nb/via/vx900: Ensure framebuffer size is within limits

- Use log2() when rounding down size_mb to the closest power of 2.
  Do a sanity check beforehand that size_mb is nonzero, else log2()
  will return -1 and there will be an undefined integer shift.
- The framebuffer size needs to be between 8 and 512 MiB, so check
  after all the calculations are done to make sure this is the case.

Change-Id: I3962e5cdc094c8da22d8dbadf16637e02fa98689
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
Found-by: Coverity CID 1391086
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34355
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/northbridge/via/vx900/memmap.c

@@ -21,6 +21,7 @@
 #include <device/pci_ops.h>
 #include <cbmem.h>
 #include <console/console.h>
+#include <lib.h>
 
 #include "vx900.h"
 
@@ -78,12 +79,16 @@ void vx900_set_chrome9hd_fb_size(u32 size_mb)
 		size_mb = max_size_mb;
 	}
 
-	/* Now round the framebuffer size to the closest power of 2 */
+	/* Now round down the framebuffer size to the closest power of 2 */
-	u8 fb_pow = 0;
+	if (size_mb == 0)
-	while (size_mb >> fb_pow)
+		die("Framebuffer size is 0\n");
-		fb_pow++;
+
-	fb_pow--;
+	int fb_pow = log2(size_mb);
-	size_mb = (1 << fb_pow);
+
+	size_mb = 1U << fb_pow;
+
+	if (size_mb < CHROME_9_HD_MIN_FB_SIZE || size_mb > CHROME_9_HD_MAX_FB_SIZE)
+		die("Framebuffer size %u is out of range\n", size_mb);
 
 	pci_update_config8(MCU, 0xa1, ~(7 << 4), (fb_pow - 2) << 4);
 }