[poky] BeagleBoard using GCC 4.6.0

Thu Jun 9 04:32:49 PDT 2011

On 06/09/2011 05:20 AM, Gary Thomas wrote:
> On 06/09/2011 12:48 AM, Khem Raj wrote:
>> On 06/08/2011 07:24 PM, Gary Thomas wrote:
>>> On 06/08/2011 06:42 PM, Khem Raj wrote:
>>>> On 06/08/2011 11:20 AM, Saul Wold wrote:
>>>>> On 06/08/2011 10:43 AM, Gary Thomas wrote:
>>>>>> Now that Poky is using GCC 4.6.0, has anyone actually checked the
>>>>>> operation on the BeagleBoard? I suspect that you'll find that the
>>>>>> EHCI USB does not work. I can't really check here as I don't trust
>>>>>> the EHCI on my BeagleBoard rev C3 (not xM).
>>>>>>
>>>>>> That said, I've tried this new compiler (previously reported) on
>>>>>> my own OMAP/3530 board which uses the same 2.6.37 kernel (just not
>>>>>> the linux-yocto version). When I build my kernel with GCC 4.5.2,
>>>>>> it works perfectly, but fails with GCC 4.6.0 (I trust the hardware).
>>>>>>
>>>>>> I've isolated it down to at least the function 'ehci_hub_control'
>>>>>> (but I suspect the problem is more fundamental). Comparing the code
>>>>>> generated by the two compilers with the same source tree, this function
>>>>>> is dramatically different. I can see why it's failing, I just don't
>>>>>> know why the compiler is doing what it's doing.
>>>>>>
>>>>>
>>>>> Gary,
>>>>>
>>>>> I am not sure if Khem Raj is on this list, so I am forwarding it to him,
>>>>> he might have some insight.
>>>>>
>>>>> Sau!
>>>>>
>>>>>
>>>>>> The lines at drivers/usb/host/ehci-hub.c:841
>>>>>> case GetPortStatus:
>>>>>> if (!wIndex || wIndex > ports)
>>>>>> goto error;
>>>>>> wIndex--;
>>>>>> status = 0;
>>>>>> temp = ehci_readl(ehci, status_reg);
>>>>>>
>>>>>> are being compiled very differently.
>>>>>>
>>>>>> With GCC 4.5.2:
>>>>>> 0xc0229810 <ehci_hub_control+672>: cmp r3, #0 ; 0x0
>>>>>> 0xc0229814 <ehci_hub_control+676>: beq 0xc0229df8
>>>>>> <ehci_hub_control+2184>
>>>>>> 0xc0229818 <ehci_hub_control+680>: cmp r3, r0
>>>>>> 0xc022981c <ehci_hub_control+684>: bgt 0xc0229df8
>>>>>> <ehci_hub_control+2184>
>>>>>> 0xc0229820 <ehci_hub_control+688>: sub r8, r3, #1 ; 0x1
>>>>>> 0xc0229824 <ehci_hub_control+692>: ldr r5, [r7, #4]
>>>>>> 0xc0229828 <ehci_hub_control+696>: uxth r8, r8
>>>>>> 0xc022982c <ehci_hub_control+700>: dmb sy
>>>>>>
>>>>>> With GCC 4.6.0:
>>>>>> 0xc0221630 <ehci_hub_control+808>: cmp r3, #0 ; 0x0
>>>>>> 0xc0221634 <ehci_hub_control+812>: beq 0xc0221d7c
>>>>>> <ehci_hub_control+2676>
>>>>>> 0xc0221638 <ehci_hub_control+816>: cmp r3, r0
>>>>>> 0xc022163c <ehci_hub_control+820>: bgt 0xc0221d7c
>>>>>> <ehci_hub_control+2676>
>>>>>> 0xc0221640 <ehci_hub_control+824>: sub r7, r3, #1 ; 0x1
>>>>>> 0xc0221644 <ehci_hub_control+828>: add r3, r11, #16 ; 0x10
>>>>>> 0xc0221648 <ehci_hub_control+832>: add r3, r8, r3, lsl #2
>>>>>> 0xc022164c <ehci_hub_control+836>: uxth r7, r7
>>>>>> 0xc0221650 <ehci_hub_control+840>: ldrb r5, [r3, #5]
>>>>>> 0xc0221654 <ehci_hub_control+844>: ldrb r2, [r3, #4]
>>>>>> 0xc0221658 <ehci_hub_control+848>: orr r5, r2, r5, lsl #8
>>>>>> 0xc022165c <ehci_hub_control+852>: ldrb r2, [r3, #6]
>>>>>> 0xc0221660 <ehci_hub_control+856>: ldrb r3, [r3, #7]
>>>>>> 0xc0221664 <ehci_hub_control+860>: orr r5, r5, r2, lsl #16
>>>>>> 0xc0221668 <ehci_hub_control+864>: orr r5, r5, r3, lsl #24
>>>>>> 0xc022166c <ehci_hub_control+868>: dmb sy
>>>>>>
>>>>>> As you can see, the old compiler accesses the ehci status register
>>>>>> in a single access, the new compiler dances around and makes multiple
>>>>>> accesses, which in the end get very incorrect data (I think that this
>>>>>> register is like many which clear bits on reads).
>>>>>>
>>>>>> Any ideas where I can go with this? I'm really trying to keep up with
>>>>>> Poky/Yocto, but this move to GCC 4.6.0 has broken my ARM targets :-(
>>>>>> I do have PowerPC targets as well - they seem fine (from limited
>>>>>> testing)
>>>>>> with the new compiler.
>>>>>>
>>>>>> Note: if you want to see the whole function disassembly, look at
>>>>>> http://www.mlbassoc.com/poky/ehci_hub_control-disassembly-gcc-4.5.2
>>>>>> http://www.mlbassoc.com/poky/ehci_hub_control-disassembly-gcc-4.6.0
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>
>>>> OK. thanks for sending the sample code.
>>>> Can you add -fstrict-volatile-bitfields option to CFLAGS while
>>>> compiling this file or even the kernel for test sake ?
>>>> and see if the problem goes away ?
>>>
>>> Sorry, this doesn't seem to make any difference in the generated code.
>>> Hardly surprising as the register in question is a simple u32, not a
>>> bitfield.
>>>
>>
>> I reduced the testcase a bit further and it seems the bug is similar to http://gcc.gnu.org/bugzilla/PR45819
>>
>> but a bit different that in this case -fstrict-volatile-bitfields does not help
>>
>> a fix for this particular problem is
>>
>> diff --git a/include/linux/usb/ehci_def.h b/include/linux/usb/ehci_def.h
>> index 6563802..b8c1833 100644
>> --- a/include/linux/usb/ehci_def.h
>> +++ b/include/linux/usb/ehci_def.h
>> @@ -194,7 +194,7 @@ struct ehci_dbg_port {
>> u32 data47;
>> u32 address;
>> #define DBGP_EPADDR(dev, ep) (((dev)<<8)|(ep))
>> -} __attribute__ ((packed));
>> +} __attribute__ ((packed,aligned(__alignof__(int))));
>>
>> #ifdef CONFIG_EARLY_PRINTK_DBGP
>> #include <linux/init.h>
>>
>>
>> since this particular structure all elements are integers but there might be more packed structures which have say char or short ints
>> and are not necessarily ordered in the descending order of alignment
>>
>> Try it out and see if it helps
>
> Yes, this does fix the problem :-) The EHCI USB port on the OMAP
> is working again.

Note: it also works properly if the __attribute__ ((packed)) is simply removed.
Looking at the layout of this structure, I don't see what that attribute is
trying to accomplish - every item within the structure is u32[], so IMO there
is nothing to pack.  Perhaps this observation can help the GCC folks figure
out why things are getting confused.

Thanks again

-- 
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Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
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