linux-kernel - Re: [PATCH 09/11] UAPI: ndctl: Fix g++-unsupported initialisation in headers [ver #2]

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Date:   Tue, 25 Sep 2018 13:22:21 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Linux API <linux-api@...r.kernel.org>,
        linux-kbuild@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/11] UAPI: ndctl: Fix g++-unsupported initialisation in
 headers [ver #2]

On Thu, Sep 6, 2018 at 2:19 AM David Howells <dhowells@...hat.com> wrote:
>
> The following code in the linux/ndctl header file:
>
>         static inline const char *nvdimm_bus_cmd_name(unsigned cmd)
>         {
>                 static const char * const names[] = {
>                         [ND_CMD_ARS_CAP] = "ars_cap",
>                         [ND_CMD_ARS_START] = "ars_start",
>                         [ND_CMD_ARS_STATUS] = "ars_status",
>                         [ND_CMD_CLEAR_ERROR] = "clear_error",
>                         [ND_CMD_CALL] = "cmd_call",
>                 };
>
>                 if (cmd < ARRAY_SIZE(names) && names[cmd])
>                         return names[cmd];
>                 return "unknown";
>         }
>
> is broken in a number of ways:
>
>  (1) ARRAY_SIZE() is not generally defined.
>
>  (2) g++ does not support "non-trivial" array initialisers fully yet.
>
>  (3) Every file that calls this function will acquire a copy of names[].
>
> The same goes for nvdimm_cmd_name().
>
> Fix all three by converting to a switch statement where each case returns a
> string.  That way if cmd is a constant, the compiler can trivially reduce it
> and, if not, the compiler can use a shared lookup table if it thinks that is
> more efficient.
>
> A better way would be to remove these functions and their arrays from the
> header entirely.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Dan Williams <dan.j.williams@...el.com>

Acked-by: Dan Williams <dan.j.williams@...el.com>

...again let me know if you'll take this with g++ series or want me to
carry it directly.