Re: [PATCH 08/16 v2] pramfs: headers

[Ryan Mallon]

On 11/09/2010 09:19 PM, Marco Stornelli wrote:
> 2010/11/8 Ryan Mallon <ryan@xxxxxxxxxxxxxxxx>:
>> 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.
> 
> Not always. Indeed, I didn't say *all* cpu used are big-endian.

My point is that little endian embedded machines make up a large portion
of the devices that this filesystem will be useful on. On those devices
it will also have an (unnecessary) conversion overhead.

>>
>> 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.

You can still do all of those things without having a fixed endianess.
You just have to have one extra step of telling the external tools what
the endianess is. IMHO, it is better to have the overhead of the endian
conversion in the tools since it is less costly there than an the
embedded system.

I'm just trying to understand why the fixed endianess rule cannot be
bent for such a specialised filesystem.

~Ryan

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