Re: [PATCH v7 3/7] mtd: spi-nor: sfdp: expose the SFDP as a read-only NVMEM device

[Michael Walle]

Hi,

On Wed Aug 12, 2026 at 12:49 PM 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@xxxxxxxxxxxxx>
> ---
>  drivers/mtd/spi-nor/core.c | 78 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 78 insertions(+)
>
> diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
> index ccf4396cdcd0..0425af6e898f 100644
> --- a/drivers/mtd/spi-nor/core.c
> +++ b/drivers/mtd/spi-nor/core.c
> @@ -17,6 +17,7 @@
>  #include <linux/mtd/mtd.h>
>  #include <linux/mtd/spi-nor.h>
>  #include <linux/mutex.h>
> +#include <linux/nvmem-provider.h>
>  #include <linux/of.h>
>  #include <linux/regulator/consumer.h>
>  #include <linux/sched/task_stack.h>
> @@ -3001,6 +3002,75 @@ static void spi_nor_init_fixup_flags(struct spi_nor *nor)
>  		nor->flags |= SNOR_F_IO_MODE_EN_VOLATILE;
>  }
>  
> +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;
> +}
> +
> +/**
> + * 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.
> + */
> +static int spi_nor_register_sfdp_nvmem(struct spi_nor *nor)
> +{
> +	struct device *dev = nor->dev;
> +	struct nvmem_config config = { };
> +	struct nvmem_device *nvmem;
> +	struct device_node *np;
> +
> +	if (!nor->sfdp)
> +		return 0;
> +
> +	np = of_get_compatible_child(dev_of_node(dev), "jedec,sfdp");
> +	if (!np)
> +		return 0;

What if there is no jedec,sfdp node? Should this be added anyways?

> +
> +	config.dev = dev;
> +	config.of_node = np;

What about just:
	config.of_node = of_get_compatible_child(dev_of_node(dev), "jedec,sfdp");

> +	config.name = "sfdp";
> +	config.id = NVMEM_DEVID_AUTO;
> +	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);
> +	of_node_put(np);

This now leaves the nvmem device with a dangling pointer. What I
meant in my previous reply, was that you'll add an of_node_get() to
the nvmem_register(), so the nvmem subsystem will have its own
tracking.

-michael

> +	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;
> +}
> +
>  /**
>   * spi_nor_late_init_params() - Late initialization of default flash parameters.
>   * @nor:	pointer to a 'struct spi_nor'
> @@ -3204,6 +3274,14 @@ 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;

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