Re: [PATCH RFC linux] dt-bindings: nvmem: Add binding for U-Boot environment NVMEM provider

On Thu, 14 Oct 2021 11:30:13 +0100
Srinivas Kandagatla [off-list ref] wrote:

On 14/10/2021 11:06, Marek Behún wrote:

quoted

On Thu, 14 Oct 2021 09:26:27 +0100
Srinivas Kandagatla [off-list ref] wrote:
  

quoted

On 14/10/2021 00:20, Marek Behún wrote:  

quoted

Add device tree bindings for U-Boot environment NVMEM provider.

U-Boot environment can be stored at a specific offset of a MTD
device, EEPROM, MMC, NAND or SATA device, on an UBI volume, or in
a file on a filesystem.

The environment can contain information such as device's MAC
address, which should be used by the ethernet controller node.
     

Have you looked at
./Documentation/devicetree/bindings/mtd/partitions/nvmem-cells.yaml
?  

Hello srini,

yes, I have. What about it? :)

That binding won't work for u-boot-env, because the data are stored
in a different way. A cell does not have a constant predetermined
offset on the MTD.  

Can't you dynamically update the nodes before nvmem-provider is
registered?

Are you talking about dynamically updating nvmem-cell OF nodes, adding
reg properties with actual offsets and lengths found after parsing?

quoted

The variables are stored as a sequence of values of format
"name=value", separated by '\0's, for example:
   board=turris_mox\0ethaddr=00:11:22:33:44:55\0bootcmd=run
distro_bootcmd\0.... Chaning lengths of values of variables, or
deleting variables, moves the data around. Integers and MAC
addresses are stored as strings, and so on. 

Do you already have a provider driver for handing this.

Not yet, I will send the proposal together with a driver in next
round.

How is pre parsing cell info and post processing data planned to be
handled?

My plan was to read the variables from the u-boot-env partition, create
a nvmem-cell for each variable, and then pair the ones mentioned in
device tree with their DT nodes, and post-process according to type
(post-processing would be done only for those mentioned in device tree,
others would be left as strings).

Currently in nvmem core we do check for "reg" property for each cell, 
unless the provider driver is adding/updating dt entries dynamically 
before registering nvmem provider

I don't think updaring DT entries dynamically is a correct solution at
all. Is this done in Linux? Updating device properties is something
different, but changing DT properties seems wrong to me.

It will not work as it is. Alteast this is what I suggested in similar
case where cell information is in tlv format.

Hmm. OK, I shall try to implement a driver for this and then will
return to you.

Secondly mac-address seems to be delimited, we recently introduced
post processing callback for provider driver [1], which should help
in this case.

Cool, I shall use that.

If the nvmem-cell names are standard like "mac-address" then you do
not need to add a new "type" binding to cell too, you can do
post-processing based on name.

I plan to add functions
  of_nvmem_get_mac_address()
  nvmem_get_mac_address()
which would look at (in this order):
  mac-address, address, mac-address-backup
We have to keep the name "address" for backwards compatibility with one
driver that uses this (drivers/net/ethernet/ni/nixge.c)

Thanks.

Marek