drivers/option: Add CBFS file based option backend

Add a new option backend that uses values stored in CBFS files, similar
to the SeaBIOS runtime config options stored in files with the etc/
prefix. Options should be stored in CBFS with the option/ prefix. Values
can be set using `cbfstool coreboot.rom add-int -n option/<option-name>
-i <value>`. For simplicity, options should be stored in the COREBOOT
(RO) FMAP region, which is the default for cbfstool. This backend is not
available in SMM due to CBFS dependencies on vboot functions which are
not added to SMM, and thus the fallback will be returned by calls to
get_uint_option() in SMM.

Tested with QEMU Q35 by setting various options for "sata_mode" and
observing the console output for the SATA controller mode during
i82801ix_sata initialization.

Change-Id: Ifc0439ee42f13f49ae54d4855d1d9333c39b01f5
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/85905
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>

src/Kconfig

@@ -149,6 +149,7 @@ config UTIL_GENPARSER
 
 choice
 	prompt "Option backend to use"
+	default USE_CBFS_FILE_OPTION_BACKEND if CHROMEOS && PLATFORM_USES_FSP2_0
 	default USE_MAINBOARD_SPECIFIC_OPTION_BACKEND if HAVE_MAINBOARD_SPECIFIC_OPTION_BACKEND
 	default USE_OPTION_TABLE if NVRAMCUI_SECONDARY_PAYLOAD
 	default USE_UEFI_VARIABLE_STORE if DRIVERS_EFI_VARIABLE_STORE && \
@@ -164,6 +165,12 @@ config USE_OPTION_TABLE
 	  Enable this option if coreboot shall read options from the "CMOS"
 	  NVRAM instead of using hard-coded values.
 
+config USE_CBFS_FILE_OPTION_BACKEND
+	bool "Use CBFS files for configuration values"
+	help
+	  Enable this option if coreboot shall read options from files in CBFS.
+	  Options can be set using `cbfstool add-int -n option/<option-name> -i value`.
+
 config USE_UEFI_VARIABLE_STORE
 	bool "Use UEFI variable-store in SPI flash as option backend"
 	depends on DRIVERS_EFI_VARIABLE_STORE

src/drivers/option/Makefile.mk

@@ -1,3 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-only
 
 ramstage-$(CONFIG_DRIVERS_OPTION_CFR) += cfr.c
+
+all-$(CONFIG_USE_CBFS_FILE_OPTION_BACKEND) += cbfs_file_option.c

src/drivers/option/cbfs_file_option.c

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <cbfs.h>
+#include <endian.h>
+#include <option.h>
+
+unsigned int get_uint_option(const char *name, const unsigned int fallback)
+{
+	size_t size;
+	uint64_t value;
+	char full_name[CBFS_METADATA_MAX_SIZE];
+	snprintf(full_name, sizeof(full_name), "option/%s", name);
+
+	void *p = cbfs_ro_map(full_name, &size);
+	if (!p || size < sizeof(value)) {
+		value = fallback;
+	} else {
+		value = le64dec(p);
+		cbfs_unmap(p);
+	}
+	return (unsigned int)value;
+}
+
+enum cb_err set_uint_option(const char *name, unsigned int value)
+{
+	return CB_ERR_NOT_IMPLEMENTED;
+}

src/include/option.h

@@ -7,7 +7,9 @@
 
 void sanitize_cmos(void);
 
-#if CONFIG(OPTION_BACKEND_NONE)
+/* The CBFS file option backend cannot be used in SMM due to vboot
+ * dependencies, which are not added to SMM */
+#if CONFIG(OPTION_BACKEND_NONE) || (CONFIG(USE_CBFS_FILE_OPTION_BACKEND) && ENV_SMM)
 
 static inline unsigned int get_uint_option(const char *name, const unsigned int fallback)
 {