Documentation: riscv: add patch acceptance guidelines

Formalize, in kernel documentation, the patch acceptance policy for
arch/riscv. In summary, it states that as maintainers, we plan to
only accept patches for new modules or extensions that have been
frozen or ratified by the RISC-V Foundation.

We've been following these guidelines for the past few months. In the
meantime, we've received quite a bit of feedback that it would be
helpful to have these guidelines formally documented.

Based on a suggestion from Matthew Wilcox, we also add a link to this
file to Documentation/process/index.rst, to make this document easier
to find. The format of this document has also been changed to align
to the format outlined in the maintainer entry profiles, in accordance
with comments from Jon Corbet and Dan Williams.

Signed-off-by: Paul Walmsley <[e-mail]>
Reviewed-by: Palmer Dabbelt <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Albert Ou <[e-mail]>
Cc: Krste Asanovic <[e-mail]>
Cc: Andrew Waterman <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>

Documentation/process/index.rst

···
    volatile-considered-harmful
    botching-up-ioctls
    clang-format
+   ../riscv/patch-acceptance
 
 .. only::  subproject and html
 

Documentation/riscv/index.rst

···
 
     boot-image-header
     pmu
+    patch-acceptance
 
 .. only::  subproject and html
 

Documentation/riscv/patch-acceptance.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+arch/riscv maintenance guidelines for developers
+================================================
+
+Overview
+--------
+The RISC-V instruction set architecture is developed in the open:
+in-progress drafts are available for all to review and to experiment
+with implementations.  New module or extension drafts can change
+during the development process - sometimes in ways that are
+incompatible with previous drafts.  This flexibility can present a
+challenge for RISC-V Linux maintenance.  Linux maintainers disapprove
+of churn, and the Linux development process prefers well-reviewed and
+tested code over experimental code.  We wish to extend these same
+principles to the RISC-V-related code that will be accepted for
+inclusion in the kernel.
+
+Submit Checklist Addendum
+-------------------------
+We'll only accept patches for new modules or extensions if the
+specifications for those modules or extensions are listed as being
+"Frozen" or "Ratified" by the RISC-V Foundation.  (Developers may, of
+course, maintain their own Linux kernel trees that contain code for
+any draft extensions that they wish.)
+
+Additionally, the RISC-V specification allows implementors to create
+their own custom extensions.  These custom extensions aren't required
+to go through any review or ratification process by the RISC-V
+Foundation.  To avoid the maintenance complexity and potential
+performance impact of adding kernel code for implementor-specific
+RISC-V extensions, we'll only to accept patches for extensions that
+have been officially frozen or ratified by the RISC-V Foundation.
+(Implementors, may, of course, maintain their own Linux kernel trees
+containing code for any custom extensions that they wish.)

MAINTAINERS

···
 M:	Palmer Dabbelt <[e-mail]>
 M:	Albert Ou <[e-mail]>
 L:	[e-mail]
+P:	Documentation/riscv/patch-acceptance.rst
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git
 S:	Supported
 F:	arch/riscv/