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

[Louis Chauvet <louis.chauvet@bootlin.com>]

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,

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].

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].

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.

[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