drm: Add DRM client cap for aspect-ratio

To enable aspect-ratio support in DRM, blindly exposing the aspect
ratio information along with mode, can break things in existing
non-atomic user-spaces which have no intention or support to use this
aspect ratio information.

To avoid this, a new drm client cap is required to enable a non-atomic
user-space to advertise if it supports modes with aspect-ratio. Based
on this cap value, the kernel will take a call on exposing the aspect
ratio info in modes or not.

This patch adds the client cap for aspect-ratio.

Since no atomic-userspaces blow up on receiving aspect-ratio
information, the client cap for aspect-ratio is always enabled
for atomic clients.

Cc: Ville Syrjala <[e-mail]>
Cc: Shashank Sharma <[e-mail]>
Signed-off-by: Ankit Nautiyal <[e-mail]>

V3: rebase
V4: As suggested by Marteen Lankhorst modified the commit message
explaining the need to use the DRM cap for aspect-ratio. Also,
tweaked the comment lines in the code for better understanding and
clarity, as recommended by Shashank Sharma.
V5: rebase
V6: rebase
V7: rebase
V8: rebase
V9: rebase
V10: rebase
V11: rebase
V12: As suggested by Daniel Vetter and Ville Syrjala,
always enable aspect-ratio client cap for atomic userspaces,
if no atomic userspace breaks on aspect-ratio bits.
V13: rebase
V14: rebase

Reviewed-by: Shashank Sharma <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Signed-off-by: Maarten Lankhorst <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_ioctl.c

···
 			return -EINVAL;
 		file_priv->atomic = req->value;
 		file_priv->universal_planes = req->value;
+		/*
+		 * No atomic user-space blows up on aspect ratio mode bits.
+		 */
+		file_priv->aspect_ratio_allowed = req->value;
+		break;
+	case DRM_CLIENT_CAP_ASPECT_RATIO:
+		if (req->value > 1)
+			return -EINVAL;
+		file_priv->aspect_ratio_allowed = req->value;
 		break;
 	default:
 		return -EINVAL;

include/drm/drm_file.h

···
 	unsigned atomic:1;
 
 	/**
+	 * @aspect_ratio_allowed:
+	 *
+	 * True, if client can handle picture aspect ratios, and has requested
+	 * to pass this information along with the mode.
+	 */
+	unsigned aspect_ratio_allowed:1;
+
+	/**
 	 * @is_master:
 	 *
 	 * This client is the creator of @master. Protected by struct

include/uapi/drm/drm.h

···
  */
 #define DRM_CLIENT_CAP_ATOMIC	3
 
+/**
+ * DRM_CLIENT_CAP_ASPECT_RATIO
+ *
+ * If set to 1, the DRM core will provide aspect ratio information in modes.
+ */
+#define DRM_CLIENT_CAP_ASPECT_RATIO    4
+
 /** DRM_IOCTL_SET_CLIENT_CAP ioctl argument type */
 struct drm_set_client_cap {
 	__u64 capability;