linux-kernel - Re: [PATCH 1/2] debugfs: improve DEFINE_DEBUGFS_ATTRIBUTE for !CONFIG_DEBUGFS

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Date:   Thu, 13 Oct 2016 12:29:44 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Nicolai Stange <nicstange@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>,
        Darren Hart <dvhart@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] debugfs: improve DEFINE_DEBUGFS_ATTRIBUTE for !CONFIG_DEBUGFS_FS

On Thursday, October 13, 2016 11:59:54 AM CEST Nicolai Stange wrote:
> >  
> > +ssize_t debugfs_attr_read(struct file *file, char __user *buf,
> > +                     size_t len, loff_t *ppos);
> > +ssize_t debugfs_attr_write(struct file *file, const char __user *buf,
> > +                     size_t len, loff_t *ppos);
> > +
> > +#define DEFINE_DEBUGFS_ATTRIBUTE(__fops, __get, __set, __fmt)                \
> > +static int __fops ## _open(struct inode *inode, struct file *file)   \
> > +{                                                                    \
> > +     __simple_attr_check_format(__fmt, 0ull);                        \
> > +     return simple_attr_open(inode, file, __get, __set, __fmt);      \
> > +}                                                                    \
> > +static const struct file_operations __fops = {                               \
> > +     .owner   = THIS_MODULE,                                         \
> > +     .open    = __fops ## _open,                                     \
> > +     .release = simple_attr_release,                                 \
> > +     .read    = debugfs_attr_read,                                   \
> > +     .write   = debugfs_attr_write,                                  \
> 
> This depends on GCC dead code elimination to always work for this
> situation, otherwise we'd get undefined references to
> debugfs_attr_read/write(), right?

Correct.

> In order to avoid having to test your patch against all those older
> versions of GCC, can we have a safety net here and define some dummy
> debugfs_attr_read/write() for the !CONFIG_DEBUGFS case?

The question of dead-code elimination in older gcc versions comes up
occasionally, and I think all versions that are able to build the
kernel these days get this right all the time, otherwise any code
using IS_ENABLED() helpers to control the calling of external interfaces
would be broken.

We could probably use that macro here if you think that's better
and do:

static const struct file_operations __fops = {
    .owner   = THIS_MODULE,
    .open    = IS_ENABLED(CONFIG_DEBUGFS_FS) ? __fops ## _open : NULL,                                     
    ...

> If nothing else, it would IMHO make the !CONFIG_DEBUGFS case more
> understandable because one had not to figure out that this actually
> relies on dead code elimination to work.

Sure, that's fine. Can you do the new version of that patch with
the change then?

	Arnd