[Buildroot] endian issue
Grant Edwards
grant.b.edwards at gmail.com
Sat May 15 14:09:11 UTC 2010
On 2010-05-15, Marcel <korgull at home.nl> wrote:
> I'm using an Atmel sam9g45 using buildroot with linux 2.6.33.
>
> I currently have most of my things working but run into an issue
> which is endian related.
>
> My driver outputs it's data in big-endian mode (ADC data).
Oops. The 9g45 is a little-endian part.
> This is fine with me.
Apparently not, since you're complaining about how it breaks things. ;)
> I added a package to buildroot with my own software that talks to the
> driver. This package than transfers the results either over ethernet,
> usb or rs232. This all works great as long as I don't perform any
> calculations in the package that I added.
That's because your processor is running in little-endian mode. Doing
calculations on big-endian data using a little-endian part (or vice
versa) doesn't work.
> When I do so, all data will be messed up and I'm sure it's related to
> endiannes.
n
Indeed it is.
> I think my package is compiled in little-endian mode
I would hope so, since you're running it on a little-endian part.
> and I'm quite sure this is the issue.
The issue is your driver. Change your driver to return data in the
format expected by your processor (that's little-endian in the case of
the 9g45). Then use standard macros (hton(), htons(), htonl()) to
convert data to "network order" before you ship it out via Ethernet or
USB.
[Don't ask me why Atmel chose little-endian when they designed the
AT91 peripherals. For anything with a network interface, big-endian
makes a lot more sense to me. But that debate has been going on for
40 years...]
> I did some byte swapping to proof it's the issue and this byte
> swapping makes things fine.
>
> Is there any way to compile my package in big-endian mode from
> buildroot ?
Yes.
But doing so will result in object files that don't work. In fact
you won't even be able to link them.
> Or is there another way I should force this ?
No, you shouldn't force this. You should fix your driver. Really.
--
Grant
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