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Re: what is the purpose of the following LE->BE patch to arch/mips/inclu

To: Matt Turner <mattst88@gmail.com>
Subject: Re: what is the purpose of the following LE->BE patch to arch/mips/include/asm/io.h?
From: "Robert P. J. Day" <rpjday@crashcourse.ca>
Date: Sat, 21 Feb 2015 03:11:51 -0500 (EST)
Cc: "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>
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On Fri, 20 Feb 2015, Matt Turner wrote:

> On Fri, Feb 20, 2015 at 8:00 PM, Matt Turner <mattst88@gmail.com> wrote:
> > On Fri, Feb 20, 2015 at 1:53 AM, Robert P. J. Day <rpjday@crashcourse.ca> 
> > wrote:
> >>
> >>   was recently handed a MIPS-based dev board (can't name the vendor,
> >> NDA) that *typically* runs in LE mode but, because of a proprietary
> >> binary that must be run on the board and was compiled as BE, has to be
> >> run in BE mode.
> >>
> >>   the vendor supplied a yoctoproject layer that seems to work fine
> >> but, in changing the DEFAULTTUNE to big-endian, the following patch
> >> had to be applied to the 3.14 kernel tree to the file
> >> arch/mips/include/asm/io.h in order to get output from the console
> >> port as the system was booting:
> >>
> >> 326c326,333
> >> <               *__mem = __val;                                         \
> >> ---
> >>>       {                                                                   
> >>>             \
> >>>               if (sizeof(type) == sizeof(u32))                \
> >>>               {                                                           
> >>>             \
> >>>                       *__mem = __cpu_to_le32(__val);  \
> >
> > They're byte swapping a value if they're in big endian mode.
> >
> >>>               }                                                           
> >>>             \
> >>>               else                                                        
> >>>     \
> >>>                       *__mem = __val;                                     
> >>>     \
> >
> > And they don't seem to really understand the __cpu_to_le32 macro...
>
> Sorry, I should be more precise. They're byte swapping 32-bit values
> if they're in big endian mode, and copying everything else without
> conversion.

  i understand *in general* what the above is doing ... what i don't
understand is why it's necessary to hack a fundamental kernel header
file this way in order to run this board in BE versus LE mode.

  the kernel was already configured for BE mode, which i would have
thought would be sufficient, so it's a mystery to me why one would
still have to *further* hack the io.h file this way -- if the above is
a necessity, shouldn't it be a conditional change based on selecting
BE configuration?

  has anyone else ever needed to do this? or is this some weird,
one-off hack that perhaps applies *only* to some bizarre feature of
this board?

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
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