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SubjectRe: [PATCH 6/8] nvmem: transformations: ethernet address offset support
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On 28.12.2021 15:25, Michael Walle wrote:
> An nvmem cell might just contain a base MAC address. To generate a
> address of a specific interface, add a transformation to add an offset
> to this base address.
>
> Add a generic implementation and the first user of it, namely the sl28
> vpd storage.
>
> Signed-off-by: Michael Walle <michael@walle.cc>
> ---
> drivers/nvmem/transformations.c | 45 +++++++++++++++++++++++++++++++++
> 1 file changed, 45 insertions(+)
>
> diff --git a/drivers/nvmem/transformations.c b/drivers/nvmem/transformations.c
> index 61642a9feefb..15cd26da1f83 100644
> --- a/drivers/nvmem/transformations.c
> +++ b/drivers/nvmem/transformations.c
> @@ -12,7 +12,52 @@ struct nvmem_transformations {
> nvmem_cell_post_process_t pp;
> };
>
> +/**
> + * nvmem_transform_mac_address_offset() - Add an offset to a mac address cell
> + *
> + * A simple transformation which treats the index argument as an offset and add
> + * it to a mac address. This is useful, if the nvmem cell stores a base
> + * ethernet address.
> + *
> + * @index: nvmem cell index
> + * @data: nvmem data
> + * @bytes: length of the data
> + *
> + * Return: 0 or negative error code on failure.
> + */
> +static int nvmem_transform_mac_address_offset(int index, unsigned int offset,
> + void *data, size_t bytes)
> +{
> + if (bytes != ETH_ALEN)
> + return -EINVAL;
> +
> + if (index < 0)
> + return -EINVAL;
> +
> + if (!is_valid_ether_addr(data))
> + return -EINVAL;
> +
> + eth_addr_add(data, index);
> +
> + return 0;
> +}
> +
> +static int nvmem_kontron_sl28_vpd_pp(void *priv, const char *id, int index,
> + unsigned int offset, void *data,
> + size_t bytes)
> +{
> + if (!id)
> + return 0;
> +
> + if (!strcmp(id, "mac-address"))
> + return nvmem_transform_mac_address_offset(index, offset, data,
> + bytes);
> +
> + return 0;
> +}
> +
> static const struct nvmem_transformations nvmem_transformations[] = {
> + { .compatible = "kontron,sl28-vpd", .pp = nvmem_kontron_sl28_vpd_pp },
> {}
> };

I think it's a rather bad solution that won't scale well at all.

You'll end up with a lot of NVMEM device specific strings and code in a
NVMEM core.

You'll have a lot of duplicated code (many device specific functions
calling e.g. nvmem_transform_mac_address_offset()).

I think it also ignores fact that one NVMEM device can be reused in
multiple platforms / device models using different (e.g. vendor / device
specific) cells.


What if we have:
1. Foo company using "kontron,sl28-vpd" with NVMEM cells:
a. "mac-address"
b. "mac-address-2"
c. "mac-address-3"
2. Bar company using "kontron,sl28-vpd" with NVMEM cell:
a. "mac-address"

In the first case you don't want any transformation.


If you consider using transformations for ASCII formats too then it
causes another conflict issue. Consider two devices:

1. Foo company device with BIN format of MAC
2. Bar company device with ASCII format of MAC

Both may use exactly the same binding:

partition@0 {
compatible = "nvmem-cells";
reg = <0x0 0x100000>;
label = "bootloader";

#address-cells = <1>;
#size-cells = <1>;

mac-address@100 {
reg = <0x100 0x6>;
};
};

how are you going to handle them with proposed implementation? You can't
support both if you share "nvmem-cells" compatible string.


I think that what can solve those problems is assing "compatible" to
NVMEM cells.

Let me think about details of that possible solution.

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