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

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Message-ID: <AANLkTim8rAQnVYpZdykd2VuL1N9CvkGj88Kf5qpSuJsC@mail.gmail.com>
Date:	Tue, 9 Nov 2010 09:19:32 +0100
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Ryan Mallon <ryan@...ewatersys.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

2010/11/8 Ryan Mallon <ryan@...ewatersys.com>:
> 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.

Not always. Indeed, I didn't say *all* cpu used are big-endian.

>
> Why does a filesystem need to have a specific endianess layout?
> Especially for a highly specialised filesystem like this.
>

I didn't agree with it, but in the first review there was more than
one developer that said this thing. The main reason was to read the
format (for example with JTAG) or to create an image with a fix
format. I remember that someone said that there was a similar problem
with jffs2 and the experience tell to us that a fix endianess is
important. At that point I decided to use big-endian. You can see all
the discussion in lkml. The review has been done at June 2009.

Regards,

Marco
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