[U-Boot-Users] Configuring U-Boot for MPC8349E in little endian mode

[Jerry Van Baren]

vivek.trivedi at wipro.com wrote:
> Hi,
> Thanks all for quick response.
> 
> Sorry, I forgot to tell that we are using MPC8349E processor with a
> "e300 core".
> And it is clearly mentioned in the e300 core datasheet that, it supports
> "True little-endian mode for compatibility with other true little-endian
> devices".

Put on your "marketing filter" glasses when reading this.  You will find 
words like "true" and "compatibility"  disappear and the sentence 
becomes meaningless.

The documentation says it can be done and, indeed, it *can*[1].  It 
looks like the e300 core is easier than "traditional" PowerPCs (hence 
the addition of "true" to the marketing line above).  What the 
documentation doesn't say is *how difficult* it is do to it.  Empirical 
evidence would indicate that it is hard.

> So, according to data sheet I have done following changes in U-Boot to
> configure it in true little endian mode:
> RCWH for little endian mode and enable the Little-endian mode in e300
> core registers MSR bit LE and HID2 registers bit LET.
> 
> After doing these settings core is able to fetch RCW from flash and
> execute some assembly code in Start.S. 
> But I am not able to understand why e300 core(i.e. MPC8349E in True) is
> not executing branch instruction(b , bl) only  and jumping to alignment
> exception vector handler(0x600).

Read and understand the endian discussion in the Programming 
Environments Manual as well as the e300 core.
   <http://www.freescale.com/files/product/doc/MPCFPE32B.pdf>

Flipping the endian configuration bit in the HRCW is just the start of 
your struggle (if you chose to continue).  I don't know of anybody that 
runs PowerPCs in little endian mode[1] because, while it can be done, 
the pain of making it work is much higher than byteswapping the data as 
necessary.

Note that the exception handling is configured separately to run in 
big/little endian.  You can be running little endian in your user 
applications and big endian in your exceptions (and OS).  Ouch.

Some types of data accesses cause the exception you are seeing and it is 
your job to field the exception, determine the desired load/store 
operation, and fix it up.  This is done in the misaligned exception 
handler (while running in big endian mode).

 From the alignment exception description in the e300 Core Reference 
Manual Table 5-2. Interrupts and Exception Conditions:

An alignment interrupt is caused when the core cannot perform a memory 
access for any of the reasons described below:
* The instruction is lswi, lswx, stswi, stswx, and the core is in 
little-endian mode. Note that PowerPC little-endian mode is not 
supported on the e300 core.

See also in the e300 Core Reference Manual section 3.2.4.3.7 Integer 
Load and Store String Instructions

> Do, I need to enable some -m alignment flag in U-Boot build so that
> e300-core can execute branch instructions.
> Is the e300-core is not able to interpret branch instruction(b , bl) in
> true little endian mode...?

Nope, not that simple.

> Has any one tried to configure and run U-Boot for MPC8349E

Extremely unlikely, especially since nobody responded to your initial 
email with useful advice on how to implement LE.

Wolfgang's advice (and others) is very good advice: you really don't 
want to do little endian.

> processor(e300 core) in true little endian mode...?
> Are missing any other setting in U-Boot for little endian operation...?

Nope, not that simple.

> We want to configure MPC8349E in little endian as some other PCI
> devices(DSPs) are operating in little endian.

Byteswapping as necessary is very likely to be a simpler solution. 
Nearly everybody byteswaps rather than running in little endian mode. 
Heed the wisdom of the crowds.

Note that there are byteswap instructions in the PowerPC instruction 
set.  For your DSP data, you will likely find it is easier to run 
through it and byteswap it as the first (incoming) or last (outgoing) 
thing done to it than try to run little endian.

> Any help/suggestion will be appreciated.
> 
> 
> Thanks,
> Vivek Trivedi

Best regards,
gvb

[1] Windows NT 3.1 ran little endian on PowerPC.  That is the *only* 
PowerPC little endian use that I'm aware of.

Ref:
   <http://support.microsoft.com/kb/102025>