topic Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel in i.MX Processors

"Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

DonaldR_PooleJr — Wed, 30 Nov 2011 18:06:45 GMT

Hello All,

I've downloaded and compiled Freescale's 2.6.35 kernel from their opensource gitweb, but when I boot the kernel on my i.MX53QSB I get a series of 8 Unhandled fault: alignment exception errors. I've attached my complete boot output, but the first occurrence of the boot error is shown below:

MMC read: dev # 0, block # 2048, count 6144 ... 6144 blocks read: OK
## Booting kernel from Legacy Image at 70800000 ...
Image Name: Linux-2.6.35.3
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 2917388 Bytes = 2.8 MiB
Load Address: 70008000
Entry Point: 70008000
Verifying Checksum ... OK
Loading Kernel Image ... OK
OK

Starting kernel ...

Linux version 2.6.35.3 (victory@ubuntu) (gcc version 4.5.2 (Sourcery G++ Lite 2011.03-41) ) #1 PREEMPT Wed Nov 30 11:30:54 CST 2011
CPU: ARMv7 Processor [412fc085] revision 5 (ARMv7), cr=10c53c7f
CPU: VIPT nonaliasing data cache, VIPT nonaliasing instruction cache
Machine: Freescale MX53 LOCO Board
Memory policy: ECC disabled, Data cache writeback
Built 1 zonelists in Zone order, mobility grouping on. Total pages: 250880
Kernel command line: console=ttymxc0,115200 root=/dev/mmcblk0p1 rw rootfstype=ext3 rootwait video=mxcdi0fb:RGB24,1024x768M@60
PID hash table entries: 4096 (order: 2, 16384 bytes)
Dentry cache hash table entries: 131072 (order: 7, 524288 bytes)
Inode-cache hash table entries: 65536 (order: 6, 262144 bytes)
Memory: 480MB 512MB = 992MB total
Memory: 998300k/998300k available, 17508k reserved, 0K highmem
Virtual kernel memory layout:
vector : 0xffff0000 - 0xffff1000 ( 4 kB)
fixmap : 0xfff00000 - 0xfffe0000 ( 896 kB)
DMA : 0xf9e00000 - 0xffe00000 ( 96 MB)
vmalloc : 0xe0800000 - 0xf4000000 ( 312 MB)
lowmem : 0x80000000 - 0xe0000000 (1536 MB)
pkmap : 0x7fe00000 - 0x80000000 ( 2 MB)
modules : 0x7f000000 - 0x7fe00000 ( 14 MB)
.init : 0x80008000 - 0x80033000 ( 172 kB)
.text : 0x80033000 - 0x807e0000 (7860 kB)
.data : 0x80804000 - 0x8085b4a0 ( 350 kB)
SLUB: Genslabs=11, HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
Hierarchical RCU implementation.
.RCU-based detection of stalled CPUs is disabled.
.Verbose stalled-CPUs detection is disabled.
NR_IRQS:368MXC GPIO hardware
MXC IRQ initialized
MXC_Early serial console at MMIO 0x53fbc000 (options '115200')
bootconsole [ttymxc0] enabled
Console: colour dummy device 80x30
Calibrating delay loop... 799.53 BogoMIPS (lpj=3997696)
pid_max: default: 32768 minimum: 301
Mount-cache hash table entries: 512
CPU: Testing write buffer coherency: ok
Unhandled fault: alignment exception (0x001) at 0x8079112a
Internal error: : 1 [#1] PREEMPT
last sysfs file:
Modules linked in:
CPU: 0 Not tainted (2.6.35.3 #1)
PC is at devtmpfs_init+0xc/0xa4
LR is at driver_init+0x8/0x28
pc : [<8001dea4>] lr : [<8001de54>] psr: 80000013
sp : df039fd0 ip : 00000000 fp : 00000000
r10: 00000000 r9 : 00000000 r8 : 00000000
r7 : 00000013 r6 : 800349fc r5 : 800088f0 r4 : 800321a8
r3 : 8079112a r2 : 800a2218 r1 : 00000000 r0 : 8083f1fc
Flags: Nzcv IRQs on FIQs on Mode SVC_32 ISA ARM Segment kernel
Control: 10c5387f Table: 70004019 DAC: 00000017
Process swapper (pid: 1, stack limit = 0xdf0382e8)
Stack: (0xdf039fd0 to 0xdf03a000)
9fc0: 00000000 00000000 800a2218 00000000
9fe0: 800321a8 8001de54 800321a8 8000896c 00000000 800349fc 01207b65 89251006
[<8001dea4>] (devtmpfs_init+0xc/0xa4) from [<8001de54>] (driver_init+0x8/0x28
[<8001de54>] (driver_init+0x8/0x28) from [<8000896c>] (kernel_init+0x7c/0x168
[<8000896c>] (kernel_init+0x7c/0x168) from [<800349fc>] (kernel_thread_exit+0x0/0x8)
Code: 8083f1ec e92d401f e59f3080 e59f0080 (e5932000)

The kernel eventually stops displaying output and appears to hang. Has anyone encountered this kind of error booting the Freescale kernel? I'm at a lose right now since I don't know what angle to attack this problem from. Thanks in advance!

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

DonaldR_PooleJr — Thu, 01 Dec 2011 23:15:48 GMT

Update: It looks like the problem was with my cross-compile toolchain.

Technically, there is a bit called the "A-bit" in the Control Register (CR) of processor that controls how unaligned memory faults are handled: 0=strict alignment fault checking disabled, 1=strict alignment fault enabled. The CodeSourcery 2011.03-41 version of the toolchain is built with "Unaligned access support" optimization, which assumes that the hardware (A-bit=0) will handle unaligned memory loads. But, the linux kernel by default uses software (A-bit=1) handlers (traps) for unaligned memory access faults. So, although the kernel was compiled and optimized to assume no strict alignment fault checking, it's still configured to set the "A-bit" to check for memory alignment faults. I found all this out after reading the Cortex-A8 Technical Reference Manual and doing some googling.

But, I said all of that to say this: If you run into this problem and you are using the CodeSourcery 2011.03-41 tool-chain, you can either use and older version of the tool-chain or use a different tool-chain altogether. I ended up using the Ubuntu's GNU C/C++ Compiler/Preprocessor for ARM found in ubuntu software center.

I hope that this is able to help somebody if they encounter the same situation.

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

Michael_McTerna — Fri, 06 Jan 2012 09:08:43 GMT

Donald R. Poole, Jr. said:

Update: It looks like the problem was with my cross-compile toolchain.
...
The CodeSourcery 2011.03-41 version of the toolchain is built with "Unaligned access support" optimization,

You can disable this behaviour by building with -mno-unaligned-access e.g.

make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- EXTRA_CFLAGS=-mno-unaligned-access

There's a good description of this here: http://blog.galemin.com/tag/codesourcery/

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

DonaldR_PooleJr — Fri, 06 Jan 2012 14:48:48 GMT

Thanks for the reply Micheal. I just came across that flag yesterday when I was reading through the changelog for codesourcery toolchain. Wish I would've known earlier because it would've saved me a lot of time.

Michael McTernan said:

Donald R. Poole, Jr. said:
Update: It looks like the problem was with my cross-compile toolchain.
...
The CodeSourcery 2011.03-41 version of the toolchain is built with "Unaligned access support" optimization,
You can disable this behaviour by building with -mno-unaligned-access e.g.

make ARCH=arm CROSS_COMPILE=arm-none-linux-gnueabi- EXTRA_CFLAGS=-mno-unaligned-access

There's a good description of this here: http://blog.galemin.com/tag/codesourcery/

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

sidebranch — Fri, 06 Jan 2012 20:29:10 GMT

I ran into that problem also, I just read this forum posting much later, so I went of I solved it on my own.

I first traced the bug down to the init function of devtmpfs (i.e. this is where the unaligned access occurs).

So could you confirm (or deny) that you had enabled devtmpfs in the config (Freescale has it off by default AFAIK).

Regards,

Leon.

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

Michael_McTerna — Thu, 29 Oct 2020 09:45:01 GMT

Leon Woestenberg said:

So could you confirm (or deny) that you had enabled devtmpfs in the config (Freescale has it off by default AFAIK).

I do have that, so tried a build without CONFIG_DEVTMPFS set, and sadly that's not it, or at least not the only non-aligned access.

In my case, I find the problem really early on, as the boot output looks like this:

Uncompressing Linux... done, booting the kernel.

So I'll stick with EXTRA_CFLAGS=-mno-unaligned-access since it's more convenient than having yet another ARM tool chain installed

Also note that this isn't a stock BSP kernel although it does have the Freescale patch set in it.

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

sidebranch — Mon, 09 Jan 2012 14:24:10 GMT

Michael,

yes your approach seems the safest one. I just found out that the new Freescale kernel patch set (2011 December) has additional alignment issues (and other new issues regarding the IPU...).

What kernel source base are you using?

Regards,

Leon.

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

Michael_McTerna — Mon, 09 Jan 2012 14:32:53 GMT

Leon Woestenberg said:

yes your approach seems the safest one. I just found out that the new Freescale kernel patch set (2011 December) has additional alignment issues (and other new issues regarding the IPU...).

Good to know - thanks! I'm definitely sticking with the switch for now :-D

What kernel source base are you using?

3.0.0.

Re: "Unhandled fault: Alignment Exception" Error booting Freescale 2.6.35 Kernel

Michael_McTerna — Fri, 17 Aug 2012 12:34:28 GMT

Addendum: some kernels (at least 2.6.35.3 and 2.6.38 from the Freescale Quick Start Board BSP) override EXTRA_CFLAGS in the Makefiles, so needs an extra patch to make the kernel bootable when using a newer toolchain with EXTRA_CFLAGS=-mno-unaligned-access.

The main culprit seems to be in arch/arm/boot/compressed/Makefile:

-EXTRA_CFLAGS := -fpic -fno-builtin
-EXTRA_ASFLAGS := -Wa,-march=all
+ccflags-y := -fpic -fno-builtin
+asflags-y := -Wa,-march=all