Re: [PATCH RESEND v7 0/2] Pass down hot-plug CONNECTOR ID to user-space

[Alex Deucher]

On Fri, Apr 17, 2026 at 3:49 AM Michel Dänzer
<michel.daenzer@xxxxxxxxxxx> wrote:
>
> On 4/16/26 15:16, Julian Orth wrote:
> > On Thu, Apr 16, 2026 at 9:46 AM Nicolas Frattaroli
> > <nicolas.frattaroli@xxxxxxxxxxxxx> wrote:
> >> On Wednesday, 15 April 2026 20:57:53 Central European Summer Time Julian Orth wrote:
> >>> On Wed, Apr 15, 2026 at 8:19 PM Nicolas Frattaroli
> >>> <nicolas.frattaroli@xxxxxxxxxxxxx> wrote:
> >>>>
> >>>> This series addresses a shortcoming whereby a hot plug event is sent
> >>>> without it being passed the actual connector that caused it. This takes
> >>>> into consideration both the polling path and the HPD (Hot Plug Detect)
> >>>> path. It also adds support for the vkms driver (using ConfigFS) for
> >>>> propagating the connector ID when changing the connector's status.
> >>>>
> >>>> The motivation is that user-space applications such as Weston would
> >>>> previously receive non-connector-specific hotplug events, and then have
> >>>> to figure out themselves which connector needs to have a modeset
> >>>> executed on. This notably did not work when the hotplug events came in
> >>>> too fast, resulting in Weston missing an on-off-on transition of a
> >>>> connector, seeing that its state was unchanged from "on" so can't be the
> >>>> one that was hotplugged, and skipping reinitialising it as it looks
> >>>> through the other connectors that could've caused it.
> >>>
> >>> Have you considered adding a u64 serial number as a DRM connector
> >>> property that is incremented every time the connector changes in some
> >>> way? Userspace could then check this serial number to see if the
> >>> connector has changed since the last time it queried the serial
> >>> number.
> >>
> >> The connector internally already has an epoch_counter member which
> >> could be used for this. However, for the particular thing this
> >> series fixes, I don't think exposing it through the uAPI is necessary
> >> or desirable. Sending hotplug events specific to the connector does
> >> not need any additional handling on the userspace side as long as it
> >> already listens to the per-connector hotplug events in order to
> >> avoid the pitfall described in the cover letter.
> >
> > I currently do not handle per-connector hotplug events. Instead,
> > whenever I get a UDEV change event for a device, I re-fetch the entire
> > kernel state for the device. That is
> >
> > - DRM_IOCTL_MODE_GETRESOURCES
> > - DRM_IOCTL_MODE_OBJ_GETPROPERTIES for each connector, crtc, plane
> > - DRM_IOCTL_MODE_GETCONNECTOR for each connector
> > - DRM_IOCTL_MODE_GETPROPERTY for each connector property
> > - DRM_IOCTL_MODE_GETPROPBLOB for the EDID
> >
> > Once I have the new state, I compare it against the desired compositor
> > state and perform a modeset if necessary.
>
> mutter is doing something similar as well.
>
>
> Note that some are arguing a modeset is always required after a hotplug event, even if the state hasn't changed.
>
> The most convincing argument I've seen is the scenario of a GPU reset, after which a modeset is required to light up the displays again. A hotplug event seems the only mechanism available for the kernel to request a modeset from the compositor. (The kernel may not be able to reliably do the modeset on its own, e.g. due to interactions with user-space atomic commits)
>
>
> If this "modeset required after hotplug event" rule is confirmed, it means that after a hotplug event without connector ID, the compositor must do a modeset for all connectors.
>

One thing we've run into is how to deal with hotplugs over suspend.
If you suspend your system, amdgpu sends a hotplug event on resume in
case anything got changed while the system was suspended.  However, we
can also suspend the driver at runtime and we use the same path so
when the driver runtime resumes, it also sends a hotplug event.  This
is probably fine in most cases, but there are some problematic corner
cases.  E.g., while the GPU is runtime suspended, if the monitors are
in DPMS off, and then a rendering command comes in or something else
that wakes the GPU, we send a hotplug event on resume and then
userspace lights up the monitors again which is not what the user
wants if the monitors are in DPMS off and the topology hasn't changed.

Alex

>
> P.S. https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/14420#note_2984697 even argued that two modesets are required after a hotplug event, one which turns things off and another one which turns them on again. I don't agree with that though, a single modeset should suffice.
>
>
> --
> Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
> https://redhat.com             \               Libre software enthusiast