linux-kernel - Re: [PATCH v2] debugfs: improve DEFINE_DEBUGFS_ATTRIBUTE for !CONFIG_DEBUG

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Date:   Fri, 21 Oct 2016 12:22:58 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Nicolai Stange <nicstange@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>,
        Darren Hart <dvhart@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] debugfs: improve DEFINE_DEBUGFS_ATTRIBUTE for
 !CONFIG_DEBUG_FS

On Thu, 2016-10-20 at 22:07 +0200, Nicolai Stange wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> The slp_s0_residency_usec debugfs file currently uses
> DEFINE_DEBUGFS_ATTRIBUTE(), but that macro cannot really be used to
> define files outside of the debugfs code, as it has no reference to
> the get/set functions if CONFIG_DEBUG_FS is not defined:
> 
> drivers/platform/x86/intel_pmc_core.c:80:12: error:
> ‘pmc_core_dev_state_get’ defined but not used [-Werror=unused-
> function]
> 
> This fixes the macro to always contain the reference, and instead rely
> on the stubbed-out debugfs_create_file to not actually refer to
> its arguments so the compiler can still drop the reference.
> This works because the attribute definition is always 'static',
> and the dead-code removal silently drops all static symbols
> that are not used.

Thanks for the fix! Looks good to me.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> 
> Fixes: c64688081490 ("debugfs: add support for self-protecting
> attribute file fops")
> Fixes: df2294fb6428 ("intel_pmc_core: Convert to
> DEFINE_DEBUGFS_ATTRIBUTE")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> [nicstange@...il.com: Add dummy implementations of debugfs_attr_read()
> and
>   debugfs_attr_write() in order to protect against possibly broken
> dead
>   code elimination and to improve readability.
>   Correct CONFIG_DEBUGFS_FS -> CONFIG_DEBUG_FS typo in changelog.]
> Signed-off-by: Nicolai Stange <nicstange@...il.com>
> ---
> Compile-tested on top of next-20161020 with gcc-4.9.0 and gcc-6.2.1,
> CONFIG_INTEL_PMC_CORE=y and both, CONFIG_DEBUG_FS=n and
> CONFIG_DEBUG_FS=y.
> 
> I verified that there isn't anything in the intel_pmc_core.o that
> shouldn'tbe there with CONFIG_DEBUG_FS=n.
> 
> Changes to v1:
>  - Add dummy implementations of
> debugfs_attr_read()/debugfs_attr_write()
>    for the CONFIG_DEBUG_FS=n case.
>  - Fix a typo in the changelog.
> 
> 
>  include/linux/debugfs.h | 44 +++++++++++++++++++++++++++-------------
> ----
>  1 file changed, 27 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
> index 4d3f0d1..1b413a9 100644
> --- a/include/linux/debugfs.h
> +++ b/include/linux/debugfs.h
> @@ -62,6 +62,21 @@ static inline const struct file_operations
> *debugfs_real_fops(struct file *filp)
>  	return filp->f_path.dentry->d_fsdata;
>  }
>  
> +#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,				
> 	\
> +	.llseek  = generic_file_llseek,				
> 	\
> +}
> +
>  #if defined(CONFIG_DEBUG_FS)
>  
>  struct dentry *debugfs_create_file(const char *name, umode_t mode,
> @@ -99,21 +114,6 @@ ssize_t debugfs_attr_read(struct file *file, char
> __user *buf,
>  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,				
> 	\
> -	.llseek  = generic_file_llseek,				
> \
> -}
> -
>  struct dentry *debugfs_rename(struct dentry *old_dir, struct dentry
> *old_dentry,
>                  struct dentry *new_dir, const char *new_name);
>  
> @@ -233,8 +233,18 @@ static inline void debugfs_use_file_finish(int
> srcu_idx)
>  	__releases(&debugfs_srcu)
>  { }
>  
> -#define DEFINE_DEBUGFS_ATTRIBUTE(__fops, __get, __set, __fmt)	
> \
> -	static const struct file_operations __fops = { 0 }
> +static inline ssize_t debugfs_attr_read(struct file *file, char
> __user *buf,
> +					size_t len, loff_t *ppos)
> +{
> +	return -ENODEV;
> +}
> +
> +static inline ssize_t debugfs_attr_write(struct file *file,
> +					const char __user *buf,
> +					size_t len, loff_t *ppos)
> +{
> +	return -ENODEV;
> +}
>  
>  static inline struct dentry *debugfs_rename(struct dentry *old_dir,
> struct dentry *old_dentry,
>                  struct dentry *new_dir, char *new_name)

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy