[PATCH 3/3] dlfb: introduce a rendering mutex

[Mikulas Patocka <mpatocka@redhat.com>]

Rendering calls may be done simultaneously from the workqueue,
dlfb_ops_write, dlfb_ops_ioctl, dlfb_ops_set_par and dlfb_dpy_deferred_io.
The code is robust enough so that it won't crash on concurrent rendering.

However, concurrent rendering may cause display corruption if the same
pixel is simultaneously being rendered. In order to avoid this corruption,
this patch adds a mutex around the rendering calls.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org

---
 drivers/video/fbdev/udlfb.c |   41 ++++++++++++++++++++++++++++++-----------
 include/video/udlfb.h       |    1 +
 2 files changed, 31 insertions(+), 11 deletions(-)

Index: linux-4.19.25/drivers/video/fbdev/udlfb.c
=================================--- linux-4.19.25.orig/drivers/video/fbdev/udlfb.c	2019-02-25 16:24:05.000000000 +0100
+++ linux-4.19.25/drivers/video/fbdev/udlfb.c	2019-02-25 16:27:35.000000000 +0100
@@ -596,7 +596,7 @@ static int dlfb_render_hline(struct dlfb
 
 static int dlfb_handle_damage(struct dlfb_data *dlfb, int x, int y, int width, int height)
 {
-	int i;
+	int i, ret;
 	char *cmd;
 	cycles_t start_cycles, end_cycles;
 	int bytes_sent = 0;
@@ -606,21 +606,29 @@ static int dlfb_handle_damage(struct dlf
 
 	start_cycles = get_cycles();
 
+	mutex_lock(&dlfb->render_mutex);
+
 	aligned_x = DL_ALIGN_DOWN(x, sizeof(unsigned long));
 	width = DL_ALIGN_UP(width + (x-aligned_x), sizeof(unsigned long));
 	x = aligned_x;
 
 	if ((width <= 0) ||
 	    (x + width > dlfb->info->var.xres) ||
-	    (y + height > dlfb->info->var.yres))
-		return -EINVAL;
+	    (y + height > dlfb->info->var.yres)) {
+		ret = -EINVAL;
+		goto unlock_ret;
+	}
 
-	if (!atomic_read(&dlfb->usb_active))
-		return 0;
+	if (!atomic_read(&dlfb->usb_active)) {
+		ret = 0;
+		goto unlock_ret;
+	}
 
 	urb = dlfb_get_urb(dlfb);
-	if (!urb)
-		return 0;
+	if (!urb) {
+		ret = 0;
+		goto unlock_ret;
+	}
 	cmd = urb->transfer_buffer;
 
 	for (i = y; i < y + height ; i++) {
@@ -654,7 +662,11 @@ error:
 		    >> 10)), /* Kcycles */
 		   &dlfb->cpu_kcycles_used);
 
-	return 0;
+	ret = 0;
+
+unlock_ret:
+	mutex_unlock(&dlfb->render_mutex);
+	return ret;
 }
 
 static void dlfb_init_damage(struct dlfb_data *dlfb)
@@ -782,17 +794,19 @@ static void dlfb_dpy_deferred_io(struct
 	int bytes_identical = 0;
 	int bytes_rendered = 0;
 
+	mutex_lock(&dlfb->render_mutex);
+
 	if (!fb_defio)
-		return;
+		goto unlock_ret;
 
 	if (!atomic_read(&dlfb->usb_active))
-		return;
+		goto unlock_ret;
 
 	start_cycles = get_cycles();
 
 	urb = dlfb_get_urb(dlfb);
 	if (!urb)
-		return;
+		goto unlock_ret;
 
 	cmd = urb->transfer_buffer;
 
@@ -825,6 +839,8 @@ error:
 	atomic_add(((unsigned int) ((end_cycles - start_cycles)
 		    >> 10)), /* Kcycles */
 		   &dlfb->cpu_kcycles_used);
+unlock_ret:
+	mutex_unlock(&dlfb->render_mutex);
 }
 
 static int dlfb_get_edid(struct dlfb_data *dlfb, char *edid, int len)
@@ -986,6 +1002,8 @@ static void dlfb_ops_destroy(struct fb_i
 
 	cancel_work_sync(&dlfb->damage_work);
 
+	mutex_destroy(&dlfb->render_mutex);
+
 	if (info->cmap.len != 0)
 		fb_dealloc_cmap(&info->cmap);
 	if (info->monspecs.modedb)
@@ -1682,6 +1700,7 @@ static int dlfb_usb_probe(struct usb_int
 	dlfb->ops = dlfb_ops;
 	info->fbops = &dlfb->ops;
 
+	mutex_init(&dlfb->render_mutex);
 	dlfb_init_damage(dlfb);
 	spin_lock_init(&dlfb->damage_lock);
 	INIT_WORK(&dlfb->damage_work, dlfb_damage_work);
Index: linux-4.19.25/include/video/udlfb.h
=================================--- linux-4.19.25.orig/include/video/udlfb.h	2019-02-25 16:24:05.000000000 +0100
+++ linux-4.19.25/include/video/udlfb.h	2019-02-25 16:24:05.000000000 +0100
@@ -48,6 +48,7 @@ struct dlfb_data {
 	int base8;
 	u32 pseudo_palette[256];
 	int blank_mode; /*one of FB_BLANK_ */
+	struct mutex render_mutex;
 	int damage_x;
 	int damage_y;
 	int damage_x2;