Hi
Am 16.07.24 um 11:01 schrieb Maxime Ripard:
Hi,
On Mon, Jul 15, 2024 at 11:38:58AM GMT, Thomas Zimmermann wrote:
For CRTCs with multiple outputs (i.e., encoders plus connectors),
only report at most a single connected output to userspace. Make
this configurable via CONFIG_DRM_REPORT_SINGLE_CONNECTED_CRTC_OUTPUT.
Having multiple connected outputs on the same CRTC complicates
display-mode and format selection, so most userspace does not
support this. This is mostly not a problem in practice, as modern
display hardware provides a separate CRTC for each output.
Do they?
That was my understanding from discussions with Gnome devs. Or at least,
it's what they officially support.
At least the RaspberryPi has multiple connectors on a single CRTC, and
for multiple CRTCs.
I think userspace wants at least one CRTC per output, but doesn't care
about details.
My understanding is that it's definitely expected, and any decent
user-space should expect it.
Did you have any bug with this?
https://gitlab.gnome.org/GNOME/mutter/-/issues/3157
There was also a lengthy discussion on IRC a few months ago IIRC.
And if it was the case, we wouldn't support cloning at all. I couldn't
really find how cloning works exactly, but my understanding was that
it's the difference between drm_encoder.possible_crtcs and
possible_clones: possible_crtcs lists the CRTCs it can be connected to,
and possible_clones the other encoders that can run with in parallel.
I'd prefer to solve this with the possible_clones field. But it didn't
work. Any ideas on that?
On old hardware or hardware with BMCs, a single CRTC often drives
multiple displays. Only reporting one of them as connected makes
the hardware compatible with common userspace.
Add the field prioritized_connectors to struct drm_connector. The
bitmask signals which other connectors have priority. Also provide
the helper drm_probe_helper_prioritize_connectors() that sets
default priorities for a given set of connectors. Calling the
helper should be enough to set up the functionality for most drivers.
With the prioritization bits in place, update connector-status
detection to test against prioritized conenctors. So when the probe
helpers detect a connector as connected, test against the prioritized
connectors. If any is also connected, set the connector status to
disconnected.
Please note that this functionality is a workaround for limitations
in userspace. If your compositor supports multiple outputs per CRTC,
CONFIG_DRM_REPORT_SINGLE_CONNECTED_CRTC_OUTPUT should be disabled.
Signed-off-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
The whole "is it actually needed" discussion aside, I'm not sure it's a
good idea to use a config option for that. Chances are distros will
either enable it or disable it depending on what they/their customers
workload will typically look like, and it won't make the kernel or
compositor's job any easier.
Could we use a client capability for that maybe?
I like this idea.
Best regards
Thomas
Maxime
--
--
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)
