linux-kernel - Re: [PATCH 06/14] Pramfs: Include files

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Message-ID: <4A3E68CD.6080803@gmail.com>
Date:	Sun, 21 Jun 2009 19:07:25 +0200
From:	Marco <marco.stornelli@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Sam Ravnborg <sam@...nborg.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linux Embedded <linux-embedded@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/14] Pramfs: Include files

Arnd Bergmann wrote:
> On Saturday 13 June 2009, Sam Ravnborg wrote:
>>> +     union {
>>> +             struct {
>>> +                     /*
>>> +                      * ptr to row block of 2D block pointer array,
>>> +                      * file block #'s 0 to (blocksize/4)^2 - 1.
>>> +                      */
>>> +                     off_t row_block;
>> It is my understanding that we shall use: __kernel_off_t
>> in exported headers.
> 
> That is a correct understanding in general, however this case is
> different, because it describes an on-disk data structure,
> not a kernel to user space interface. Here, __kernel_off_t is just
> as wrong as off_t, because it will differ between 32 and 64 bit
> architectures, making the fs layout incompatible. I'd suggest
> simply defining this as __u64.
> 
> Moreover, file system layout should be described in terms of
> big-endian or little-endian types (e.g. __be64 or __le64),
> together with the right accessor functions.
> 
> 	Arnd <><
> 

I was thinking about your comment and I think I'll use __kernel_off_t
for the exported headers. I know that it will differ between 32 and 64
bit architectures, but for this kind of fs there isn't any compatibility
problem at layout level. You cannot remove a chip of RAM from a board
32bit little endian and attach it to a board with a cpu 64bit big
endian, the memory isn't a disk. Indeed, I see that tmpfs uses simply
"unsigned long" in the exported header file without any problems to
little or big endian.

Regards,

Marco
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