lib/cbfs_spi: provide boot_device_rw() support

Provide the RW boot device operations for the common cbfs
SPI wrapper. The RW region_device is the same as the read-only
one. As noted in the boot_device_rw() introduction patch the
mmap() support should not be used in conjuction with writing
as that results in incoherent operations. That's fine as the
current mmap() support is only used in the cbfs layer which
does not support writing, i.e. no cbfs regions would be
written to with any previous or outstanding mmap() calls.

BUG=chrome-os-partner:56151

Change-Id: I7cc7309a68ad23b30208ac961b1999a79626b307
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16199
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/drivers/spi/cbfs_spi.c

@@ -34,10 +34,29 @@ static ssize_t spi_readat(const struct region_device *rd, void *b,
 	return size;
 }
 
+static ssize_t spi_writeat(const struct region_device *rd, const void *b,
+				size_t offset, size_t size)
+{
+	if (spi_flash_info->write(spi_flash_info, offset, size, b))
+		return -1;
+	return size;
+}
+
+static ssize_t spi_eraseat(const struct region_device *rd,
+				size_t offset, size_t size)
+{
+	if (spi_flash_info->erase(spi_flash_info, offset, size))
+		return -1;
+	return size;
+}
+
+/* Provide all operations on the same device. */
 static const struct region_device_ops spi_ops = {
 	.mmap = mmap_helper_rdev_mmap,
 	.munmap = mmap_helper_rdev_munmap,
 	.readat = spi_readat,
+	.writeat = spi_writeat,
+	.eraseat = spi_eraseat,
 };
 
 static struct mmap_helper_region_device mdev =
@@ -78,3 +97,9 @@ const struct region_device *boot_device_ro(void)
 
 	return &mdev.rdev;
 }
+
+/* The read-only and read-write implementations are symmetric. */
+const struct region_device *boot_device_rw(void)
+{
+	return boot_device_ro();
+}