MIPS questions

[Jay Monkman]

On Fri, Oct 24, 2003 at 02:18:02PM -0400, gregory.menke at gsfc.nasa.gov wrote:
> 
> Jay Monkman writes:
>  > I'm working on a MIPS BSP, and have a couple questions.
>  > 
>  > Does anybody have TCP/IP working? Unless I make changes, I get
>  > alignment errors. During processing, the stack overlays the TCP source
>  > and dest port numbers with the address of the mbuf. Since there's no
>  > guarantee those stay aligned, how is TCP working on MIPS? It looks
>  > like the fixes we put in for ARM will fix this also.
> 
> Could you be more specific here?  What driver are you using, where is
> the alignment error happening, etc...

It's a new driver for the Au1500. I don't remember where I catch the
error, but the macro REASS_MBUF is what does the overlaying. There's a
handful of places in tcp_input.c where it happens. The device I'm
using causes the IP packet to be 2 bytes off of 4 byte alignment,
since the ethernet header is 14 bytes long. This ensures the source
and destination ports in the TCP header are misaligned (unless there
are IP options?). Does TCP work on any other MIPS CPUs? Do they have
misaligned IP packets? Is this only a problem on MIPS32?


>  > 
>  > In _CPU_ISR_Set_level in cpukit/score/cpu/mips/cpu.c, there's a chunk
>  > of code:
>  >   sr = srbits | ((new_level==0)? (0xfc00 | SR_EXL | SR_IE): \
>  >                  (((new_level<<9) & 0xfc00) | \
>  >                   (new_level & 1)?(SR_EXL | SR_IE):0));
>  > 
>  > I think there needs to be parentheses around the last line. Without
>  > the parentheses, the statement turns into:
>  >   sr = srbits | ((new_level==0)? (0xfc00 | SR_EXL | SR_IE): \
>  >	             (SR_EXL | SR_IE));
>  > 
>  > Or am I missing something?
> 
> I think you're missing something, the false term of the first ?: is in
> parens.  The 9 bit rightshift and mask yields a constant or'ed into
> the expression.  The 2nd ?: tests for 1, which if false yields 0
> else the (SR_EXL | SR_IE).  But it is kind of messy and extra parens
> would be helpful.

Is the second test supposed to be
     (new_level & 1)
or
     ( (new_level<<9) & 0xfc00) | (new_level & 1)

I think according to C's precedence rules ( '|' is higher than '?:') ,
the test is the latter, but we want the former.

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