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

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Message-ID: <87r37kmsqb.fsf@gmail.com>
Date:   Thu, 13 Oct 2016 12:46:36 +0200
From:   Nicolai Stange <nicstange@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nicolai Stange <nicstange@...il.com>,
        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

Arnd Bergmann <arnd@...db.de> writes:

> 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?

I'd be happy to (won't be able to do this before tomorrow though).

Thanks,

Nicolai