From: Oleksandr Andrushchenko <oleksandr_andrushche...@epam.com>

Initial handling for Xen bus states: implement
Xen bus state machine for the frontend driver according to
the state diagram and recovery flow from display para-virtualized
protocol: xen/interface/io/displif.h.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushche...@epam.com>
---
 drivers/gpu/drm/xen/xen_drm_front.c | 124 +++++++++++++++++++++++++++++++++++-
 drivers/gpu/drm/xen/xen_drm_front.h |  18 ++++++
 2 files changed, 141 insertions(+), 1 deletion(-)
 create mode 100644 drivers/gpu/drm/xen/xen_drm_front.h

diff --git a/drivers/gpu/drm/xen/xen_drm_front.c 
b/drivers/gpu/drm/xen/xen_drm_front.c
index 03bc50b128fd..520b5b34520b 100644
--- a/drivers/gpu/drm/xen/xen_drm_front.c
+++ b/drivers/gpu/drm/xen/xen_drm_front.c
@@ -16,19 +16,141 @@
 
 #include <xen/interface/io/displif.h>
 
+#include "xen_drm_front.h"
+
+static void xen_drv_remove_internal(struct xen_drm_front_info *front_info)
+{
+}
+
+static int displback_initwait(struct xen_drm_front_info *front_info)
+{
+       return 0;
+}
+
+static int displback_connect(struct xen_drm_front_info *front_info)
+{
+       return 0;
+}
+
+static void displback_disconnect(struct xen_drm_front_info *front_info)
+{
+       xenbus_switch_state(front_info->xb_dev, XenbusStateInitialising);
+}
+
 static void displback_changed(struct xenbus_device *xb_dev,
                enum xenbus_state backend_state)
 {
+       struct xen_drm_front_info *front_info = dev_get_drvdata(&xb_dev->dev);
+       int ret;
+
+       DRM_DEBUG("Backend state is %s, front is %s\n",
+                       xenbus_strstate(backend_state),
+                       xenbus_strstate(xb_dev->state));
+
+       switch (backend_state) {
+       case XenbusStateReconfiguring:
+               /* fall through */
+       case XenbusStateReconfigured:
+               /* fall through */
+       case XenbusStateInitialised:
+               break;
+
+       case XenbusStateInitialising:
+               /* recovering after backend unexpected closure */
+               displback_disconnect(front_info);
+               break;
+
+       case XenbusStateInitWait:
+               /* recovering after backend unexpected closure */
+               displback_disconnect(front_info);
+               if (xb_dev->state != XenbusStateInitialising)
+                       break;
+
+               ret = displback_initwait(front_info);
+               if (ret < 0)
+                       xenbus_dev_fatal(xb_dev, ret,
+                                       "initializing frontend");
+               else
+                       xenbus_switch_state(xb_dev, XenbusStateInitialised);
+               break;
+
+       case XenbusStateConnected:
+               if (xb_dev->state != XenbusStateInitialised)
+                       break;
+
+               ret = displback_connect(front_info);
+               if (ret < 0)
+                       xenbus_dev_fatal(xb_dev, ret,
+                                       "initializing DRM driver");
+               else
+                       xenbus_switch_state(xb_dev, XenbusStateConnected);
+               break;
+
+       case XenbusStateClosing:
+               /*
+                * in this state backend starts freeing resources,
+                * so let it go into closed state, so we can also
+                * remove ours
+                */
+               break;
+
+       case XenbusStateUnknown:
+               /* fall through */
+       case XenbusStateClosed:
+               if (xb_dev->state == XenbusStateClosed)
+                       break;
+
+               displback_disconnect(front_info);
+               break;
+       }
 }
 
 static int xen_drv_probe(struct xenbus_device *xb_dev,
                const struct xenbus_device_id *id)
 {
-       return 0;
+       struct xen_drm_front_info *front_info;
+
+       front_info = devm_kzalloc(&xb_dev->dev,
+                       sizeof(*front_info), GFP_KERNEL);
+       if (!front_info)
+               return -ENOMEM;
+
+       front_info->xb_dev = xb_dev;
+       dev_set_drvdata(&xb_dev->dev, front_info);
+       return xenbus_switch_state(xb_dev, XenbusStateInitialising);
 }
 
 static int xen_drv_remove(struct xenbus_device *dev)
 {
+       struct xen_drm_front_info *front_info = dev_get_drvdata(&dev->dev);
+       int to = 100;
+
+       xenbus_switch_state(dev, XenbusStateClosing);
+
+       /*
+        * On driver removal it is disconnected from XenBus,
+        * so no backend state change events come via .otherend_changed
+        * callback. This prevents us from exiting gracefully, e.g.
+        * signaling the backend to free event channels, waiting for its
+        * state to change to XenbusStateClosed and cleaning at our end.
+        * Normally when front driver removed backend will finally go into
+        * XenbusStateInitWait state.
+        *
+        * Workaround: read backend's state manually and wait with time-out.
+        */
+       while ((xenbus_read_unsigned(front_info->xb_dev->otherend,
+                       "state", XenbusStateUnknown) != XenbusStateInitWait) &&
+                       to--)
+               msleep(10);
+
+       if (!to)
+               DRM_ERROR("Backend state is %s while removing driver\n",
+                       xenbus_strstate(xenbus_read_unsigned(
+                                       front_info->xb_dev->otherend,
+                                       "state", XenbusStateUnknown)));
+
+       xen_drv_remove_internal(front_info);
+       xenbus_frontend_closed(dev);
        return 0;
 }
 
diff --git a/drivers/gpu/drm/xen/xen_drm_front.h 
b/drivers/gpu/drm/xen/xen_drm_front.h
new file mode 100644
index 000000000000..8b8d4d1e0816
--- /dev/null
+++ b/drivers/gpu/drm/xen/xen_drm_front.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+
+/*
+ *  Xen para-virtual DRM device
+ *
+ * Copyright (C) 2016-2018 EPAM Systems Inc.
+ *
+ * Author: Oleksandr Andrushchenko <oleksandr_andrushche...@epam.com>
+ */
+
+#ifndef __XEN_DRM_FRONT_H_
+#define __XEN_DRM_FRONT_H_
+
+struct xen_drm_front_info {
+       struct xenbus_device *xb_dev;
+};
+
+#endif /* __XEN_DRM_FRONT_H_ */
-- 
2.7.4

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