linux-kernel - Re: [PATCH v2 09/10] sysfs: bin_attribute: add const read/write callback variants

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Message-ID: <7ed3b713f8901398f52d7485d59613c19ea0e752.camel@HansenPartnership.com>
Date: Tue, 03 Dec 2024 11:06:16 -0500
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: linux@...ssschuh.net
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Subject: Re: [PATCH v2 09/10] sysfs: bin_attribute: add const read/write
 callback variants

> diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
> index
> d17c473c1ef292875475bf3bdf62d07241c13882..d713a6445a6267145a7014f308d
> f3bb25b8c3287 100644
> --- a/include/linux/sysfs.h
> +++ b/include/linux/sysfs.h
> @@ -305,8 +305,12 @@ struct bin_attribute {
>  	struct address_space *(*f_mapping)(void);
>  	ssize_t (*read)(struct file *, struct kobject *, struct
> bin_attribute *,
>  			char *, loff_t, size_t);
> +	ssize_t (*read_new)(struct file *, struct kobject *, const
> struct bin_attribute *,
> +			    char *, loff_t, size_t);
>  	ssize_t (*write)(struct file *, struct kobject *, struct
> bin_attribute *,
>  			 char *, loff_t, size_t);
> +	ssize_t (*write_new)(struct file *, struct kobject *,
> +			     const struct bin_attribute *, char *,
> loff_t, size_t);
>  	loff_t (*llseek)(struct file *, struct kobject *, const
> struct bin_attribute *,
>  			 loff_t, int);
>  	int (*mmap)(struct file *, struct kobject *, const struct
> bin_attribute *attr,
> @@ -325,11 +329,28 @@ struct bin_attribute {
>   */
>  #define sysfs_bin_attr_init(bin_attr) sysfs_attr_init(&(bin_attr)-
> >attr)
>  
> +typedef ssize_t __sysfs_bin_rw_handler_new(struct file *, struct
> kobject *,
> +					   const struct
> bin_attribute *, char *, loff_t, size_t);
> +
>  /* macros to create static binary attributes easier */
>  #define __BIN_ATTR(_name, _mode, _read, _write, _size)
> {		\
>  	.attr = { .name = __stringify(_name), .mode = _mode
> },		\
> -	.read	=
> _read,						\
> -	.write	=
> _write,						\
> +	.read =
> _Generic(_read,						\
> +		__sysfs_bin_rw_handler_new * :
> NULL,			\
> +		default :
> _read						\
> +	),							
> 	\
> +	.read_new =
> _Generic(_read,					\
> +		__sysfs_bin_rw_handler_new * :
> _read,			\
> +		default :
> NULL						\
> +	),							
> 	\
> +	.write =
> _Generic(_write,					\
> +		__sysfs_bin_rw_handler_new * :
> NULL,			\
> +		default :
> _write					\
> +	),							
> 	\
> +	.write_new =
> _Generic(_write,					\
> +		__sysfs_bin_rw_handler_new * :
> _write,			\
> +		default :
> NULL						\
> +	),							
> 	\
>  	.size	=
> _size,						\
>  }

It's probably a bit late now, but you've done this the wrong way
around.  What you should have done is added the const to .read/.write
then added a .read_old/.write_old with the original function prototype
and used _Generic() to switch between them.  Then when there are no
more non const left, you can simply remove .read_old and .write_old
without getting Linus annoyed by having to do something like this:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e70140ba0d2b1a30467d4af6bcfe761327b9ec95

Regards,

James