[poky] BeagleBoard using GCC 4.6.0

[Khem Raj]

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.
>
No. Its not a simple u32
its a pointer and points to address of a member of structure type 
ehci_hcd which can have bitfields. There has been bug reports in this 
area in GCC which
have been fixed and some are not. I was merely trying to reduce the 
problem. So you tried compiling kernel with this option and you found
that it does not work ? Did I understand it correct