linux-kernel - Re: [PATCH 08/16 v2] pramfs: headers

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Message-ID: <4CD85489.8000503@bluewatersys.com>
Date:	Tue, 09 Nov 2010 08:50:33 +1300
From:	Ryan Mallon <ryan@...ewatersys.com>
To:	Marco Stornelli <marco.stornelli@...il.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Embedded <linux-embedded@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Tim Bird <tim.bird@...sony.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 08/16 v2] pramfs: headers

On 11/08/2010 08:49 PM, Marco Stornelli wrote:
> 2010/11/7 Ryan Mallon <ryan@...ewatersys.com>:
>> On 11/06/2010 09:58 PM, Marco Stornelli wrote:
>>> From: Marco Stornelli <marco.stornelli@...il.com>
>>>
>>> Definitions for the PRAMFS filesystem.
>>>
>>> Signed-off-by: Marco Stornelli <marco.stornelli@...il.com>
>>> ---
>>> 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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