vbnv: check alignment of nvram in advance

Currently, erase operation only works if the region is sector-aligned.
These asserts ensure we can erase the region when it's all used up.

Erase operation can be updated to handle unaligned erases by read,
update, write-back cycle. However, these asserts will still remain useful
in case the adjacent region contains critical data and mis-updating it
can cause a critical failure.

Additionaly we should write a FAFT test but it's more reliable to catch
it here since FAFT can fail in many ways.

BUG=none
BRANCH=master
TEST=tested on samus using misaligned nvram region

Change-Id: I3add4671ed354d9763e21bf96616c8aeca0cb777
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: fc001a4d3446cf96b76367dde492c3453aa948c6
Original-Change-Id: Ib4df8f620bf7531b345364fa4c3e274aba09f677
Original-Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/297801
Reviewed-on: http://review.coreboot.org/11654
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/vendorcode/google/chromeos/vbnv_flash.c

@@ -19,6 +19,7 @@
  * TODO: Make this CAR-friendly in case we use it on x86 some day.
  */
 
+#include <assert.h>
 #include <console/console.h>
 #include <spi_flash.h>
 #include <string.h>
@@ -111,6 +112,23 @@ static int init_vbnv(void)
 	return 0;
 }
 
+static void vbnv_is_erasable(void)
+{
+	/*
+	 * We check whether the region is aligned or not in advance to ensure
+	 * we can erase the region when it's all used up.
+	 *
+	 * The region offset & size are determined by fmap.dts yet the check can
+	 * be confidently done only by the spi flash driver. We use the same
+	 * check as the one used by spi_flash_cmd_erase, which happens to be
+	 * common to all the spi flash parts we support.
+	 *
+	 * TODO: Check by calling can_erase implemented by each spi flash driver
+	 */
+	assert(!(region_device_offset(&nvram_region) % spi_flash->sector_size));
+	assert(!(region_device_sz(&nvram_region) % spi_flash->sector_size));
+}
+
 static int vbnv_flash_probe(void)
 {
 	if (!spi_flash) {
@@ -119,6 +137,11 @@ static int vbnv_flash_probe(void)
 			printk(BIOS_ERR, "failed to probe spi flash\n");
 			return 1;
 		}
+		/*
+		 * Called here instead of init_vbnv to reduce impact on boot
+		 * speed.
+		 */
+		vbnv_is_erasable();
 	}
 	return 0;
 }