[RFC] eSPI: Exploring support and possible options in Linux

["M, Krishnamoorthi" <krishnamoorthi.m@amd.com>]

Hi All,

I would like to start a discussion around the growing need for eSPI 
support in Linux, particularly on AMD platforms.

We are seeing increasing adoption of eSPI for multiple use cases beyond 
flash access, including channel-specific functionality such as 
Peripheral, Virtual Wire, and OOB communication. In some of our initial 
work, we explored extending the existing SPI subsystem to support parts 
of these requirements. While this works for certain cases, it also 
exposes some limitations as additional eSPI functionality and 
integrations are introduced.

Unlike traditional SPI transfers, eSPI provides a more structured 
protocol with multiple logical channels, negotiated capabilities, 
message-oriented communication, and asynchronous events. Because of 
this, some of the newer eSPI use cases do not appear to fit naturally 
into the SPI subsystem model.

As more channel-specific functionality is being adopted, I wanted to 
understand whether the current direction of extending SPI is the 
preferred long-term approach, or if there are alternative kernel 
abstractions/frameworks that may be better suited for handling eSPI 
semantics.

I would appreciate any feedback or pointers from the community, 
particularly around the following:

   * Whether there has been any prior work, RFCs, discussions, or
     attempts toward generic eSPI framework support in Linux.
   * Would continuing to extend SPI be the recommended approach, or are
     there other subsystem models that may fit better?
   * Any historical context, architectural considerations, or lessons
     learned from previous attempts in this area.

Any guidance or suggestions would be very helpful in understanding the 
right direction for supporting these evolving eSPI use cases.

Thanks in advance for your time and feedback.

Thanks,
Krishna