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

[Geert Uytterhoeven]

On Tue, Nov 9, 2010 at 21:35, Ryan Mallon <ryan@xxxxxxxxxxxxxxxx> wrote:
> 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    Â1970-01-01 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 {
>>>>>> + Â Â __be16 Âs_sum; Â Â Â Â Â/* checksum of this sb, including padding */
>>>>>> + Â Â __be64 Âs_size; Â Â Â Â /* total size of fs in bytes */
>>>>>> + Â Â __be32 Âs_blocksize; Â Â/* blocksize in bytes */
>>>>>> + Â Â __be32 Âs_inodes_count; /* total inodes count (used or free) */
>>>>>> + Â Â __be32 Âs_free_inodes_count;/* free inodes count */
>>>>>> + Â Â __be32 Âs_free_inode_hint; Â/* start hint for locating free inodes */
>>>>>> + Â Â __be32 Âs_blocks_count; /* total data blocks count (used or free) */
>>>>>> + Â Â __be32 Âs_free_blocks_count;/* free data blocks count */
>>>>>> + Â Â __be32 Âs_free_blocknr_hint;/* free data blocks count */
>>>>>> + Â Â __be64 Âs_bitmap_start; /* data block in-use bitmap location */
>>>>>> + Â Â __be32 Âs_bitmap_blocks;/* size of bitmap in number of blocks */
>>>>>> + Â Â __be32 Âs_mtime; Â Â Â Â/* Mount time */
>>>>>> + Â Â __be32 Âs_wtime; Â Â Â Â/* Write time */
>>>>>> + Â Â __be16 Âs_magic; Â Â Â Â/* Magic signature */
>>>>>> +   char  Âs_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.

When it was decided that filesystems should be fixed-endian and support for
big-endian ext2 was dropped, the overhead of doing the fixed conversions was
deetermined negligible due to compiler optimization.
That was ages ago, and current embedded systems run circles around the
machines of those days.

Note that this is about metadata only. Actual file contents are always just
byte streams.

Gr{oetje,eeting}s,

            Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
             Â Â -- Linus Torvalds
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