Hi Michael, Srinivas, + Thomas and Robert michael@xxxxxxxx wrote on Fri, 2 Sep 2022 00:18:37 +0200: > This is now the third attempt to fetch the MAC addresses from the VPD > for the Kontron sl28 boards. Previous discussions can be found here: > https://lore.kernel.org/lkml/20211228142549.1275412-1-michael@xxxxxxxx/ > > > NVMEM cells are typically added by board code or by the devicetree. But > as the cells get more complex, there is (valid) push back from the > devicetree maintainers to not put that handling in the devicetree. > > Therefore, introduce NVMEM layouts. They operate on the NVMEM device and > can add cells during runtime. That way it is possible to add more complex > cells than it is possible right now with the offset/length/bits > description in the device tree. For example, you can have post processing > for individual cells (think of endian swapping, or ethernet offset > handling). > > The imx-ocotp driver is the only user of the global post processing hook, > convert it to nvmem layouts and drop the global post pocessing hook. Please > note, that this change is only compile-time tested. These layouts are an excellent idea. I actually have a new use case for them. In modern Ethernet switches which follow the ONIE standard [1] there is an nvmem device which contains a standardized type-length-value array with many information about manufacturing and mac addresses. There is no "static" pattern there and anyway so many possible entries that it would be very tedious to list all of them in the bindings, as each manufacturer chooses what it want to export on each of its devices (although reading the data sequentially and extracting the cells is rather straightforward). Moreover, the specification [1] does not define any storage device type, so it can be eg. an MTD device or an EEPROM. Having an nvmem device provider separated from the nvmem cells provider makes complete sense, the "layout" drivers idea proposed by Michael seem to be a perfect fit. Srinivas, can you give us an update on what you think about this series (not a commitment, just how you feel it overall)? Michael, is there a v3 in preparation? I'll try to write something on top of your v2 otherwise. > You can also have cells which have no static offset, like the > ones in an u-boot environment. The last patches will convert the current > u-boot environment driver to a NVMEM layout and lifting the restriction > that it only works with mtd devices. But as it will change the required > compatible strings, it is marked as RFC for now. It also needs to have > its device tree schema update which is left out here. These two patches > are not expected to be applied, but rather to show another example of > how to use the layouts. Actually I think these two matches make complete sense, right now one can only use the u-boot-env cells if the environment is stored in an mtd device, of course this covers many cases but not all of them and it would be really nice to have this first layout example merged, not only on the mailing list. > For now, the layouts are selected by a specific compatible string in a > device tree. E.g. the VPD on the kontron sl28 do (within a SPI flash node): > compatible = "kontron,sl28-vpd", "user-otp"; > or if you'd use the u-boot environment (within an MTD patition): > compatible = "u-boot,env", "nvmem"; > > The "user-otp" (or "nvmem") will lead to a NVMEM device, the > "kontron,sl28-vpd" (or "u-boot,env") will then apply the specific layout > on top of the NVMEM device. Thanks, Miquèl
