linux-kernel - Re: [PATCH 1/2] mtd: spi-nor: sst: register SFDP region into NVMEM framework to read MAC Address

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Message-Id: <D891QPQIT13L.1S55ZM1J286PE@kernel.org>
Date: Thu, 06 Mar 2025 09:39:31 +0100
From: "Michael Walle" <mwalle@...nel.org>
To: "Miquel Raynal" <miquel.raynal@...tlin.com>,
 <Manikandan.M@...rochip.com>
Cc: <robh@...nel.org>, <krzk+dt@...nel.org>, <conor+dt@...nel.org>,
 <Nicolas.Ferre@...rochip.com>, <alexandre.belloni@...tlin.com>,
 <claudiu.beznea@...on.dev>, <tudor.ambarus@...aro.org>,
 <pratyush@...nel.org>, <richard@....at>, <vigneshr@...com>,
 <devicetree@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
 <linux-kernel@...r.kernel.org>, <linux-mtd@...ts.infradead.org>,
 <Varshini.Rajendran@...rochip.com>
Subject: Re: [PATCH 1/2] mtd: spi-nor: sst: register SFDP region into NVMEM
 framework to read MAC Address

> >>> +static int sst26vf_nor_post_sfdp(struct spi_nor *nor)
> >>> +{
> >>> +     struct nvmem_device *nvmem;
> >>> +
> >>> +     sst26vf_sfdp_nvmem_config.dev = nor->dev;
> >>> +     sst26vf_sfdp_nvmem_config.size = nor->sfdp->num_dwords * sizeof(*nor->sfdp->dwords);
> >>> +     sst26vf_sfdp_nvmem_config.priv = nor;
> >>> +     sst26vf_sfdp_nvmem_config.reg_read = sst26vf_sfdp_mac_addr_read;
> >>> +
> >>> +     nvmem = devm_nvmem_register(nor->dev, &sst26vf_sfdp_nvmem_config);
> >>> +     if (IS_ERR(nvmem)) {
> >>> +             dev_err(nor->dev, "failed to register NVMEM device: %ld\n", PTR_ERR(nvmem));
> >>> +             return PTR_ERR(nvmem);
> >> 
> >> I don't think it makes sense to have this one-off in a particular
> >> driver. If at all, this should be handled in the core. Sorry, but
> >> this really looks like an ugly hack.
> >> 
> >
> > Because the EUI identifier within the SFDP is unique to the 
> > SST26VF064BEUI flash, I opted to handle it here rather than in the core.
> >
> > Also here the MAC address data resides within the 0x260-0x26F range, I 
> > will resize the nvmem_config.size to 0x10 instead of registering the 
> > full SFDP region as NVMEM.
>
> Open question to all parties in this thread: how do we give an offset in
> the device tree that is relative to the sfdp region and not the data
> region? I believe we care not to mix these areas while describing.

You don't do it, because there is not even a relative offset that is
fixed. There should be a pointer to the vendor table inside the SFDP
structure. Thus, you need to properly parse it.

Regarding how to reference it within the device tree, I'd assume
something along 'compatible = "jedec,sfdp-vendor-table-NNN";' or
similar. But no static/relative offsets.

-michael