netdev - Re: [PATCH net-next v4 6/7] docs: netlink: document struct support for genetlink-legacy

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Message-ID: <20230324205224.18f31f14@kernel.org>
Date:   Fri, 24 Mar 2023 20:52:24 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Donald Hunter <donald.hunter@...il.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        donald.hunter@...hat.com
Subject: Re: [PATCH net-next v4 6/7] docs: netlink: document struct support
 for genetlink-legacy

On Fri, 24 Mar 2023 19:18:59 +0000 Donald Hunter wrote:
>  Legacy families can define C structures both to be used as the contents
> -of an attribute and as a fixed message header. The plan is to define
> -the structs in ``definitions`` and link the appropriate attrs.
> +of an attribute and as a fixed message header. Structs are defined
> +in ``definitions`` and referenced in operations or attributes.

We should call out that the structs in YAML are implicitly "packed"
(in the C sense of the word), so struct { u8 a; u16 b; u8 c; } is 
4 bytes not 6 bytes.

Any padding must be explicitly, C-like languages should infer the need
for explicit packing from whether the members are naturally aligned.

> +.. code-block:: yaml
> +
> +  definitions:
> +    -
> +      name: message-header
> +      type: struct
> +      members:
> +        -
> +          name: a
> +          type: u32
> +        -
> +          name: b
> +          type: string

Maybe not the most fortunate example :) cause I think that for
string/binary we'll need an explicit length. Maybe not for
last one if it's a flexible array... but that's rare in NL.

The rest LGTM, thanks!