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Provide a simple introduction
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Provide a simple introduction the platform spec, and a brief
description of the overall structure.  This is essentially v3
of the proposal as submitted to the mailing list, along with
corrections and suggestions from all v3 reviewers, but in patch
form.

Signed-off-by: Al Stone <[email protected]>
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//
// introduction.adoc: provide an introduction to the document
//
// Provide a basic introduction to the overall document, defining some
// key terms and usages.
// Provide a basic introduction to the overall document.
//

# Introduction

This document is the RISC-V Profiles and Platform Specification (RVP2S).

This specification defines the hardware and firmware required to be able
to install and use one or more operating environments (OEs) on a platform
built around the RISC-V Instruction Set Architecture (ISA). The intent
is to capture the constraints necessary for a hardware platform so that
an operating environment can be assured it is compatible with that platform,
if the platform is in compliance with this specification.
to install and use one or more operating environments (OEs) on a computing
system built around the RISC-V Instruction Set Architecture (ISA). The
intent is to capture the constraints necessary for hardware so that an
operating environment can be assured it is compatible with that hardware,
if it is in compliance with the specification.

For the purposes of this specification an _operating environment_ can
range from a single application running on a bare metal system to a full
scale server operating system (OS) running a Linux distribution on a
commercial off-the shelf (COTS) system.

Constraints on the hardware and firmware comprising part of a platform
are key to this specification. For example, an OS needs to be able to
make some basic assumptions about the platform in order to boot; if there
are too many variations possible, it becomes unmanageable for the OS vendor
and could preclude using the platform at all. We would like to ensure this
does not happen.

This specification also sets out any necessary constraints on the RISCV-V
are key to this specification. However, these constraints are only meant
to define a baseline set of functionality. Vendors may add whatever
advanced functionality they wish beyond that, as long as it is discoverable
in some way. For example, any given platform may need an MMU or an IRQ
controller to boot, but may not require an I2C bus; the vendor can of course
add that bus if they wish, but it might not be part of the required baseline
for that platform.

This specification also sets out any necessary constraints on the RISC-V
Instruction Set Architecture (ISA) -- at all privilege levels -- that are
required to provide a consistent and predictable environment for running
operating systems.

The intent is to provide only the constraints necessary, and not to restrict
creativity.

## Platforms
The term _platform_ has already been used several times. The definition
used here is that a platform is the sum total of the hardware, firmware,
and software comprising a usable computer system. Examples include
smartphones, laptops, data center servers, single-board computers, tablets,
and almost anything in between. While this is not terribly precise, this
is intentional; this specification is not meant to constrain what a
platform is, but to clarify what software can expect of it.

## Profiles
The term _profile_ is frequently used in the RISC-V world. It is time
to provide a reasonably precise definition of the term and how it
is to be used. We do this since the specification itself sets down
requirements on the basis of what a profile needs.

Consider a platform as being built up by connecting together a number of
different interfaces -- the interfaces between hardware and firmware, or
firmware and software, or software and the end-user.

What we would like to do is define just the interfaces between the objects
that constitute a platform. This allows the implementations to be whatever
they need to be, as long as they abide by the interface definition. For
example, we may need a UEFI Runtime Services interface. Whether that is
implemented via EDK2 source, or is an interface to linuxboot is irrelevant.
The functionality defined by the interface is the important part.

Of particular interest in this specification are:

* The User Interface: how the end-user of a system interacts with it.
Examples might be various Linux-, BSD- or Windows based systems; they
might also include a specific application developed for a specific use.
* The Firmware Interface: how the User Interface makes use of S-mode. An
example is UEFI Runtime Services -- a list of the services needed in order
to allow the User Interface to work properly.
* The Supervisor Interface: how the Firmware Interface interacts with RISC-V
hardware threads (_harts_), providing functionality to the Firmware
Interface to start and stop them, for example. There will likely be
cases, too, where the User Interface interacts directly with this
functionality, such as when it provides mechanims needed that are not
defined by the Firmware Interface.
* The Hardware Interface: what the previously mentioned interfaces can expect
to be present; in a way this is the interface to the outside world, but most
likely it would be a list of the hardware components that are guaranteed
to be present, and what standards they comply with.
* The Processor Interface: this interface defines what to expect of the
processor. In the case of RISC-V this would include the parts of the ISA
that and are not implemented.
* The Boot Interface: this is what a user would see at a cold start of the
system -- for the most part these days, this includes U-Boot bootm/booti
or the UEFI Shell, though it could just as easily be linuxboot or coreboot.
This defines the functionality needed to boot the system, update the
firmware and possibly manage the system remotely; this will have to
interact closely with OS booting protocols.
For the above interfaces, a platform must provide an instance of each one;
a platform would not be usable without all of them. There are some interfaces
that could be optional:

* The Enumeration Interface: how the User Interface determine what hardware
is present on the platform; this will need to include mechanisms for finding
self-identifying hardware such as PCIe, and for hardware that does not
identify itself such as UARTs, the motherboard, or CPU topology. This
is typically ACPI in general purpose computing platforms, and Device Tree
in embedded systems. Note that this may include further information such
as SMBIOS data. At the same time, though, a very specific-use platform
may already encapsulate all of this knowledge in the software being run
so neither ACPI nor DT are needed.
* The Secondary Boot Loader: not all OEs require this. For the ones that
do, it must behave correctly. GRUB2 is expected by all recent Linux
distributions, for example.
For each of the interfaces listed above, there is a domain of possibilities.
Let us look at each domain of interfaces as an element in an n-tuple defined
as follows:

```
N = { user, firmware, supervisor, hardware, processor, boot,
enumeration, secondary-boot-loader }
```

Not all of the permutations of this set make sense. Not all of them are
necessarily useful. However, it does allow us to provide a more precise
definition of a profile -- a _profile_ is a value in **_N_**, a profile-tuple,
if you will.

### An Example Profile
The test of any formalism is whether or not it is usable. So, let’s define
a general purpose Linux platform that could be used to run one of the various
Linux distributions such as RHEL, Fedora, SuSE, Debian, ArchLinux or maybe
even Gentoo and Slackware. The profile might be:

```
LinuxServer = { Linux, UEFI, OpenSBI, rack-mount, RV64GC, UEFI Shell,
ACPI, GRUB2}
```

This is just an example of what is possible; it is not definitive. A proper
profile for Linux server distributions will be provided later in this
specification. Regardless, this could tell us:

* The User Interface is a Linux system of some flavor.
* The Firmware Interface is UEFI, built upon SBI.
* The Supervisor Interface is OpenSBI.
* The Hardware Interface is for rack-mount server systems -- defining
this is where we would have to list what devices that implies.
* The Processor Interface is RV64GC (U-mode, and S-mode)
* The Boot Interface is the UEFI Shell.
* The Enumeration Interface is ACPI, so we now know how to identify
all of the hardware being used.
* The Secondary Boot Loader is GRUB2, so Linux now has some idea of
how it will be booted.
And, as long as the interface definitions are fairly clear, we can now
determine what would be in a platform that supports the ```LinuxServerDistro```
profile. It is the purview of this platform specification to (a) define
the profiles of interest, and then (b) ensure that each of the terms
in the n-tuple -- the interfaces -- are clearly defined.

### Other Distros
While the definition of a ```LinuxServerDistro``` profile used above would
seem to preclude non-Linux operating systems, defining a profile as an
n-tuple gives us a way of handling this. For example, let’s suppose we
define profiles for Windows and most BSD-based systems:

```
WindowsDistro = { Windows, UEFI, OpenSBI, rack-mount, RV64GC, UEFI Shell,
ACPI, WindowsLoader}

BSDServer = { NetBSD, UEFI, OpenSBI, rack-mount, RV64GC, UEFI Shell,
ACPI, GPT}
```

Again, these are only examples, and not definitive. Given what we know
about these sorts of systems, we could assert the following:

```
LinuxServer = WindowsDistro = BSDServer
```

That is, all of these are essentially identical in the things that matter
to this specification. We know that’s not exactly true, of course, but this
does give us the tool to start reasoning about such things.

### Notation
Whilst set notation is all well and good, we need something a bit more
practical. For that, we can borrow from the example provided by GCC.
A profile tuple would then be:

```
N = user-firmware-supervisor-hardware-processor-boot-enumeration-secondary
```

Or, for the example profiles listed above:

```
linux-server = linux-uefi-opensbi-rackmount-rv64gc-uefi-acpi-grub2
windows-distro = windows-uefi-opensbi-rackmount-rv64gc-uefi-acpi-windows
bsd-server = bsd-uefi-opensbi-rackmount-rv64gc-uefi-acpi-gpt
```

For the long term, these could be converted into values and captured in
firmware to be relayed to the OS, perhaps as part of the Chosen node in
the DT used to pass information to the Linux kernel. In this manner, the
platform would describe itself, instead of the OS having to make an
educated guess.

### Revisions
Over time, the definitions of the elements in this profile-tuple will
change. It’s simply the nature of the problem -- new hardware will be
defined, and ultimately will be assumed to be present, or old hardware
will get deprecated (e.g., CXL, and perhaps some day UARTs, respectively).
Hence, we will need to be able to note the revision of an element definition
for a profile definition to be complete. Again, we could have the firmware
convey this information to the OS as an additional byte string (how is TBD).
For practical use, we may wish to append the revisions to the tuple notation:

```
N = user-firmware-supervisor-hardware-processor-boot-enumeration-secondary \
:ur-fr-sr-hr-pr-br-er-sr
```

Or, for the example profiles listed above:

```
linux-server = linux-uefi-opensbi-rackmount-rv64gc-uefi-acpi-grub2: \
0-0-0-0-0-0-0-0
windows-distro = windows-uefi-opensbi-rackmount-rv64gc-uefi-acpi-windows: \
0-3-5-5-0-1-0-2
bsd-server = bsd-uefi-opensbi-rackmount-rv64gc-uefi-acpi-gpt: \
1-0-3-4-0-1-0-2
```

### Some Notes
* Profiles are _only_ a property of platforms, _not_ harts.
* This section uses example profiles; these are not intended to be definitive.
Proper profile definitions will appear in following sections of the
specification.
* This section presents the current definition of a platform-tuple. This
may change as we know more.


# Profiles
## Linux Profiles
### Linux Development SBC Profile
### Linux Laptop Profile
### Linux Desktop Profile
### Linux Server Profile
```
linux-server = linux-uefi-opensbi-rackmount-rv64gc-uefi-acpi-grub2
```

## BSD Profiles

## Windows Profiles

## Bare Metal SBC Profiles

# Interface Specifications

## User Interfaces
### Linux
### Windows
### BSD

## Firmware Interfaces
### UEFI
### U-Boot

## Supervisor Interfaces
### SBI

## Hardware Interfaces
### Console
### I/O

## Processor Interfaces

## Boot Interfaces
### UEFI Shell
### U-Boot bootm/booti

## Enumeration Interfaces
### ACPI
### Device Tree
### SMBIOS

## Secondary Boot Loaders
### GRUB2

The structure of the document is straightforward. First, basic terminology
is defined, in particular the terms _profile_ and _platform_ that form the
basis for the structure of the rest of the specification. Second, the
currently defined profiles are described, setting out the use of the RISC-V
architecture in a processor. Third, the current defined platforms are
described with specific hardware, firmware, and software requirements.
And finally, checklists are provided in the appendices to make it easier
to determine if a new system meets a given platform specification.

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