docs, nvme: add a feature and quirk policy document

This adds a document about what specification features are supported by
the Linux NVMe driver, and what qualifies for a quirk if an implementation
has problems following the specification.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>

Documentation/maintainer/maintainer-entry-profile.rst

@@ -104,3 +104,4 @@ to do something different in the near future.
    ../riscv/patch-acceptance
    ../driver-api/media/maintainer-entry-profile
    ../driver-api/vfio-pci-device-specific-driver-acceptance
+   ../nvme/feature-and-quirk-policy

Documentation/nvme/feature-and-quirk-policy.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=======================================
+Linux NVMe feature and and quirk policy
+=======================================
+
+This file explains the policy used to decide what is supported by the
+Linux NVMe driver and what is not.
+
+
+Introduction
+============
+
+NVM Express is an open collection of standards and information.
+
+The Linux NVMe host driver in drivers/nvme/host/ supports devices
+implementing the NVM Express (NVMe) family of specifications, which
+currently consists of a number of documents:
+
+ - the NVMe Base specification
+ - various Command Set specifications (e.g. NVM Command Set)
+ - various Transport specifications (e.g. PCIe, Fibre Channel, RDMA, TCP)
+ - the NVMe Management Interface specification
+
+See https://nvmexpress.org/developers/ for the NVMe specifications.
+
+
+Supported features
+==================
+
+NVMe is a large suite of specifications, and contains features that are only
+useful or suitable for specific use-cases. It is important to note that Linux
+does not aim to implement every feature in the specification.  Every additional
+feature implemented introduces more code, more maintenance and potentially more
+bugs.  Hence there is an inherent tradeoff between functionality and
+maintainability of the NVMe host driver.
+
+Any feature implemented in the Linux NVMe host driver must support the
+following requirements:
+
+  1. The feature is specified in a release version of an official NVMe
+     specification, or in a ratified Technical Proposal (TP) that is
+     available on NVMe website. Or if it is not directly related to the
+     on-wire protocol, does not contradict any of the NVMe specifications.
+  2. Does not conflict with the Linux architecture, nor the design of the
+     NVMe host driver.
+  3. Has a clear, indisputable value-proposition and a wide consensus across
+     the community.
+
+Vendor specific extensions are generally not supported in the NVMe host
+driver.
+
+It is strongly recommended to work with the Linux NVMe and block layer
+maintainers and get feedback on specification changes that are intended
+to be used by the Linux NVMe host driver in order to avoid conflict at a
+later stage.
+
+
+Quirks
+======
+
+Sometimes implementations of open standards fail to correctly implement parts
+of the standards.  Linux uses identifier-based quirks to work around such
+implementation bugs.  The intent of quirks is to deal with widely available
+hardware, usually consumer, which Linux users can't use without these quirks.
+Typically these implementations are not or only superficially tested with Linux
+by the hardware manufacturer.
+
+The Linux NVMe maintainers decide ad hoc whether to quirk implementations
+based on the impact of the problem to Linux users and how it impacts
+maintainability of the driver.  In general quirks are a last resort, if no
+firmware updates or other workarounds are available from the vendor.
+
+Quirks will not be added to the Linux kernel for hardware that isn't available
+on the mass market.  Hardware that fails qualification for enterprise Linux
+distributions, ChromeOS, Android or other consumers of the Linux kernel
+should be fixed before it is shipped instead of relying on Linux quirks.

MAINTAINERS

@@ -14827,6 +14827,7 @@ L:	linux-nvme@lists.infradead.org
 S:	Supported
 W:	http://git.infradead.org/nvme.git
 T:	git://git.infradead.org/nvme.git
+F:	Documentation/nvme/
 F:	drivers/nvme/host/
 F:	drivers/nvme/common/
 F:	include/linux/nvme*