Hi
(This was discussed on #dri-devel, but I'll reiterate here as well).
There are two problems at hand; one is the race condition during boot
when the login screen (or whatever display server appears first) is
launched with simpledrm, only some moments later having the real GPU
driver appear.
The other is general purpose GPU hotplugging, including the unplugging
the GPU decided by the compositor to be the primary one.
The situation of booting with simpledrm (problem 2) is a special case of
problem 1. From the kernel's perspective, unloading simpledrm is the
same as what you call general purpose GPU hotplugging. Even through
there is not a full GPU, but a trivial scanout buffer. In userspace, you
see the same sequence of events as in the general case.
The latter is something that should be handled in userspace, by
compositors, etc, I agree.
The former, however, is not properly solved by userspace learning how to
deal with primary GPU unplugging and switching to using a real GPU
driver, as it'd break the booting and login experience.
When it works, i.e. the race condition is not hit, is this:
* System boots
* Plymouth shows a "splash" screen
* The login screen display server is launched with the real GPU driver
* The login screen interface is smoothly animating using hardware
accelerating, presenting "advanced" graphical content depending on
hardware capabilities (e.g. high color bit depth, HDR, and so on)
If the race condition is hit, with a compositor supporting primary GPU
hotplugging, it'll work like this:
* System boots
* Plymouth shows a "splash" screen
* The login screen display server is launched with simpledrm
* Due to using simpldrm, the login screen interface is not animated and
just plops up, and no "advanced" graphical content is enabled due to
apparent missing hardware capabilities
* The real GPU driver appears, the login screen now starts to become
animated, and may suddenly change appearance due to capabilties
having changed
Thus, by just supporting hotplugging the primary GPU in userspace, we'll
still end up with a glitchy boot experience, and it forces userspace to
add things like sleep(10) to work around this.
In other words, fixing userspace is *not* a correct solution to the
problem, it's a work around (albeit a behaivor we want for other
reasons) for the race condition.
To really fix the flickering, you need to read the old DRM device's
atomic state and apply it to the new device. Then tell the desktop and
applications to re-init their rendering stack.
Depending on the DRM driver and its hardware, it might be possible to do
this without flickering. The key is to not loose the original scanout
buffer, while not probing the new device driver. But that needs work in
each individual DRM driver.
Arguably, the only place a more educated guess about whether to wait or
not, and if so how long, is the kernel.
As I said before, driver modules come and go and hardware devices come
and go.
To detect if there might be a native driver waiting to be loaded, you
can test for
- 'nomodeset' on the command line -> no native driver
- 'systemd-load-modules' not started -> maybe wait
- look for drivers under /lib/modules/<version>/kernel/drivers/gpu/drm/
-> maybe wait
- maybe udev can tell you more
- it might for detection help that recently simpledrm devices refer to
their parent PCI device
- maybe systemd tracks the probed devices
Best regards
Thomas
Jonas
The next best solution is to keep the final DRM device open until a new one
shows up. All DRM graphics drivers with hotplugging support are required to
accept commands after their hardware has been unplugged. They simply won't
display anything.
Best regards
Thomas
Thanks
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
