xen, fbfront: add support for specifying size via xenstore

Today xen-fbfront supports specifying the display size via module
parameters only. Add support for specifying the size via Xenstore in
order to enable doing this easily via the domain's Xen configuration.

Add an error message in case the configured display size conflicts
with video memory size.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

drivers/video/fbdev/xen-fbfront.c

@@ -18,6 +18,8 @@
  * frame buffer.
  */
 
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
 #include <linux/console.h>
 #include <linux/kernel.h>
 #include <linux/errno.h>
@@ -380,10 +382,18 @@ static int xenfb_probe(struct xenbus_device *dev,
 			video[KPARAM_MEM] = val;
 	}
 
+	video[KPARAM_WIDTH] = xenbus_read_unsigned(dev->otherend, "width",
+						   video[KPARAM_WIDTH]);
+	video[KPARAM_HEIGHT] = xenbus_read_unsigned(dev->otherend, "height",
+						    video[KPARAM_HEIGHT]);
+
 	/* If requested res does not fit in available memory, use default */
 	fb_size = video[KPARAM_MEM] * 1024 * 1024;
 	if (video[KPARAM_WIDTH] * video[KPARAM_HEIGHT] * XENFB_DEPTH / 8
 	    > fb_size) {
+		pr_warn("display parameters %d,%d,%d invalid, use defaults\n",
+			video[KPARAM_MEM], video[KPARAM_WIDTH],
+			video[KPARAM_HEIGHT]);
 		video[KPARAM_WIDTH] = XENFB_WIDTH;
 		video[KPARAM_HEIGHT] = XENFB_HEIGHT;
 		fb_size = XENFB_DEFAULT_FB_LEN;