Re: [musl] cortex-m support?

[Rob Landley <rob-VoJi6FS/r0vR7s880joybQ@public.gmane.org>]

On 12/08/2016 03:11 PM, Rich Felker wrote:
> On Tue, Dec 06, 2016 at 11:52:29PM -0600, Rob Landley wrote:
>> Index: src/setjmp/arm/setjmp.S
>> ===================================================================
>> --- src/setjmp/arm/setjmp.S	(revision 0)
>> +++ src/setjmp/arm/setjmp.S	(revision 4920)
>> @@ -0,0 +1,45 @@
>> +.global __setjmp
>> +.global _setjmp
>> +.global setjmp
>> +.type __setjmp,%function
>> +.type _setjmp,%function
>> +.type setjmp,%function
>> +__setjmp:
>> +_setjmp:
>> +setjmp:
>> +	mov ip,r0
>> +#if defined(__thumb__)
>> +	stmia ip!,{v1,v2,v3,v4,v5,v6,sl,fp,lr}
>> +        str sp, [ip], #4
>> +#else
>> +	stmia ip!,{v1,v2,v3,v4,v5,v6,sl,fp,sp,lr}
>> +#endif
> 
> My leaning is to just always use the version that works on thumb,

Presumaby there's a longjmp.S bit too because:

        ldmia ip!, {v1,v2,v3,v4,v5,v6,sl,fp,sp,lr}

Make sure you're aware of erratum 752419:

  https://sourceware.org/ml/binutils/2011-05/msg00087.html

On a board with a lot of serial traffic, it takes about 3 minutes for
uClibc (yes, including the current -ng) to crash (by loading the link
register into the stack pointer and stomping code before the setjmp).

The emcraft guys fixed this in their cortex-m uClibc fork way back when
(http://www.emcraft.com/som/building-uclibc), and of _course_ never
bothered to send it upstream. (Or put it on github with their kernel and
uboot.) But the source is in their distro tarball, and their patch to
uClibc's __longjmp.S is basically:

--- uclibc-1.0.17/libc/sysdeps/linux/arm/__longjmp.S
+++ emcraft/libc/sysdeps/linux/arm/__longjmp.S
@@ -56,9 +60,15 @@
 #if defined(__thumb2__)
 	/* Thumb-2 does not allow loading sp with ldm.  */
 	ldmia	ip!,	{v1-v6, sl, fp}
+#if 0
 	ldr	sp, [ip], #4
 	ldr	lr, [ip], #4
 #else
+	ldr	sp, [ip, #0]
+	ldr	lr, [ip, #4]
+	add	ip, #8
+#endif
+#else
 	ldmia	ip!,	{v1-v6, sl, fp, sp, lr}
 #endif


(Dunno why the "add ip, #8" because it's a scratch register and this is
the last use in the function, but maybe they just made it do exactly
what the #else case does?)

cc-ing buildroot because this is still broken in their november release.

Rob