[PATCH] drm/connector: demote connector force-probes for non-master clients

[Daniel Vetter]

On Fri, Apr 02, 2021 at 01:22:12PM +0200, Simon Ser wrote:
> Force-probing a connector can be slow and cause flickering. As this
> affects the global KMS state, let's make it so only the DRM master
> can force-probe a connector.
> 
> Non-master DRM clients won't be able to force-probe a connector
> anymore. Instead, KMS will perform a regular read-only connector
> query.
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>
> ---
>  drivers/gpu/drm/drm_connector.c | 11 ++++++++---
>  include/uapi/drm/drm_mode.h     |  7 ++++---
>  2 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 7631f76e7f34..2f70a52a892b 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -20,6 +20,7 @@
>   * OF THIS SOFTWARE.
>   */
>  
> +#include <drm/drm_auth.h>
>  #include <drm/drm_connector.h>
>  #include <drm/drm_edid.h>
>  #include <drm/drm_encoder.h>
> @@ -2374,9 +2375,13 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
>  
>  	mutex_lock(&dev->mode_config.mutex);
>  	if (out_resp->count_modes == 0) {
> -		connector->funcs->fill_modes(connector,
> -					     dev->mode_config.max_width,
> -					     dev->mode_config.max_height);
> +		if (drm_is_current_master(file_priv))
> +			connector->funcs->fill_modes(connector,
> +						     dev->mode_config.max_width,
> +						     dev->mode_config.max_height);
> +		else
> +			drm_dbg_kms(dev, "User-space requested a forced probe on [CONNECTOR:%d:%s] but is not the DRM master, demoting to read-only probe",
> +				    connector->base.id, connector->name);
>  	}
>  
>  	out_resp->mm_width = connector->display_info.width_mm;
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index a5e76aa06ad5..3efa2e38d89b 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -413,9 +413,10 @@ enum drm_mode_subconnector {
>   *
>   * **Force-probing a connector**
>   *
> - * If the @count_modes field is set to zero, the kernel will perform a forced
> - * probe on the connector to refresh the connector status, modes and EDID.
> - * A forced-probe can be slow, might cause flickering and the ioctl will block.
> + * If the @count_modes field is set to zero and the DRM client is the DRM

*current* DRM master

The drm master/client relationship survives a DROPMASTER ioctl, but also
it's only really relevant for the old authmagic dance. But just to be
consistent here.

> + * master, the kernel will perform a forced probe on the connector to refresh
> + * the connector status, modes and EDID. A forced-probe can be slow, might
> + * cause flickering and the ioctl will block.

Do we have an igt for this? Timing test should do the job I think,
assuming we have at least one output which requires an edid read (so maybe
skip the test if a forced probe takes less than 10ms or so).

Patch lgtm, but igt would be really nice here.

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

>   *
>   * User-space needs to force-probe connectors to ensure their metadata is
>   * up-to-date at startup and after receiving a hot-plug event. User-space
> -- 
> 2.31.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch