[PATCH] docs, nvme: add a feature and quirk policy document

[Randy Dunlap]

Hi--

Just a couple of small nits below...

On 12/15/22 04:51, Christoph Hellwig wrote:
> 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 at kernel.dk>
> Signed-off-by: Keith Busch <kbusch at kernel.org>
> Signed-off-by: Sagi Grimberg <sagi at grimberg.me>
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  Documentation/process/index.rst               |  1 +
>  .../process/nvme-feature-and-quirk-policy.rst | 77 +++++++++++++++++++
>  MAINTAINERS                                   |  1 +
>  3 files changed, 79 insertions(+)
>  create mode 100644 Documentation/process/nvme-feature-and-quirk-policy.rst
> 
> diff --git a/Documentation/process/index.rst b/Documentation/process/index.rst
> index d4b6217472b0a0..0dc33994ddefc5 100644
> --- a/Documentation/process/index.rst
> +++ b/Documentation/process/index.rst
> @@ -50,6 +50,7 @@ Other guides to the community that are of interest to most developers are:
>     embargoed-hardware-issues
>     maintainers
>     researcher-guidelines
> +   nvme-feature-and-quirk-policy
>  
>  These are some overall technical guides that have been put here for now for
>  lack of a better place.
> diff --git a/Documentation/process/nvme-feature-and-quirk-policy.rst b/Documentation/process/nvme-feature-and-quirk-policy.rst
> new file mode 100644
> index 00000000000000..eee19f3d9904bd
> --- /dev/null
> +++ b/Documentation/process/nvme-feature-and-quirk-policy.rst
> @@ -0,0 +1,77 @@
> +.. 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 identifiers based quirks to work around such

                      Linux uses identifier-based quirks

> +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 rely on Linux quirk.

I would s/rely/relying/, but either way:

Reviewed-by: Randy Dunlap <rdunlap at infradead.org>

Thanks.

> diff --git a/MAINTAINERS b/MAINTAINERS
> index bb77a3ed9d5423..59e9f2dfa842ad 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -14827,6 +14827,7 @@ L:	linux-nvme at lists.infradead.org
>  S:	Supported
>  W:	http://git.infradead.org/nvme.git
>  T:	git://git.infradead.org/nvme.git
> +F:	Documentation/process/nvme-feature-and-quirk-policy.rst
>  F:	drivers/nvme/host/
>  F:	drivers/nvme/common/
>  F:	include/linux/nvme*

-- 
~Randy