1.. SPDX-License-Identifier: GPL-2.0
2
3=======================================
4Linux NVMe feature and and quirk policy
5=======================================
6
7This file explains the policy used to decide what is supported by the
8Linux NVMe driver and what is not.
9
10
11Introduction
12============
13
14NVM Express is an open collection of standards and information.
15
16The Linux NVMe host driver in drivers/nvme/host/ supports devices
17implementing the NVM Express (NVMe) family of specifications, which
18currently consists of a number of documents:
19
20 - the NVMe Base specification
21 - various Command Set specifications (e.g. NVM Command Set)
22 - various Transport specifications (e.g. PCIe, Fibre Channel, RDMA, TCP)
23 - the NVMe Management Interface specification
24
25See https://nvmexpress.org/developers/ for the NVMe specifications.
26
27
28Supported features
29==================
30
31NVMe is a large suite of specifications, and contains features that are only
32useful or suitable for specific use-cases. It is important to note that Linux
33does not aim to implement every feature in the specification.  Every additional
34feature implemented introduces more code, more maintenance and potentially more
35bugs.  Hence there is an inherent tradeoff between functionality and
36maintainability of the NVMe host driver.
37
38Any feature implemented in the Linux NVMe host driver must support the
39following requirements:
40
41  1. The feature is specified in a release version of an official NVMe
42     specification, or in a ratified Technical Proposal (TP) that is
43     available on NVMe website. Or if it is not directly related to the
44     on-wire protocol, does not contradict any of the NVMe specifications.
45  2. Does not conflict with the Linux architecture, nor the design of the
46     NVMe host driver.
47  3. Has a clear, indisputable value-proposition and a wide consensus across
48     the community.
49
50Vendor specific extensions are generally not supported in the NVMe host
51driver.
52
53It is strongly recommended to work with the Linux NVMe and block layer
54maintainers and get feedback on specification changes that are intended
55to be used by the Linux NVMe host driver in order to avoid conflict at a
56later stage.
57
58
59Quirks
60======
61
62Sometimes implementations of open standards fail to correctly implement parts
63of the standards.  Linux uses identifier-based quirks to work around such
64implementation bugs.  The intent of quirks is to deal with widely available
65hardware, usually consumer, which Linux users can't use without these quirks.
66Typically these implementations are not or only superficially tested with Linux
67by the hardware manufacturer.
68
69The Linux NVMe maintainers decide ad hoc whether to quirk implementations
70based on the impact of the problem to Linux users and how it impacts
71maintainability of the driver.  In general quirks are a last resort, if no
72firmware updates or other workarounds are available from the vendor.
73
74Quirks will not be added to the Linux kernel for hardware that isn't available
75on the mass market.  Hardware that fails qualification for enterprise Linux
76distributions, ChromeOS, Android or other consumers of the Linux kernel
77should be fixed before it is shipped instead of relying on Linux quirks.
78