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

[<Manikandan.M@microchip.com>]

Hi Michael,

Sorry for the late reply.

On 9/7/26 6:30 PM, Michael Walle wrote:
> 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@microchip.com>
>> ---
>>   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");
> 
will do that, Thanks
>> +	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.
> 
And Sorry, I overlooked that — I moved the of_node_put() instead of 
adding the of_node_get() to nvmem_register().
I'll fold it into v8 and send it shortly — let me know if you'd rather 
see the core patch posted separately first.
> -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;
> 


-- 
Thanks and Regards,
Manikandan M.