[OCP-LOCK]: METD and AUX registers should be write-only

[james000]

In the latest specification, the MEK register is defined as write-only, but the METD and AUX registers are defined as read-write. In what use case would the Caliptra firmware read back a value from the METD and AUX registers? How would the device hardware know which entry to return?

The current LOCK commands are all one way: device firmware provides the values and Caliptra firmware issues the DMA transactions to write these blobs into the registers. There are no LOCK commands to support reading. I propose the METD and AUX registers should also be write-only.

[clundin25]

Hey @james000

MEK is a secret, while METD and AUX are not. Can you share any use cases for this proposal?

@GwangbaeCHOI PTAL

[GwangbaeCHOI]

Hi @james000,

There is no use case in which the Caliptra RT reads back from the METD and AUX registers. So, even if the METD and AUX registers are write-only, it will not cause any issue.

Within the spec, they are defined as RW as we were assuming that the default register access is RW and changing it into write-only or read-only would require additional integration effort.

[james000]

It sounds like the specification should make them write-only as there is no use case for them to be read back.