Re: [RFC PATCH 2/4] btrfs: create structure to encode checksum type and length

[David Sterba <dsterba@suse.cz>]

On Thu, Jul 25, 2019 at 11:33:49AM +0200, Johannes Thumshirn wrote:
> Create a structure to encode the type and length for the known on-disk
> checksums. Also add a table and a convenience macro for adding the
> checksum types to the table.
> 
> This makes it easier to add new checksums later.
> 
> Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
> ---
>  fs/btrfs/ctree.h | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index da97ff10f421..099401f5dd47 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -82,9 +82,15 @@ struct btrfs_ref;
>   */
>  #define BTRFS_LINK_MAX 65535U
>  
> -/* four bytes for CRC32 */
> -static const int btrfs_csum_sizes[] = { 4 };
> -static const char *btrfs_csum_names[] = { "crc32c" };
> +#define BTRFS_CHECKSUM_TYPE(_type, _size, _name) \
> +	[_type] = { .size = _size, .name = _name }

I think the macro initializer might be an overkill here, there are only
3 items to initialize.

> +
> +static const struct btrfs_csums {
> +	u16		size;
> +	const char	*name;
> +} btrfs_csums[] = {
> +	BTRFS_CHECKSUM_TYPE(BTRFS_CSUM_TYPE_CRC32, 4, "crc32c"),
> +};
>  
>  #define BTRFS_EMPTY_DIR_SIZE 0
>  
> @@ -2373,13 +2379,13 @@ static inline int btrfs_super_csum_size(const struct btrfs_super_block *s)
>  	/*
>  	 * csum type is validated at mount time
>  	 */
> -	return btrfs_csum_sizes[t];
> +	return btrfs_csums[t].size;
>  }
>  
>  static inline const char *btrfs_super_csum_name(u16 csum_type)
>  {
>  	/* csum type is validated at mount time */
> -	return btrfs_csum_names[csum_type];
> +	return btrfs_csums[csum_type].name;
>  }

This has been in the code already, but shouldn't the btrfs_csums table
be declared in ctree.h and defined in eg. in ctree.c? As ctree.h is
included by everything we have multiple copies of it in the .o files.