Re: [PATCH v4 3/3] drm/vkms: Add polling for HPD

[Marius Vlad <marius.vlad@collabora.com>]

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On Tue, Nov 04, 2025 at 11:21:28AM +0100, Louis Chauvet wrote:
> 
> 
> Le 03/11/2025 à 18:45, Marius Vlad a écrit :
> > vkms is missing any kind of HPD (polling/irq), so add polling to handle
> > hotplug events.
> 
> Hi,
Hi Louis, thanks for the quick reply!
> 
> I am a bit surprised by this addition, we currently have hotplug/unplug in
> VKMS using configFS, and it seems to work. The current "irq" system is
> writing in configFS the status and call drm_kms_helper_hotplug_event after
> the status is updated [1].
The situation is that the VKMS connectors/card are also being exposed
through sysfs similar to real devices. That's where we trigger/simulate
a connector hot-plug without physically unplugging cables. Wasn't aware
that VKMS had something similar to that with ConfigFS, so that needs
adjusting.
> 
> I think the polling will not work, I read in the drm_kms_helper_poll_init
> documentation that the connector is not polled if some connector flags are
> not set [2]/[3].
In this case the loop will be no-op, but prior to the loop
`changed` is set to `changed = dev->mode_config.delayed_event` [1] so
the code at [2] will do run.
> 
> If the polling works, I don't think it will be enough for the current VKMS
> implementation. Currently setting status using configFS will do everything
> synchronously:
> - Set the connector status (next call to connector->detect will have the new
> value)
> - Call drm_kms_helper_hotplug_event
> 
> With your implementation, I think you expect something to update
> connector->status_changed in between, using polling if I understood
> correctly, which will probably happen after the call to
> drm_kms_helper_hotplug_event.
Right, that's another path for VKMS.

[1] https://elixir.bootlin.com/linux/v6.17.7/source/drivers/gpu/drm/drm_probe_helper.c#L769
[2] https://elixir.bootlin.com/linux/v6.17.7/source/drivers/gpu/drm/drm_probe_helper.c#L846

> 
> [1]:https://gitlab.freedesktop.org/drm/misc/kernel/-/blob/drm-misc-next/drivers/gpu/drm/vkms/vkms_connector.c?ref_type=heads#L91-96
> [2]:https://elixir.bootlin.com/linux/v6.17.7/source/drivers/gpu/drm/drm_probe_helper.c#L917-L918
> [3]:https://elixir.bootlin.com/linux/v6.17.7/source/drivers/gpu/drm/drm_probe_helper.c#L793
> 
> > Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
> > ---
> >   drivers/gpu/drm/vkms/vkms_drv.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.c b/drivers/gpu/drm/vkms/vkms_drv.c
> > index e8472d9b6e3b..ec815c42ef04 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.c
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.c
> > @@ -198,6 +198,9 @@ static int vkms_create(struct vkms_config *config)
> >   	if (ret)
> >   		goto out_devres;
> > +	/* init kms poll for handling hpd */
> > +	drm_kms_helper_poll_init(&vkms_device->drm);
> > +
> >   	drm_client_setup(&vkms_device->drm, NULL);
> >   	return 0;
> > @@ -240,6 +243,7 @@ static void vkms_destroy(struct vkms_config *config)
> >   	fdev = config->dev->faux_dev;
> > +	drm_kms_helper_poll_fini(&config->dev->drm);
> >   	drm_dev_unregister(&config->dev->drm);
> >   	drm_atomic_helper_shutdown(&config->dev->drm);
> >   	devres_release_group(&fdev->dev, NULL);
> 
> -- 
> --
> Louis Chauvet, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com
> 

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