought
> there was more. But been a while I last looked at this all in detail.
>
> Wrt per-output switching WIP branch. That would be something you'd
> need to type ofc, I was just laying out what I think would make sense
> as a possible path to integrate this into upstream.
> -Daniel

Okay. I misunderstood. When you said that vga-switcheroo could switch 
individual outputs and do so without powering down the 
switched-away-from GPU, I took that to mean that this feature had 
already been implemented somewhere, despite appearances to the contrary 
upstream. I agree that adding per-output switching support to 
vga-switcheroo would be a sensible path.

Does this sound like a sensible high-level design?

* vga-switcheroo-capable GPU drivers can register muxable outputs.
* Each GPU driver must register each unique muxable output with the same 
identifier. The outputs will be registered together with individual 
devices they can be muxed to, in order to support e.g. muxing between 
different GPU devices driven by the same vendor. (I'm not aware of any 
designs that actually support this, but it seems reasonable to design 
for.) The output identifier may be platform-specific (e.g. ACPI handle, 
simple index). For example, two GPU drivers may each register an 
internal panel with identifier "0" and an external port with identifier "1".
* For each output that was registered with more than one device, 
vga-switcheroo will expose a sysfs file listing the devices that output 
can be muxed to, and a sysfs file allowing the selection of a particular 
device. Perhaps these could be the same file (e.g., on reading it prints 
a list with an indicator to mark the currently selected device, write a 
device ID to switch to that device).

I think this would allow flexibility across N muxable outputs that can 
each be switched to one of M GPU devices, without requiring that each of 
the outputs be switchable to all M possible GPUs.

In addition to the above mechanism for advertising the switchability of 
individual outputs and providing an interface, the following would also 
be needed to support PSR:

* A new callback for GPU drivers so vga-switcheroo can tell them an 
output is being switched to a GPU.
* vga-switcheroo can call this callback for all devices once before 
switching the mux, and then again after the mux switch is complete. A 
parameter of the callback would indicate whether the mux switch is 
pending, completed, or failed, allowing GPU drivers to do any necessary 
pre-switch setup, post-switch cleanup, or failed-switch recovery in 
response to events from vga-switcheroo.
* Maybe a similar callback for mux handlers so vga-switcheroo can signal 
pre- and post-switch events to the handler driver. The PSR designs I'm 
aware of are GPU-driven, but I can imagine a design that is driven by 
some component other than a GPU, in which case it would make sense for 
the mux handler to drive PSR. I suppose this could be left out for now 
and then added later if such a design emerges.
* It would probably be good to let userspace drive the pre/switch/post 
operations independently, e.g. so that userspace can prepare the first 
frame for presentation on the switched-to GPU before PSR is disabled.

Now, I'm not sure if it makes more sense to also implement the AUX 
channel proxying that Lukas had prototyped a while back, or to update 
i915 and any other vga-switcheroo-compatible drivers that assume that 
eDP is always connected to allow for the possibility of eDP being 
disconnected due to being muxed away. I personally think disconnecting 
the eDP output makes more sense, since it more accurately reflects 
reality than proxying the AUX channel through to the GPU that's muxed 
in, but if there's another reason the AUX proxying makes more sense, we 
could plan on making that work (on a per-output basis, since I think 
e.g. regular DP for external connectors wouldn't need this) as well.

>>> 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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>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch