video: hyperv: hyperv_fb: refresh the VM screen by force on VM panic

Currently the VSC has no chance to notify the VSP of the dirty rectangle on VM
panic because the notification work is done in a workqueue, and in panic() the
kernel typically ends up in an infinite loop, and a typical kernel config has
CONFIG_PREEMPT_VOLUNTARY=y and CONFIG_PREEMPT is not set, so a context switch
can't happen in panic() and the workqueue won't have a chance to run. As a
result, the VM Connection window can't refresh until it's closed and we
re-connect to the VM.

We can register a handler on panic_notifier_list: the handler can notify
the VSC and switch the framebuffer driver to a "synchronous mode", meaning
the VSC flushes any future framebuffer change to the VSP immediately.

v2: removed the MS-TFS line in the commit message
v3: remove some 'unlikely' markings
v4: avoid global variables as Tomi Valkeinen suggested

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/fbdev/hyperv_fb.c

@@ -224,6 +224,11 @@ struct hvfb_par {
 	u32 pseudo_palette[16];
 	u8 init_buf[MAX_VMBUS_PKT_SIZE];
 	u8 recv_buf[MAX_VMBUS_PKT_SIZE];
+
+	/* If true, the VSC notifies the VSP on every framebuffer change */
+	bool synchronous_fb;
+
+	struct notifier_block hvfb_panic_nb;
 };
 
 static uint screen_width = HVFB_WIDTH;
@@ -532,6 +537,19 @@ static void hvfb_update_work(struct work_struct *w)
 		schedule_delayed_work(&par->dwork, HVFB_UPDATE_DELAY);
 }
 
+static int hvfb_on_panic(struct notifier_block *nb,
+			 unsigned long e, void *p)
+{
+	struct hvfb_par *par;
+	struct fb_info *info;
+
+	par = container_of(nb, struct hvfb_par, hvfb_panic_nb);
+	par->synchronous_fb = true;
+	info = par->info;
+	synthvid_update(info);
+
+	return NOTIFY_DONE;
+}
 
 /* Framebuffer operation handlers */
 
@@ -582,14 +600,44 @@ static int hvfb_blank(int blank, struct fb_info *info)
 	return 1;	/* get fb_blank to set the colormap to all black */
 }
 
+static void hvfb_cfb_fillrect(struct fb_info *p,
+			      const struct fb_fillrect *rect)
+{
+	struct hvfb_par *par = p->par;
+
+	cfb_fillrect(p, rect);
+	if (par->synchronous_fb)
+		synthvid_update(p);
+}
+
+static void hvfb_cfb_copyarea(struct fb_info *p,
+			      const struct fb_copyarea *area)
+{
+	struct hvfb_par *par = p->par;
+
+	cfb_copyarea(p, area);
+	if (par->synchronous_fb)
+		synthvid_update(p);
+}
+
+static void hvfb_cfb_imageblit(struct fb_info *p,
+			       const struct fb_image *image)
+{
+	struct hvfb_par *par = p->par;
+
+	cfb_imageblit(p, image);
+	if (par->synchronous_fb)
+		synthvid_update(p);
+}
+
 static struct fb_ops hvfb_ops = {
 	.owner = THIS_MODULE,
 	.fb_check_var = hvfb_check_var,
 	.fb_set_par = hvfb_set_par,
 	.fb_setcolreg = hvfb_setcolreg,
-	.fb_fillrect = cfb_fillrect,
-	.fb_copyarea = cfb_copyarea,
-	.fb_imageblit = cfb_imageblit,
+	.fb_fillrect = hvfb_cfb_fillrect,
+	.fb_copyarea = hvfb_cfb_copyarea,
+	.fb_imageblit = hvfb_cfb_imageblit,
 	.fb_blank = hvfb_blank,
 };
 
@@ -801,6 +849,11 @@ static int hvfb_probe(struct hv_device *hdev,
 
 	par->fb_ready = true;
 
+	par->synchronous_fb = false;
+	par->hvfb_panic_nb.notifier_call = hvfb_on_panic;
+	atomic_notifier_chain_register(&panic_notifier_list,
+				       &par->hvfb_panic_nb);
+
 	return 0;
 
 error:
@@ -820,6 +873,9 @@ static int hvfb_remove(struct hv_device *hdev)
 	struct fb_info *info = hv_get_drvdata(hdev);
 	struct hvfb_par *par = info->par;
 
+	atomic_notifier_chain_unregister(&panic_notifier_list,
+					 &par->hvfb_panic_nb);
+
 	par->update = false;
 	par->fb_ready = false;