Re: [PATCH v5 3/7] mtd: spi-nor: sfdp: expose the SFDP as a read-only NVMEM device

["Michael Walle" <mwalle@kernel.org>]

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On Tue Jul 21, 2026 at 7:28 AM CEST, Manikandan Muralidharan wrote:
> The SPI NOR core already reads the SFDP tables during enumeration and
> caches them in nor->sfdp->dwords (see spi_nor_parse_sfdp()). Re-expose
> that cached data as a read-only NVMEM device, in on-flash byte order,
> rooted at the flash's SFDP child node (compatible "jedec,sfdp").
>
> This lets NVMEM cells reference any SFDP data: a fixed-layout for
> parameters at a known offset, or an nvmem-layout parser for vendor data
> whose location must be discovered at runtime.The device is only registered
> when an "sfdp" node is present in the device tree.
>
> Signed-off-by: Manikandan Muralidharan <manikandan.m@microchip.com>
> ---
>  drivers/mtd/spi-nor/core.c |  8 ++++
>  drivers/mtd/spi-nor/core.h |  1 +
>  drivers/mtd/spi-nor/sfdp.c | 86 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 95 insertions(+)
>
> diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
> index ccf4396cdcd0..b833d8ec2d65 100644
> --- a/drivers/mtd/spi-nor/core.c
> +++ b/drivers/mtd/spi-nor/core.c
> @@ -3204,6 +3204,14 @@ static int spi_nor_init_params(struct spi_nor *nor)
>  		spi_nor_init_params_deprecated(nor);
>  	}
>  
> +	/*
> +	 * Expose the SFDP table as an NVMEM device only when
> +	 * the flash actually provides one
> +	 */
> +	ret = spi_nor_register_sfdp_nvmem(nor);
> +	if (ret)
> +		return ret;
> +
>  	ret = spi_nor_late_init_params(nor);
>  	if (ret)
>  		return ret;
> diff --git a/drivers/mtd/spi-nor/core.h b/drivers/mtd/spi-nor/core.h
> index ba2d1a862c9d..0a6484298c5c 100644
> --- a/drivers/mtd/spi-nor/core.h
> +++ b/drivers/mtd/spi-nor/core.h
> @@ -698,6 +698,7 @@ int spi_nor_controller_ops_write_reg(struct spi_nor *nor, u8 opcode,
>  
>  int spi_nor_check_sfdp_signature(struct spi_nor *nor);
>  int spi_nor_parse_sfdp(struct spi_nor *nor);
> +int spi_nor_register_sfdp_nvmem(struct spi_nor *nor);

That's probably not needed if..

>  static inline struct spi_nor *mtd_to_spi_nor(struct mtd_info *mtd)
>  {
> diff --git a/drivers/mtd/spi-nor/sfdp.c b/drivers/mtd/spi-nor/sfdp.c
> index 4600983cb579..704799fe92ae 100644
> --- a/drivers/mtd/spi-nor/sfdp.c
> +++ b/drivers/mtd/spi-nor/sfdp.c
> @@ -6,6 +6,8 @@
>  
>  #include <linux/bitfield.h>
>  #include <linux/mtd/spi-nor.h>
> +#include <linux/nvmem-provider.h>
> +#include <linux/of.h>
>  #include <linux/slab.h>
>  #include <linux/sort.h>
>  
> @@ -1612,3 +1614,87 @@ int spi_nor_parse_sfdp(struct spi_nor *nor)
>  	kfree(param_headers);
>  	return err;
>  }
> +
> +static int spi_nor_sfdp_reg_read(void *priv, unsigned int offset,
> +				 void *val, size_t bytes)
> +{
> +	struct spi_nor *nor = priv;
> +	struct sfdp *sfdp = nor->sfdp;
> +	size_t sfdp_size = sfdp->num_dwords * sizeof(*sfdp->dwords);
> +
> +	if (offset >= sfdp_size || bytes > sfdp_size - offset)
> +		return -EINVAL;
> +
> +	/* The cached SFDP is kept in on-flash (little-endian) byte order. */
> +	memcpy(val, (u8 *)sfdp->dwords + offset, bytes);
> +
> +	return 0;
> +}
> +
> +static void spi_nor_sfdp_nvmem_put_np(void *data)
> +{
> +	of_node_put(data);
> +}
> +
> +/**
> + * spi_nor_register_sfdp_nvmem() - expose the SFDP as a read-only NVMEM device
> + * @nor:	pointer to a 'struct spi_nor'
> + *
> + * Expose the whole SFDP, in on-flash byte order, as a read-only NVMEM device
> + * rooted at the flash's SFDP child node (compatible "jedec,sfdp"). This lets
> + * generic (fixed-layout) or vendor (nvmem-layout) cells reference any SFDP
> + * data. The device is only registered when a child node with the "jedec,sfdp"
> + * compatible is described in the device tree.
> + *
> + * Return: 0 on success or if there is nothing to do, -errno otherwise.
> + */
> +int spi_nor_register_sfdp_nvmem(struct spi_nor *nor)

.. you move all this into the core, as the sfdp.c is just for
parsing the tables.

> +{
> +	struct device *dev = nor->dev;
> +	struct nvmem_config config = { };
> +	struct nvmem_device *nvmem;
> +	struct device_node *np;
> +	int ret;
> +
> +	if (!nor->sfdp)
> +		return 0;
> +
> +	for_each_available_child_of_node(dev_of_node(dev), np)
> +		if (of_device_is_compatible(np, "jedec,sfdp"))
> +			break;

There is already of_get_compatible_child() doing exactly this.

> +	if (!np)
> +		return 0;
> +
> +	/*
> +	 * Register the put before devm_nvmem_register() so it runs last on
> +	 * detach, after the NVMEM device that uses the node is gone.
> +	 */
> +	ret = devm_add_action_or_reset(dev, spi_nor_sfdp_nvmem_put_np, np);
> +	if (ret)
> +		return ret;
> +
> +	config.dev = dev;
> +	config.of_node = np;
> +	config.name = "sfdp";
> +	config.id = NVMEM_DEVID_AUTO;

Or rather NVEMEM_DEVID_NONE? There will ever be just one SFDP nvmem
device.

How does the sysfs path looks like?

-michael

> +	config.owner = THIS_MODULE;
> +	config.read_only = true;
> +	config.word_size = 1;
> +	config.stride = 1;
> +	config.size = (int)(nor->sfdp->num_dwords * sizeof(*nor->sfdp->dwords));
> +	config.reg_read = spi_nor_sfdp_reg_read;
> +	config.priv = nor;
> +
> +	nvmem = devm_nvmem_register(dev, &config);
> +	if (IS_ERR(nvmem)) {
> +		/* NVMEM support is optional. */
> +		if (PTR_ERR(nvmem) == -EOPNOTSUPP)
> +			return 0;
> +		return dev_err_probe(dev, PTR_ERR(nvmem),
> +				     "failed to register SFDP NVMEM device\n");
> +	}
> +
> +	dev_dbg(dev, "exposed %d-byte SFDP as an NVMEM device\n", config.size);
> +
> +	return 0;
> +}

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