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

[Julian Orth]

On Fri, Apr 17, 2026 at 12:58 PM Ville Syrjälä
<ville.syrjala@xxxxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 17, 2026 at 09:49:36AM +0200, Michel Dänzer 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.
>
> GPU reset should relight the display on its own really. That's what
> i915 does, albeit somewhat badly at the moment.
>
> > 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)
>
> There's nothing preventing the kernel from doing extra atomic
> commits whenever it wants. But if you want to punt the thing to
> userspace then the kernel must set the link-status property to
> bad, and then fire the hotplug uevent.
>
> > 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.
>
> Only for connectors where things changed, or the link-status shows bad.
>
> >
> >
> > 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.
>
> The actual argument is that you should not defer the hotplug
> handling when things get disconnected, mainly because of crap type-c
> firmware.
>
> I think the userspace behaviour there was that you get a disconnected,
> defer processing it, and then you get a reconnect, and then decide that
> nothing actually changed and a modeset is not needed after all. That is
> not correct IMO. Clearly a ->disconnect->reconnect should count as a
> change in the connector's state, and a full modeset is thus required.

The connector state, AIUI from userspace, is the set of all DRM
connector properties. If the true state changes in a way that requires
userspace to act but the DRM properties are unchanged, then I would
argue that that is a kernel bug.

The link-status property seems like it is perfect for this.

> The kernel can of course then decide that the full modeset is not
> actually required and skip the modeset part during the commit. But
> userspace cannot make that determination.
>
> I suppose what we could maybe do there to force userspace's hand is set
> the link-status to bad already when the thing gets disconnected, and keep
> it like that until a full disable+re-enable cycle has been done. Then
> userspace could not think that the ->disconnect->reconnect is a NOP
> (which I still think is incorrect behaviour).
>
> --
> Ville Syrjälä
> Intel