On Thu, Nov 05, 2015 at 05:25:14PM +0100, Gilles Chanteperdrix wrote:
> I am sure. Xenomai does not use H_DMA on armv7, so, it is not used
> if your code, it is not used at all.
Well we are using it for one allocation for talking to a network port.
If I remove it, I get a NULL pointer dereference, so it clearly matters
to something in our code.
> > It does appear that when LPAE is enabled H_DMA does do something on arm,
> > but only when LPAE is enabled since that enables ZONE_DMA.
>
> I never said that H_DMA did nothing.
OK I misunderstood then.
Setting /proc/cpu/alignment to 0 gives the kernel message:
alignment: ignoring faults is unsafe on this CPU. Defaulting to fixup mode.
Apparently armv6+ with CR_U set is not allowed to turn off fixups.
The fixups don't work for the xenomai code though it would seem.
I get:
Alignment trap: not handling instruction f8d03002 at [<00020be0>]
Unhandled fault: alignment exception (0x221) at 0xb56ba006
Code looks like this:
20bd8: 6802 ldr r2, [r0, #0]
20bda: 680b ldr r3, [r1, #0]
20bdc: 429a cmp r2, r3
20bde: d107 bne.n 20bf0 <_ZNK11rc_mac_addreqERKS_+0x18>
-> 20be0: f8d0 3002 ldr.w r3, [r0, #2]
20be4: f8d1 0002 ldr.w r0, [r1, #2]
20be8: 1a1b subs r3, r3, r0
20bea: 4258 negs r0, r3
20bec: 4158 adcs r0, r3
20bee: 4770 bx lr
20bf0: 2000 movs r0, #0
20bf2: 4770 bx lr
As far as I understand it, ldr.w should work for unaligned reads as long
as the page if flagged normal. Still trying to determine what the page
is marked as in this case.
And looking at the code compiled with gcc-4.6 it is obviously different
and would not have alignment issues (but is much less efficient):
49a46: b5f0 push {r4, r5, r6, r7, lr}
49a48: 7847 ldrb r7, [r0, #1]
49a4a: 7803 ldrb r3, [r0, #0]
49a4c: 784e ldrb r6, [r1, #1]
49a4e: ea43 2707 orr.w r7, r3, r7, lsl #8
49a52: 780b ldrb r3, [r1, #0]
49a54: 7884 ldrb r4, [r0, #2]
49a56: ea43 2606 orr.w r6, r3, r6, lsl #8
49a5a: 788b ldrb r3, [r1, #2]
49a5c: 78c5 ldrb r5, [r0, #3]
49a5e: 78ca ldrb r2, [r1, #3]
49a60: ea47 4704 orr.w r7, r7, r4, lsl #16
49a64: ea46 4603 orr.w r6, r6, r3, lsl #16
49a68: ea47 6705 orr.w r7, r7, r5, lsl #24
49a6c: ea46 6602 orr.w r6, r6, r2, lsl #24
49a70: 42b7 cmp r7, r6
49a72: d114 bne.n 49a9e <_ZNK11rc_mac_addreqERKS_+0x58>
49a74: ea43 2302 orr.w r3, r3, r2, lsl #8
49a78: 790a ldrb r2, [r1, #4]
49a7a: ea44 2405 orr.w r4, r4, r5, lsl #8
49a7e: 7905 ldrb r5, [r0, #4]
49a80: 7940 ldrb r0, [r0, #5]
49a82: ea43 4302 orr.w r3, r3, r2, lsl #16
49a86: 794a ldrb r2, [r1, #5]
49a88: ea44 4405 orr.w r4, r4, r5, lsl #16
49a8c: ea44 6000 orr.w r0, r4, r0, lsl #24
49a90: ea43 6302 orr.w r3, r3, r2, lsl #24
49a94: 1ac1 subs r1, r0, r3
49a96: 4248 negs r0, r1
49a98: eb40 0001 adc.w r0, r0, r1
49a9c: bdf0 pop {r4, r5, r6, r7, pc}
49a9e: 2000 movs r0, #0
49aa0: bdf0 pop {r4, r5, r6, r7, pc}
Stupid alignment problems. :)
I tried booting with 'noalign' kernel argument which appears in the code
to clear the CR_A flag, but that didn't make any difference.
--
Len Sorensen
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