mb/prodrive/hermes: Make board settings less error-prone

First of all, make sure that `get_board_settings()` never returns NULL.
If there's a problem, return predefined values for board settings.

If the board settings definition differs between coreboot and the BMC,
the CRC will not match. Allow coreboot to use the BMC settings provided
by older BMC firmware revisions which have less settings, if the CRC of
the first N bytes matches the expected CRC.

TEST=Boot coreboot master with BMC FW R04.05, observe board settings
being honored even though coreboot's definition has an extra option.

Change-Id: I0f009b21ef0850a2af6edef1818c770171358314
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67381
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

1 files changed, 52 insertions, 16 deletions

diff --git a/src/mainboard/prodrive/hermes/eeprom.c b/src/mainboard/prodrive/hermes/eeprom.c
index 5defff6fc1..4daef43b1d 100644
--- a/src/mainboard/prodrive/hermes/eeprom.c
+++ b/src/mainboard/prodrive/hermes/eeprom.c
@@ -38,37 +38,73 @@ int check_signature(const size_t offset, const uint64_t signature)
  * Read board settings from the EEPROM and verify their checksum.
  * If checksum is valid, we assume the settings are sane as well.
  */
-static bool get_board_settings_from_eeprom(struct eeprom_board_settings *board_cfg)
+static size_t get_board_settings_from_eeprom(struct eeprom_board_settings *board_cfg)
 {
 	const size_t board_settings_offset = offsetof(struct eeprom_layout, board_settings);
 
 	if (eeprom_read_buffer(board_cfg, board_settings_offset, sizeof(*board_cfg))) {
 		printk(BIOS_ERR, "CFG EEPROM: Failed to read board settings\n");
-		return false;
+		return 0;
 	}
 
-	const uint32_t crc =
-		CRC(&board_cfg->raw_settings, sizeof(board_cfg->raw_settings), crc32_byte);
-
-	if (crc != board_cfg->signature) {
-		printk(BIOS_ERR, "CFG EEPROM: Board settings have invalid checksum\n");
-		return false;
+	/*
+	 * Ideally, the definition for board settings would always be the same across
+	 * coreboot and the BMC. However, this is not always the case. When there's a
+	 * mismatch, coreboot often has the newer definition with more settings. When
+	 * there's a mismatch, coreboot and the BMC calculate the CRC for a different
+	 * set of bytes, which results in two different CRC values.
+	 *
+	 * As existing board settings do not get repurposed, it is relatively easy to
+	 * make coreboot backwards compatible with older BMC firmware revisions which
+	 * do not provide the latest board settings. Starting with all board settings
+	 * coreboot knows about, if the CRC does not match, we drop the last byte and
+	 * try again until we find a match or exhaust all bytes.
+	 */
+	for (size_t len = sizeof(board_cfg->raw_settings); len > 0; len--) {
+		const uint32_t crc = CRC(&board_cfg->raw_settings, len, crc32_byte);
+		if (crc != board_cfg->signature)
+			continue;
+
+		printk(BIOS_DEBUG, "CFG EEPROM: Board settings CRC OK for %zu / %lu bytes\n",
+				len, sizeof(board_cfg->raw_settings));
+		return len;
 	}
-	return true;
+
+	printk(BIOS_ERR, "CFG EEPROM: Board settings have invalid checksum\n");
+	return 0;
 }
 
 struct eeprom_board_settings *get_board_settings(void)
 {
+	/*
+	 * Default settings to be used if the EEPROM settings are unavailable.
+	 * Unspecified settings default to 0. These default values do not get
+	 * passed to edk2 in any way, so there is no need to provide defaults
+	 * for edk2-only options like `secureboot`.
+	 */
+	const struct eeprom_board_settings default_cfg = {
+		.deep_sx_enabled      = 1,
+		.wake_on_usb          = 1,
+		.power_state_after_g3 = !CONFIG_MAINBOARD_POWER_FAILURE_STATE,
+		.blue_rear_vref       = 2,
+		.pink_rear_vref       = 2,
+	};
+
 	static struct eeprom_board_settings board_cfg = {0};
+	static bool cfg_cached = false;
 
-	/* Tri-state: -1: settings are invalid, 0: uninitialized, 1: settings are valid */
-	static int checked_valid = 0;
+	if (cfg_cached)
+		return &board_cfg;
 
-	if (checked_valid == 0) {
-		const bool success = get_board_settings_from_eeprom(&board_cfg);
-		checked_valid = success ? 1 : -1;
-	}
-	return checked_valid > 0 ? &board_cfg : NULL;
+	const size_t valid_bytes = get_board_settings_from_eeprom(&board_cfg);
+
+	/* If we could not read all settings from the EEPROM, get the rest from defaults */
+	for (size_t i = valid_bytes; i < sizeof(board_cfg.raw_settings); i++)
+		board_cfg.raw_settings[i] = default_cfg.raw_settings[i];
+
+	cfg_cached = true;
+
+	return &board_cfg;
 }
 
 struct eeprom_bmc_settings *get_bmc_settings(void)