[PATCH v8 4/4] efi_vars: Implement SPI Flash store

[Heinrich Schuchardt]

On 2/11/26 16:56, Michal Simek wrote:
> From: Shantur Rathore <i at shantur.com>
> 
> 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.
> 
> https://github.com/rhboot/efivar
> is not updated to support this location that's why you can mimic it by
> running:
> dd if=/sys/firmware/efi/efivars/VarToFile-b2ac5fc9-92b7-4acd-aeac-11e818c3130c \
>     of=/tmp/vars skip=4 bs=1
> flashcp /tmp/vars /dev/mtdX
> 
> where mtdX should match location defined by
> CONFIG_EFI_VARIABLE_SF_OFFSET/CONFIG_EFI_VAR_BUF_SIZE.
> 
> Signed-off-by: Shantur Rathore <i at shantur.com>
> Signed-off-by: Michal Simek <michal.simek at amd.com>
> Tested-by: Neil Armstrong <neil.armstrong at linaro.org> # on AML-S905D3-CC
> Acked-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> 
> ---
> 
> Changes in v8:
> - Add missing EFI_VARIABLE_SF_STORE dependency to EFI_VARIABLE_SF_DEVICE_INDEX
> 
> Changes in v7:
> - sed -i 's/efi_var_from/efi_var_from_storage/g'
> 
> Changes in v6:
> - sed -i 's/efi_var_read/efi_var_from/g'
> - sed -i 's/efi_var_write/efi_var_to_storage/g'
> 
> Changes in v4:
> - Extend Kconfig description
> - Extend commit message and describe efivar missing part
> - use unify methods for reading/writing variable
> 
> Changes in v3:
> - Fixed compiler warnings.
> 
> Changes in v2:
> - Refactored efi_var_file to move common parts out as requested
> - Changed ifdefs to use CONFIG_IS_DEFINED
> - Fixed typos
> 
>   lib/efi_loader/Kconfig      | 35 +++++++++++++-
>   lib/efi_loader/Makefile     |  1 +
>   lib/efi_loader/efi_var_sf.c | 92 +++++++++++++++++++++++++++++++++++++
>   3 files changed, 127 insertions(+), 1 deletion(-)
>   create mode 100644 lib/efi_loader/efi_var_sf.c
> 
> diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig
> index 579eed658801..0f6f927ddaf1 100644
> --- a/lib/efi_loader/Kconfig
> +++ b/lib/efi_loader/Kconfig
> @@ -124,6 +124,24 @@ 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
> @@ -153,7 +171,7 @@ endchoice
>   
>   config EFI_RT_VOLATILE_STORE
>   	bool "Allow variable runtime services in volatile storage (e.g RAM)"
> -	depends on EFI_VARIABLE_FILE_STORE
> +	depends on EFI_VARIABLE_FILE_STORE || EFI_VARIABLE_SF_STORE

Hello Michal,

If the backend store is SPI flash, we should not publish the variable 
"RTStorageVolatile" at runtime as currently defined.

For the background see this commit for the efivar library:

https://github.com/rhboot/efivar/commit/68daa04654acbe1bbaa17ebfc23c371b39e69c6b

The first three patches look correct to me and I will add them to a 
merge request for efi-next.

The usage of efi_var_mem_ins() and efi_var_mem_del() when SetVariable() 
does not change anything is not flash-friendly. Michal, do you want to 
have a look at this area?

@Ilias, @Vincent:
We currently lack documentation for the 3 variables.
Would it make sense to add the definition of the variables to chapter 
source/chapter5-variable-storage.rst.

Would an efivar helper for SPI flash make sense?
How should RTStorageVolatile describe a SPI flash location?

Best regards

Heinrich

>   	help
>   	  When EFI variables are stored on file we don't allow SetVariableRT,
>   	  since the OS doesn't know how to write that file. At the same time
> @@ -194,6 +212,21 @@ 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
> diff --git a/lib/efi_loader/Makefile b/lib/efi_loader/Makefile
> index ca1775eb03be..d73ad43951b1 100644
> --- a/lib/efi_loader/Makefile
> +++ b/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
> diff --git a/lib/efi_loader/efi_var_sf.c b/lib/efi_loader/efi_var_sf.c
> new file mode 100644
> index 000000000000..61d68f7c5c94
> --- /dev/null
> +++ b/lib/efi_loader/efi_var_sf.c
> @@ -0,0 +1,92 @@
> +// 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)
> +{
> +	efi_status_t ret;
> +	struct efi_var_file *buf;
> +	loff_t len;
> +	struct udevice *sfdev;
> +
> +	ret = efi_var_collect(&buf, &len, EFI_VARIABLE_NON_VOLATILE);
> +	if (len > EFI_VAR_BUF_SIZE) {
> +		log_err("EFI var buffer length more than target SPI Flash size");
> +		ret = EFI_OUT_OF_RESOURCES;
> +		goto error;
> +	}
> +
> +	log_debug("%s - Got buffer to write buf->len : %d\n", __func__, buf->length);
> +
> +	if (ret != EFI_SUCCESS)
> +		goto error;
> +
> +	ret = uclass_get_device(UCLASS_SPI_FLASH, CONFIG_EFI_VARIABLE_SF_DEVICE_INDEX, &sfdev);
> +	if (ret)
> +		goto error;
> +
> +	ret = spi_flash_erase_dm(sfdev, CONFIG_EFI_VARIABLE_SF_OFFSET, EFI_VAR_BUF_SIZE);
> +	log_debug("%s - Erased SPI Flash offset %x\n", __func__, CONFIG_EFI_VARIABLE_SF_OFFSET);
> +	if (ret)
> +		goto error;
> +
> +	ret = spi_flash_write_dm(sfdev, CONFIG_EFI_VARIABLE_SF_OFFSET, len, buf);
> +	log_debug("%s - Wrote buffer to SPI Flash : %ld\n", __func__, ret);
> +
> +	if (ret)
> +		goto error;
> +
> +	ret = EFI_SUCCESS;
> +error:
> +	if (ret)
> +		log_err("Failed to persist EFI variables in SF\n");
> +	free(buf);
> +	return ret;
> +}
> +
> +efi_status_t efi_var_from_storage(void)
> +{
> +	struct efi_var_file *buf;
> +	efi_status_t ret;
> +	struct udevice *sfdev;
> +
> +	buf = calloc(1, EFI_VAR_BUF_SIZE);
> +	if (!buf) {
> +		log_err("%s - Unable to allocate buffer\n", __func__);
> +		return EFI_OUT_OF_RESOURCES;
> +	}
> +
> +	ret = uclass_get_device(UCLASS_SPI_FLASH, 0, &sfdev);
> +	if (ret)
> +		goto error;
> +
> +	ret = spi_flash_read_dm(sfdev, CONFIG_EFI_VARIABLE_SF_OFFSET,
> +				EFI_VAR_BUF_SIZE, buf);
> +
> +	log_debug("%s - read buffer buf->length: %x\n", __func__, buf->length);
> +
> +	if (ret || buf->length < sizeof(struct efi_var_file)) {
> +		log_err("%s - buffer read from SPI Flash isn't valid\n", __func__);
> +		goto error;
> +	}
> +
> +	ret = efi_var_restore(buf, false);
> +	if (ret != EFI_SUCCESS)
> +		log_err("%s - Unable to restore EFI variables from buffer\n", __func__);
> +
> +	ret = EFI_SUCCESS;
> +error:
> +	free(buf);
> +	return ret;
> +}