[PATCH 1/2] drm/vkms: Add drm_dev_enter/exit guards for hot-removable paths

[Sebastian Wick]

In preparation for switching to drm_dev_unplug() for ConfigFS device
removal, add drm_dev_enter()/drm_dev_exit() guards to vkms callbacks
that can be reached while the device is being torn down.
drm_dev_unplug() relies on these SRCU guards to wait for in-flight
operations before proceeding with teardown. Without them,
synchronize_srcu() completes immediately and the teardown path can race
with in-flight operations.

Guard the following callbacks:

 - vkms_connector_detect() and vkms_conn_get_modes(): called from the
   MODE_GETCONNECTOR ioctl via fill_modes, which holds mode_config.mutex.
   Without the guard, an in-flight getconnector can hold locks that
   drm_atomic_helper_shutdown() needs.

 - vkms_wb_connector_get_modes(): same fill_modes path for the writeback
   connector.

 - vkms_set_crc_source(): called from the CRC debugfs release path. The
   debugfs proxy explicitly does not protect the release callback
   against removal races, so this needs its own guard.

Atomic commit callbacks are intentionally not guarded: they are
serialized against drm_atomic_helper_shutdown() via modeset ww_mutex
locks, and must remain functional after unplug so shutdown can disable
CRTCs.

Signed-off-by: Sebastian Wick <sebastian.wick@xxxxxxxxxx>
---
 drivers/gpu/drm/vkms/vkms_composer.c  |  8 +++++++-
 drivers/gpu/drm/vkms/vkms_connector.c | 14 +++++++++++++-
 drivers/gpu/drm/vkms/vkms_writeback.c | 13 +++++++++++--
 3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index 0fc915a954ba..e0576edd7168 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -5,6 +5,7 @@
 #include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_blend.h>
+#include <drm/drm_drv.h>
 #include <drm/drm_colorop.h>
 #include <drm/drm_fourcc.h>
 #include <drm/drm_fixed.h>
@@ -731,11 +732,16 @@ int vkms_set_crc_source(struct drm_crtc *crtc, const char *src_name)
 {
 	struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
 	bool enabled = false;
-	int ret = 0;
+	int ret, idx;
+
+	if (!drm_dev_enter(crtc->dev, &idx))
+		return -ENODEV;
 
 	ret = vkms_crc_parse_source(src_name, &enabled);
 
 	vkms_set_composer(out, enabled);
 
+	drm_dev_exit(idx);
+
 	return ret;
 }
diff --git a/drivers/gpu/drm/vkms/vkms_connector.c b/drivers/gpu/drm/vkms/vkms_connector.c
index b0a6b212d3f4..81cd845cf84f 100644
--- a/drivers/gpu/drm/vkms/vkms_connector.c
+++ b/drivers/gpu/drm/vkms/vkms_connector.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0+
 
 #include <drm/drm_atomic_helper.h>
+#include <drm/drm_drv.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_managed.h>
 #include <drm/drm_probe_helper.h>
@@ -16,6 +17,10 @@ static enum drm_connector_status vkms_connector_detect(struct drm_connector *con
 	struct vkms_connector *vkms_connector;
 	enum drm_connector_status status;
 	struct vkms_config_connector *connector_cfg;
+	int idx;
+
+	if (!drm_dev_enter(dev, &idx))
+		return connector_status_disconnected;
 
 	vkms_connector = drm_connector_to_vkms_connector(connector);
 
@@ -31,6 +36,8 @@ static enum drm_connector_status vkms_connector_detect(struct drm_connector *con
 			status = vkms_config_connector_get_status(connector_cfg);
 	}
 
+	drm_dev_exit(idx);
+
 	return status;
 }
 
@@ -44,12 +51,17 @@ static const struct drm_connector_funcs vkms_connector_funcs = {
 
 static int vkms_conn_get_modes(struct drm_connector *connector)
 {
-	int count;
+	int count, idx;
+
+	if (!drm_dev_enter(connector->dev, &idx))
+		return 0;
 
 	/* Use the default modes list from DRM */
 	count = drm_add_modes_noedid(connector, XRES_MAX, YRES_MAX);
 	drm_set_preferred_mode(connector, XRES_DEF, YRES_DEF);
 
+	drm_dev_exit(idx);
+
 	return count;
 }
 
diff --git a/drivers/gpu/drm/vkms/vkms_writeback.c b/drivers/gpu/drm/vkms/vkms_writeback.c
index ecf29a2c0c8e..e779ace7b383 100644
--- a/drivers/gpu/drm/vkms/vkms_writeback.c
+++ b/drivers/gpu/drm/vkms/vkms_writeback.c
@@ -3,6 +3,7 @@
 #include <linux/iosys-map.h>
 
 #include <drm/drm_atomic.h>
+#include <drm/drm_drv.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_fourcc.h>
 #include <drm/drm_writeback.h>
@@ -67,9 +68,17 @@ static int vkms_wb_atomic_check(struct drm_connector *connector,
 static int vkms_wb_connector_get_modes(struct drm_connector *connector)
 {
 	struct drm_device *dev = connector->dev;
+	int count, idx;
 
-	return drm_add_modes_noedid(connector, dev->mode_config.max_width,
-				    dev->mode_config.max_height);
+	if (!drm_dev_enter(dev, &idx))
+		return 0;
+
+	count = drm_add_modes_noedid(connector, dev->mode_config.max_width,
+				     dev->mode_config.max_height);
+
+	drm_dev_exit(idx);
+
+	return count;
 }
 
 static int vkms_wb_prepare_job(struct drm_writeback_connector *wb_connector,
-- 
2.56.0