netdev - Re: [PATCH 1/7] debugfs: Add debugfs_create_xul() for hexadecimal unsigned long

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Date:   Tue, 22 Oct 2019 10:03:36 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ludovic Desroches <ludovic.desroches@...rochip.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Jaehoon Chung <jh80.chung@...sung.com>,
        "David S . Miller" <davem@...emloft.net>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Johannes Berg <johannes@...solutions.net>,
        Linux MMC List <linux-mmc@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        netdev <netdev@...r.kernel.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/7] debugfs: Add debugfs_create_xul() for hexadecimal
 unsigned long

Hi Joe,

On Mon, Oct 21, 2019 at 5:37 PM Joe Perches <joe@...ches.com> wrote:
> On Mon, 2019-10-21 at 16:37 +0200, Geert Uytterhoeven wrote:
> > The existing debugfs_create_ulong() function supports objects of
> > type "unsigned long", which are 32-bit or 64-bit depending on the
> > platform, in decimal form.  To format objects in hexadecimal, various
> > debugfs_create_x*() functions exist, but all of them take fixed-size
> > types.
> >
> > Add a debugfs helper for "unsigned long" objects in hexadecimal format.
> > This avoids the need for users to open-code the same, or introduce
> > bugs when casting the value pointer to "u32 *" or "u64 *" to call
> > debugfs_create_x{32,64}().
> []
> > diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> []
> > @@ -356,4 +356,14 @@ static inline ssize_t debugfs_write_file_bool(struct file *file,
> >
> >  #endif
> >
> > +static inline void debugfs_create_xul(const char *name, umode_t mode,
> > +                                   struct dentry *parent,
> > +                                   unsigned long *value)
> > +{
> > +     if (sizeof(*value) == sizeof(u32))
> > +             debugfs_create_x32(name, mode, parent, (u32 *)value);
> > +     else
> > +             debugfs_create_x64(name, mode, parent, (u64 *)value);
>
> trivia: the casts are unnecessary.

They are necessary, in both calls (so using #ifdef as suggested below
won't help):

    include/linux/debugfs.h:375:42: error: passing argument 4 of
‘debugfs_create_x32’ from incompatible pointer type
[-Werror=incompatible-pointer-types]
       debugfs_create_x32(name, mode, parent, value);
                                              ^~~~~
    include/linux/debugfs.h:114:6: note: expected ‘u32 * {aka unsigned
int *}’ but argument is of type ‘long unsigned int *’
     void debugfs_create_x32(const char *name, umode_t mode, struct
dentry *parent,
          ^~~~~~~~~~~~~~~~~~
    include/linux/debugfs.h:377:42: error: passing argument 4 of
‘debugfs_create_x64’ from incompatible pointer type
[-Werror=incompatible-pointer-types]
       debugfs_create_x64(name, mode, parent, value);
                                              ^~~~~
    include/linux/debugfs.h:116:6: note: expected ‘u64 * {aka long
long unsigned int *}’ but argument is of type ‘long unsigned int *’
     void debugfs_create_x64(const char *name, umode_t mode, struct
dentry *parent,
          ^~~~~~~~~~~~~~~~~~

> This might be more sensible using #ifdef
>
> static inline void debugfs_create_xul(const char *name, umode_t mode,
>                                       struct dentry *parent,
>                                       unsigned long *value)
> {
> #if BITS_PER_LONG == 64
>         debugfs_create_x64(name, mode, parent, value);
> #else
>         debugfs_create_x32(name, mode, parent, value);
> #endif
> }

... at the expense of the compiler checking only one branch.

Just like "if (IS_ENABLED(CONFIG_<foo>)" (when possible) is preferred
over "#ifdef CONFIG_<foo>" because of compile-coverage, I think using
"if" here is better than using "#if".

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds