On 11/08/2010 08:49 PM, Marco Stornelli wrote:
> 2010/11/7 Ryan Mallon <ryan@xxxxxxxxxxxxxxxx>:
>> On 11/06/2010 09:58 PM, Marco Stornelli wrote:
>>> From: Marco Stornelli <marco.stornelli@xxxxxxxxx>
>>>
>>> Definitions for the PRAMFS filesystem.
>>>
>>> Signed-off-by: Marco Stornelli <marco.stornelli@xxxxxxxxx>
>>> ---
>>> diff -Nurp linux-2.6.36-orig/fs/pramfs/pram.h linux-2.6.36/fs/pramfs/pram.h
>>> --- linux-2.6.36-orig/fs/pramfs/pram.h 1970-01-01 01:00:00.000000000 +0100
>>> +++ linux-2.6.36/fs/pramfs/pram.h 2010-10-30 12:02:45.000000000 +0200
>>> @@ -0,0 +1,317 @@
>>
>>> +/*
>>> + * Structure of the super block in PRAMFS
>>> + */
>>> +struct pram_super_block {
>>> + __be16 s_sum; /* checksum of this sb, including padding */
>>> + __be64 s_size; /* total size of fs in bytes */
>>> + __be32 s_blocksize; /* blocksize in bytes */
>>> + __be32 s_inodes_count; /* total inodes count (used or free) */
>>> + __be32 s_free_inodes_count;/* free inodes count */
>>> + __be32 s_free_inode_hint; /* start hint for locating free inodes */
>>> + __be32 s_blocks_count; /* total data blocks count (used or free) */
>>> + __be32 s_free_blocks_count;/* free data blocks count */
>>> + __be32 s_free_blocknr_hint;/* free data blocks count */
>>> + __be64 s_bitmap_start; /* data block in-use bitmap location */
>>> + __be32 s_bitmap_blocks;/* size of bitmap in number of blocks */
>>> + __be32 s_mtime; /* Mount time */
>>> + __be32 s_wtime; /* Write time */
>>> + __be16 s_magic; /* Magic signature */
>>> + char s_volume_name[16]; /* volume name */
>>> +};
>>
>> Is there a particular reason to use big endian types for the data
>> structures? On a little endian machine you will end up converting values
>> everywhere. I assume that you don't expect the machine to change
>> endianess between reboots :-). If this is for generating/reading
>> filesystems from userspace, wouldn't it be better to have the userspace
>> tools specify the target endianess and do the conversions there?
>>
>> ~Ryan
>
> Yes, there is a reason. In the first review a comment was: the fs must
> have a fix endianess layout. This fs is designed for the embedded
> world mainly. Since most of cpus used in this case are big-endian, it
> means that for typical use case, there is no cost for endianess
> conversion.
ARM, which is a large portion of the embedded space, is typically little
endian.
Why does a filesystem need to have a specific endianess layout?
Especially for a highly specialised filesystem like this.
~Ryan
--
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