Re: [RFC PATCH v1 1/3] dt-bindings: net: Add 'mac-address-lookup' property

From: Rob Herring
Date: Wed Aug 15 2018 - 16:45:34 EST

On Tue, Aug 14, 2018 at 03:37:56PM -0700, Brian Norris wrote:
> Some firmwares present data tables that can be parsed to retrieve
> device-specific details, like MAC addresses. While in some cases, one
> could teach the firmware to understand the device tree format and insert
> a 'mac-address'/'local-mac-address' property into the FDT on its own,
> this method can be brittle (e.g., involving memorizing expected FDT
> structure), and it's not strictly necessary -- especially when parsers
> for such firmware formats are already needed in the OS for other
> reasons.

If you have an 'ethernetN' alias then you don't need to know the
structure. IIRC, that is what u-boot does.

> One such format: the Vital Product Data (VPD) [1] used by Coreboot. It
> supports a table of key/value pairs, and some systems keep MAC addresses
> there in a well-known format. Allow a device tree to specify
> (1) that the MAC address for a given device is stored in the VPD table
> and
> (2) what key should be used to retrieve the MAC address for said
> device (e.g., "ethernet_mac0" or "wifi_mac1").
> [1] Ref:
> TL;DR: VPD consists of a TLV-like table, with key/value pairs of
> strings. This is often stored persistently on the boot flash and
> presented via in-memory Coreboot tables, for the operating system to
> read.
> Signed-off-by: Brian Norris <briannorris@xxxxxxxxxxxx>
> ---
> Documentation/devicetree/bindings/net/ethernet.txt | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
> diff --git a/Documentation/devicetree/bindings/net/ethernet.txt b/Documentation/devicetree/bindings/net/ethernet.txt
> index cfc376bc977a..d3fd1da18bf4 100644
> --- a/Documentation/devicetree/bindings/net/ethernet.txt
> +++ b/Documentation/devicetree/bindings/net/ethernet.txt
> @@ -4,6 +4,18 @@ NOTE: All 'phy*' properties documented below are Ethernet specific. For the
> generic PHY 'phys' property, see
> Documentation/devicetree/bindings/phy/phy-bindings.txt.
> +- mac-address-lookup: string, indicating a method by which a MAC address may be
> + discovered for this device. Methods may be parameterized by some value, such
> + that the method can determine the device's MAC address using that parameter.
> + For example, a firmware might store MAC addresses in a table, keyed by some
> + predetermined string, and baked in read-only flash. A lookup method "foo"
> + with a parameter "bar" should be written "foo:bar".
> + Supported values for method:
> + "google-vpd" - Google's Vital Product Data (VPD), as used in the Coreboot
> + project. Documentation for VPD can be found at:
> +
> + Example:
> + mac-address-lookup = "google-vpd:ethernet_mac0"

This doesn't strike me as a very DT style way of describing this. I
would expect something like a phandle to the VPD and an identifier.

Also, an already common way besides local-mac-address is using nvmem
binding which wouldn't use this and perhaps could be used here? This
feels very much Google specific, not common (and maybe that is fine).