qxl: Don't notify userspace when monitors config is unchanged

When the QXL driver receives a QXL_INTERRUPT_CLIENT_MONITORS_CONFIG interrupt,
we currently always notify userspace that there was some hotplug event.

However, gnome-shell/mutter is reacting to this event by attempting a
resolution change, which it does by issueing drmModeRmFB, drmModeAddFB,
and then drmModeSetCrtc. This has the side-effect of causing
qxl_crtc_mode_set() to tell the QXL virtual hardware that a primary
surface was destroyed and created. After going through QEMU and then the
remote SPICE client, a new identical monitors config message will be
sent, resulting in a QXL_INTERRUPT_CLIENT_MONITORS_CONFIG interrupt to
be emitted, and the same scenario occurring again.

As destroying/creating the primary surface causes a visible screen
flicker, this makes the guest hard to use (
https://bugzilla.redhat.com/show_bug.cgi?id=1266484 ).

This commit checks if the screen configuration we received is the same
one as the current one, and does not notify userspace about it if that's
the case.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>
Message-id: 20161108091209.25568-7-cfergeau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/qxl/qxl_display.c

@@ -57,11 +57,18 @@ static void qxl_alloc_client_monitors_config(struct qxl_device *qdev, unsigned c
 	qdev->client_monitors_config->count = count;
 }
 
+enum {
+	MONITORS_CONFIG_MODIFIED,
+	MONITORS_CONFIG_UNCHANGED,
+	MONITORS_CONFIG_BAD_CRC,
+};
+
 static int qxl_display_copy_rom_client_monitors_config(struct qxl_device *qdev)
 {
 	int i;
 	int num_monitors;
 	uint32_t crc;
+	int status = MONITORS_CONFIG_UNCHANGED;
 
 	num_monitors = qdev->rom->client_monitors_config.count;
 	crc = crc32(0, (const uint8_t *)&qdev->rom->client_monitors_config,
@@ -70,7 +77,7 @@ static int qxl_display_copy_rom_client_monitors_config(struct qxl_device *qdev)
 		qxl_io_log(qdev, "crc mismatch: have %X (%zd) != %X\n", crc,
 			   sizeof(qdev->rom->client_monitors_config),
 			   qdev->rom->client_monitors_config_crc);
-		return 1;
+		return MONITORS_CONFIG_BAD_CRC;
 	}
 	if (num_monitors > qdev->monitors_config->max_allowed) {
 		DRM_DEBUG_KMS("client monitors list will be truncated: %d < %d\n",
@@ -79,6 +86,10 @@ static int qxl_display_copy_rom_client_monitors_config(struct qxl_device *qdev)
 	} else {
 		num_monitors = qdev->rom->client_monitors_config.count;
 	}
+	if (qdev->client_monitors_config
+	      && (num_monitors != qdev->client_monitors_config->count)) {
+		status = MONITORS_CONFIG_MODIFIED;
+	}
 	qxl_alloc_client_monitors_config(qdev, num_monitors);
 	/* we copy max from the client but it isn't used */
 	qdev->client_monitors_config->max_allowed =
@@ -88,17 +99,39 @@ static int qxl_display_copy_rom_client_monitors_config(struct qxl_device *qdev)
 			&qdev->rom->client_monitors_config.heads[i];
 		struct qxl_head *client_head =
 			&qdev->client_monitors_config->heads[i];
-		client_head->x = c_rect->left;
-		client_head->y = c_rect->top;
-		client_head->width = c_rect->right - c_rect->left;
-		client_head->height = c_rect->bottom - c_rect->top;
-		client_head->surface_id = 0;
-		client_head->id = i;
-		client_head->flags = 0;
+		if (client_head->x != c_rect->left) {
+			client_head->x = c_rect->left;
+			status = MONITORS_CONFIG_MODIFIED;
+		}
+		if (client_head->y != c_rect->top) {
+			client_head->y = c_rect->top;
+			status = MONITORS_CONFIG_MODIFIED;
+		}
+		if (client_head->width != c_rect->right - c_rect->left) {
+			client_head->width = c_rect->right - c_rect->left;
+			status = MONITORS_CONFIG_MODIFIED;
+		}
+		if (client_head->height != c_rect->bottom - c_rect->top) {
+			client_head->height = c_rect->bottom - c_rect->top;
+			status = MONITORS_CONFIG_MODIFIED;
+		}
+		if (client_head->surface_id != 0) {
+			client_head->surface_id = 0;
+			status = MONITORS_CONFIG_MODIFIED;
+		}
+		if (client_head->id != i) {
+			client_head->id = i;
+			status = MONITORS_CONFIG_MODIFIED;
+		}
+		if (client_head->flags != 0) {
+			client_head->flags = 0;
+			status = MONITORS_CONFIG_MODIFIED;
+		}
 		DRM_DEBUG_KMS("read %dx%d+%d+%d\n", client_head->width, client_head->height,
 			  client_head->x, client_head->y);
 	}
-	return 0;
+
+	return status;
 }
 
 static void qxl_update_offset_props(struct qxl_device *qdev)
@@ -124,9 +157,18 @@ void qxl_display_read_client_monitors_config(struct qxl_device *qdev)
 {
 
 	struct drm_device *dev = qdev->ddev;
-	while (qxl_display_copy_rom_client_monitors_config(qdev)) {
+	int status;
+
+	status = qxl_display_copy_rom_client_monitors_config(qdev);
+	while (status == MONITORS_CONFIG_BAD_CRC) {
 		qxl_io_log(qdev, "failed crc check for client_monitors_config,"
 				 " retrying\n");
+		status = qxl_display_copy_rom_client_monitors_config(qdev);
+	}
+	if (status == MONITORS_CONFIG_UNCHANGED) {
+		qxl_io_log(qdev, "config unchanged\n");
+		DRM_DEBUG("ignoring unchanged client monitors config");
+		return;
 	}
 
 	drm_modeset_lock_all(dev);