Re: [PATCH v2] nvmem: core: Default to read-only if wp-gpios present

[Bartosz Golaszewski]

On Sun, 26 Apr 2026 04:44:32 +0200, Marek Vasut <marex@xxxxxxxxxxxx> said:
> In case the nvmem DT node contains "wp-gpios" DT property, the device
> currently defaults to read-write and the force_ro sysfs attribute reads
> 0. Switch to the default read-only, which is both safer, and aligned
> with eMMC HW BOOT partition force_ro sysfs attribute behavior, which
> also defaults to read-only.
>
> The adjustment of nvmem->read_only value to read-only in case wp-gpios
> DT property is present must be done only after the device_add() got
> called because device_add() does internally call nvmem_bin_attr_get_umode(),
> which configures the permissions of 'nvmem' bin attr based on the value
> of nvmem->read_only that is only parsed from DT property 'read-only',
> without any adjustment. This way, if DT property 'read-only' is present,
> the 'nvmem' attribute is always read-only. Otherwise, if the device is
> writeable, then 'nvmem' attribute is writeable, and nvmem->read_only
> defaults to read-only, but can be switched to read-write at runtime via
> the 'force_ro' attribute.
>
> The updated behavior can be tested as follows:
>
> Current content:
> "
> $ cat /sys/bus/nvmem/devices/logging7/force_ro
> 1
>
> $ hexdump -C /sys/bus/nvmem/devices/logging7/nvmem
> 00000000  66 6f 6f 0a ff ff ff ff
> "
>
> Write into default-read-only device:
> "
> $ echo bar > /sys/bus/nvmem/devices/logging7/nvmem
> bash: echo: write error: Operation not permitted
> $ cat /sys/bus/nvmem/devices/logging7/force_ro
> 1
> "
>
> Unlock and write into device:
> "
> $ echo 0 > /sys/bus/nvmem/devices/logging7/force_ro
> $ cat /sys/bus/nvmem/devices/logging7/force_ro
> 0
>
> $ echo bar > /sys/bus/nvmem/devices/logging7/nvmem
> $ hexdump -C /sys/bus/nvmem/devices/logging7/nvmem
> 00000000  62 61 72 0a ff ff ff ff
> "
>
> Relock and write into device, fails because device is read-only again:
> "
> $ echo 1 > /sys/bus/nvmem/devices/logging7/force_ro
> $ echo baz > /sys/bus/nvmem/devices/logging7/nvmem
> bash: echo: write error: Operation not permitted
>
> $ hexdump -C /sys/bus/nvmem/devices/logging7/nvmem
> 00000000  62 61 72 0a ff ff ff ff
> "
>
> Signed-off-by: Marek Vasut <marex@xxxxxxxxxxxx>
> ---
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Bartosz Golaszewski <brgl@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Srinivas Kandagatla <srini@xxxxxxxxxx>
> Cc: linux-i2c@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> V2: Simplify the implementation, make devices with wp-gpios default
>     to read-only and let users unlock those devices via force_ro
>     sysfs attribute, just like eMMC HW BOOT partitions
> ---
>  drivers/nvmem/core.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> index 311cb2e5a5c02..0b1942e389113 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -1019,6 +1019,12 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>  	if (rval)
>  		goto err_remove_dev;
>
> +	/* If the device has WP GPIO, default to read-only */
> +	if (nvmem->wp_gpio) {
> +		nvmem->read_only = true;
> +		gpiod_set_value_cansleep(nvmem->wp_gpio, 1);

Why is this needed if we already did:

	nvmem->wp_gpio = gpiod_get_optional(config->dev, "wp", GPIOD_OUT_HIGH);

?

Bart

> +	}
> +
>  #ifdef CONFIG_NVMEM_SYSFS
>  	rval = nvmem_populate_sysfs_cells(nvmem);
>  	if (rval)
> --
> 2.53.0
>
>