+ Srinivas, as there are NVMEM questions

Hi Florian,

Thanks for the quick reply.

On Tue, Aug 14, 2018 at 04:00:28PM -0700, Florian Fainelli wrote:

On 08/14/2018 03:37 PM, Brian Norris wrote:

quoted

Today, we have generic support for 'mac-address' and 'local-mac-address'
properties in both Device Tree nodes and in generic Device Properties,
such that network device drivers can pick up a hardware address from
there, in cases where the MAC address isn't baked into the network card.
This method of MAC address retrieval presumes that either:
(a) there's a unique device tree (or similar) stored on a given device
    or
(b) some other entity (e.g., boot firmware) will modify device nodes
    runtime to place that MAC address into the appropriate device
    properties.

Option (a) is not feasbile for many systems.

Option (b) can work, but there are some reasons why one might not want
to do that:
 (1) This requires that system firmware understand the device tree
     structure, sometimes to the point of memorizing path names (e.g.,
     /soc/wifi@xxxxxxxx). At least for Device Tree, these path names are
     not necessarily an ABI, and so this introduces unneeded fragility.

The path to a node is something that is well defined and should be
stable given that the high level function of the node and its unit
address are not supposed to change. Under which circumstances, besides
incorrect specification of either of these two things, do they not
consist an ABI? Not refuting your statement here, just curious when/how
this can happen?

I can think of a few reasons:

 * it's not really standardized whether to use a /soc/ node or to
   just put top-level SoC blocks directly under /. There might be
   "recommendations", but I certainly have seen it both ways.

 * the "high level function name" is not set in stone AFAICT. Or at
   least, I've never seen a list of documented names. So while on one
   system I might see 'wlan@', another might use 'wifi@'.

 * in any of the above (and in any other case of lack of clarity), one
   can make slightly different choices when, e.g., submitting a device
   tree upstream vs. a downstream tree. While we may try our hardest to
   document and stick to documented bindings, I personally can't
   guarantee that one of these choices will be made differently during
   review, possibly breaking any firmware that made assumptions based on
   those choices. So I might end up with a firmware that satisfies
   documented bindings and works with a downstream device tree, but
   doesn't work with a device tree that gets submitted upstream.

Also, aliases in DT are meant to provide some stability.

How, specifically? I don't see any relevant binding description for
aliases under Documentation/devicetree/bindings/net/.

quoted

 (2) Other than this device-tree shim requirement, system firmware may
     have no reason to understand anything about network devices.

So instead, I'm looking for a way to have a device node describe where
to find its MAC address, rather than having the device node contain the
MAC address directly. Then system firmware doesn't have to manage
anything.

In particular, I add support for the Google Vital Product Data (VPD)
format, used within the Coreboot project. The format is described here:

https://chromium.googlesource.com/chromiumos/platform/vpd/+/master/README.md

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.

We already have a VPD driver that parses this table and presents it to
user space. This series extends that driver to allow in-kernel lookups
of MAC address entries.

A possible alternative approach is to have the VPD driver become a NVMEM
producer to expose the VPD keys, did you look into that and possibly
found that it was not a good model? The downside to that approach though
is that you might have to have a phandle for the VPD provider in the
Device Tree, but AFAICS this should solve your needs?

I did notice some NVMEM work. The MTD links you point at shouldn't be
relevant, since this table is already present in RAM. But I suppose I
could shoehorn the memory table into being a fake NVMEM...

And then, how would you recommend doing the parameterization I note
here? Is the expectation that I define a new cell for every single type?
Each cell might have a different binary format, so I'd have to describe:
(a) that they contain MAC addresses (so the "reader" knows to translate
    the ASCII strings into equivalent binary representation) and
(b) which key matches (it's not just "mac_address=xxxxx"; there may be
    many MAC addresses, with keys "ether_mac0", "ether_mac1",
    "wifi_mac0")
Part (a) especially doesn't really sound like the typical NVMEM, which
seems to pretend it provides raw access to these memory regions.

Additionally, VPD is not stored at a fixed address, nor are any
particular entries within it (it uses TLV), so it seems like there are
plenty of other bits of the nvmem.txt documentation I'd have to violate
to get there, such as the definition of 'reg':

reg:    specifies the offset in byte within the storage device.

And finally, this may be surmountable, but the existing APIs seem very
device tree centric. We use this same format on ACPI systems, and the
current series would theoretically work on both [1]. I'd have to rewrite
the current (OF-only) helpers to get equivalent support...

BTW, it's quite annoying that we have all of these:

fwnode_get_mac_address()
device_get_mac_address()
of_get_mac_address()
of_get_nvmem_mac_address()

and only 2 of those share any code! Brilliant!

[1]: https://patchwork.ozlabs.org/cover/956062/
[2]: https://lkml.org/lkml/2018/3/24/312

Brian

[1] Fortunately, I've only needed this on DT so far.