efi_vars: Implement SPI Flash store

Currently U-Boot uses ESP as storage for EFI variables.
Devices with SPI Flash are used for storing environment with this
commit we allow EFI variables to be stored on SPI Flash.

Signed-off-by: Shantur Rathore <i@shantur.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on AML-S905D3-CC
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

lib/efi_loader/Kconfig

@@ -126,6 +126,24 @@ config EFI_VARIABLE_FILE_STORE
 	  Select this option if you want non-volatile UEFI variables to be
 	  stored as file /ubootefi.var on the EFI system partition.
 
+config EFI_VARIABLE_SF_STORE
+	bool "Store non-volatile UEFI variables in SPI Flash"
+	depends on SPI_FLASH
+	help
+	  Select this option if you want non-volatile UEFI variables to be
+	  stored in SPI Flash.
+
+	  Define CONFIG_EFI_VARIABLE_SF_OFFSET as offset in SPI Flash to use as
+	  the storage for variables. CONFIG_EFI_VAR_BUF_SIZE defines the space
+	  needed.
+
+	  Note that SPI Flash devices have a limited number of program/erase
+	  cycles. Frequent updates to UEFI variables may cause excessive wear
+	  and can permanently damage the flash device, particularly on SPI NAND
+	  or low-end SPI NOR parts without wear leveling. This option should be
+	  used with care on such systems, and is not recommended for platforms
+	  where UEFI variables are updated frequently.
+
 config EFI_MM_COMM_TEE
 	bool "UEFI variables storage service via the trusted world"
 	depends on OPTEE
@@ -196,6 +214,21 @@ config FFA_SHARED_MM_BUF_ADDR
 	  the MM SP in secure world.
 	  It is assumed that the MM SP knows the address of the shared MM communication buffer.
 
+config EFI_VARIABLE_SF_OFFSET
+	hex "EFI variables in SPI flash offset"
+	depends on EFI_VARIABLE_SF_STORE
+	help
+	  Offset from the start of the SPI Flash where EFI variables will be stored.
+	  This should be aligned to the sector size of SPI Flash.
+
+config EFI_VARIABLE_SF_DEVICE_INDEX
+	int "Device Index for target SPI Flash"
+	depends on EFI_VARIABLE_SF_STORE
+	default 0
+	help
+	  The index of SPI Flash device used for storing EFI variables. This would be
+	  needed if there are more than 1 SPI Flash devices available to use.
+
 config EFI_VARIABLES_PRESEED
 	bool "Initial values for UEFI variables"
 	depends on !COMPILE_TEST

lib/efi_loader/Makefile

@@ -54,6 +54,7 @@ obj-y += efi_variable_tee.o
 else
 obj-y += efi_variable.o
 obj-$(CONFIG_EFI_VARIABLE_FILE_STORE) += efi_var_file.o
+obj-$(CONFIG_EFI_VARIABLE_SF_STORE) += efi_var_sf.o
 obj-$(CONFIG_EFI_VARIABLES_PRESEED) += efi_var_seed.o
 endif
 obj-y += efi_watchdog.o

lib/efi_loader/efi_var_sf.c

@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * SPI Flash interface for UEFI variables
+ *
+ * Copyright (c) 2023, Shantur Rathore
+ * Copyright (C) 2026, Advanced Micro Devices, Inc.
+ */
+
+#define LOG_CATEGORY LOGC_EFI
+
+#include <efi_loader.h>
+#include <efi_variable.h>
+#include <spi_flash.h>
+#include <dm.h>
+
+efi_status_t efi_var_to_storage(void)
+{
+	struct efi_var_file *buf;
+	struct spi_flash *flash;
+	struct udevice *sfdev;
+	efi_status_t ret;
+	size_t erase_len;
+	loff_t len;
+	int r;
+
+	ret = efi_var_collect(&buf, &len, EFI_VARIABLE_NON_VOLATILE);
+	if (ret != EFI_SUCCESS)
+		goto error;
+
+	if (len > EFI_VAR_BUF_SIZE) {
+		log_debug("EFI var buffer length more than target SPI Flash size\n");
+		ret = EFI_OUT_OF_RESOURCES;
+		goto error;
+	}
+
+	log_debug("Got buffer to write buf->len: %d\n", buf->length);
+
+	r = uclass_get_device(UCLASS_SPI_FLASH,
+			      CONFIG_EFI_VARIABLE_SF_DEVICE_INDEX, &sfdev);
+	if (r) {
+		ret = EFI_DEVICE_ERROR;
+		goto error;
+	}
+
+	flash = dev_get_uclass_priv(sfdev);
+	if (!flash) {
+		log_debug("Failed to get SPI Flash priv data\n");
+		ret = EFI_DEVICE_ERROR;
+		goto error;
+	}
+	erase_len = ALIGN(len, flash->sector_size);
+
+	r = spi_flash_erase_dm(sfdev, CONFIG_EFI_VARIABLE_SF_OFFSET,
+			       erase_len);
+	if (r) {
+		log_debug("Failed to erase SPI Flash\n");
+		ret = EFI_DEVICE_ERROR;
+		goto error;
+	}
+
+	r = spi_flash_write_dm(sfdev, CONFIG_EFI_VARIABLE_SF_OFFSET, len, buf);
+	if (r) {
+		log_debug("Failed to write to SPI Flash: %d\n", r);
+		ret = EFI_DEVICE_ERROR;
+	}
+
+error:
+	free(buf);
+	return ret;
+}
+
+efi_status_t efi_var_from_storage(void)
+{
+	struct efi_var_file *buf;
+	struct udevice *sfdev;
+	efi_status_t ret;
+	int r;
+
+	buf = calloc(1, EFI_VAR_BUF_SIZE);
+	if (!buf) {
+		log_err("Unable to allocate buffer\n");
+		return EFI_OUT_OF_RESOURCES;
+	}
+
+	r = uclass_get_device(UCLASS_SPI_FLASH,
+			      CONFIG_EFI_VARIABLE_SF_DEVICE_INDEX, &sfdev);
+	if (r) {
+		log_err("Failed to get SPI Flash device: %d\n", r);
+		ret = EFI_DEVICE_ERROR;
+		goto error;
+	}
+
+	r = spi_flash_read_dm(sfdev, CONFIG_EFI_VARIABLE_SF_OFFSET,
+			      EFI_VAR_BUF_SIZE, buf);
+	if (r) {
+		log_err("Failed to read from SPI Flash: %d\n", r);
+		ret = EFI_DEVICE_ERROR;
+		goto error;
+	}
+
+	if (efi_var_restore(buf, false) != EFI_SUCCESS) {
+		log_err("No valid EFI variables in SPI Flash\n");
+		ret = EFI_DEVICE_ERROR;
+		goto error;
+	}
+
+	ret = EFI_SUCCESS;
+error:
+	free(buf);
+	return ret;
+}