On Fri, Feb 21, 2025 at 12:29:03PM -0600, Stuart Yoder wrote: > > > On 2/21/25 8:02 AM, Sudeep Holla wrote: > > Hi Sumit, > > > > On Fri, Feb 21, 2025 at 07:16:35PM +0530, Sumit Garg wrote: > > > On Mon, Feb 17, 2025 at 10:56:58AM -0600, Stuart Yoder wrote: > > > > > > > > I don't see how changing TPM discovery to be via FF-A directly > > > > would improve maintainability. > > > > > > You are considering ACPI at this point but when people want to use this > > > TPM over FF-A on a platform using DT then it will require corresponding > > > DT bindings. After that each platform has to enable TPM over FF-A in > > > their corresponding ACPI/DT. All that won't be needed with auto > > > discovery over FF-A. > > Yes, we would need a new DT binding. > Not sure how that would look like, so I will hold off my comments on this topic. But we really should strive towards auto-discovery as much as possible. > > I hear you and completely agree. However, someone thought it was a good idea > > to align with other start methods and duplicate information in the TCG ACPI > > specification. This is definitely a bad idea, as it may contradict the > > firmware. All we needed was a simple flag to indicate whether FF-A is the > > start method. > > Do you mean a flag exposed via ACPI? If you do FF-A based discovery you > don't even need that. Everything could be determined via an FF-A > interface. > > > It sounds like a classic case of misalignment between specification authors > > and practical implementation needs. Instead of a simple flag to indicate FF-A > > as the start method, duplicating information in the TCG ACPI specification > > seems unnecessary and potentially problematic—especially if it risks > > conflicting with firmware behavior. > > There is a lot of history, but I think it was simply that ACPI > advertisement of an FF-A based TPM seemed like the approach > with the least friction. And Linux is not the only target OS. > I guess so. I understand sometimes we need to consider multiple target OS. > > Anyway, I can't comment on how we ended up here, but this seems to be the reality. > > I don't think we are locked into ACPI (or DT) only discovery. > It's possible that with a modest delta on top of this patch series > that the tpm_crb driver could also probe based on FF-A. > > The CRB over FF-A spec (DEN0138) could be extended in a backwards > compatible way to expose additional info like the base address of the > CRB. > Ideally, we should manage with dynamic buffers. But I do understand the reasons why we may need static curve outs. I prefer the ffa client driver take care of that without needing to build FF-A bindings just for that. I will wait and see how all these shape up (soon ?) -- Regards, Sudeep
