On 08/14/2018 05:22 PM, Brian Norris wrote: > + 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: >>> 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. That type of stability and standardization, ok > > * 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@'. There are some recommended function names defined in devicetree-spec, not everything is covered since the spec is still lagging a bit behind as far as recommending names for what a modern SoC/board would embed (with some definition of "modern"). > > * 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. Sure, this is kind of a self inflicted problem but agreed this does exist. > >> 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/. Indeed they are not, likewise, we should probably update devicetree-spec to come up with standard names that of_alias_get_id() already consumes. > >>> (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 I see, in your current patch series that knowledge is pushed to both the VPD producer and the specific object lookup function, so this scales better. > (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") The key to lookup is definitively node specific, it is just unfortunate that there is not a better way to infer which key to lookup for (as opposed to just having to specify it directly) based on the Device Tree topology. By that I mean, if you have a "mac-address-lookup" property associated with Wi-Fi adapter #1 (with numbering starting at 0), then this automatically means looking up for "wifi_mac1", etc. > 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... All fair points, never mind NVMEM, I was just too keen on thinking this would be finally the way to make the consumers and producers of such information into a single API, but your proposal appears valid too. Is ChromeOS' directly inspired from the PCI's spec VPD? > > 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! Sounds like you just found another area to improve on ;) > >> [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. > -- Florian
