[PATCH 2/7] drm/probe-helper: Optionally report single connected output per CRTC

[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?

At least the RaspberryPi has multiple connectors on a single CRTC, and
for multiple CRTCs.

My understanding is that it's definitely expected, and any decent
user-space should expect it.

Did you have any bug with this?

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.

> 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 at suse.de>

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?

Maxime
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