Re: memcpy on 68000

[Greg Ungerer <gerg@snapgear.com>]

Hi Matthias,

On 07/24/2011 11:50 PM, Matthias Reis wrote:
> I recently did some work to compile a kernel for MMU less Atari STs.
> However, I 'm having a problem with the memcpy code as it is compiled by
> gcc. When copying from odd addresses, gcc produces memcpy code that tries
> to access odd addresses with move.w, which is not possible on the 68000
> and therefore produces address error exceptions. Below you can find the
> debug output from the hatari emulator and the memcpy version I'm using (I
> hope it's the most recent one). Any help would be appreciated.

That looks to be the memcpy.c from linux-3.0, yes?

Looking at the memcpy.c code for m68knommu before merging
and cleanup I can see that it used to do this:

   const char *c_from = from;
   char *c_to = to;
   while (n-- > 0)
     *c_to++ = *c_from++;
   return((void *) to);

for the the M68000 case. (Actually it did that for all
non-ColdFire cases). Which obviously would always work,
but is not particularly efficient.

So, yes, the current code is broken for M68000. It doesn't
check alignment of the source ("from") address.

Heres a first attempt at a fix for this. This is only compile
tested, not run tested... Basically we check that if after
16bit aligning the destination if the source is unaligned
then we resort to a byte wise copy.

Need to check if CPU32 needs this or not too. ColdFire doesn't.


diff --git a/arch/m68k/lib/memcpy.c b/arch/m68k/lib/memcpy.c
index 0648893..10ca051 100644
--- a/arch/m68k/lib/memcpy.c
+++ b/arch/m68k/lib/memcpy.c
@@ -22,6 +22,15 @@ void *memcpy(void *to, const void *from, size_t n)
                 from = cfrom;
                 n--;
         }
+#if defined(CONFIG_M68000)
+       if ((long)from & 1) {
+               char *cto = to;
+               const char *cfrom = from;
+               for (; n; n--)
+                       *cto++ = *cfrom++;
+               return xto;
+       }
+#endif
         if (n > 2 && (long)to & 2) {
                 short *sto = to;
                 const short *sfrom = from;


Regards
Greg


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Greg Ungerer  --  Principal Engineer        EMAIL:     gerg@snapgear.com
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