Re: BeagleBoard using GCC 4.6.0

[Gary Thomas <gary@mlbassoc.com>]

On 06/08/2011 12:21 PM, Darren Hart 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).
>
> Gary,
>
> Thank you for digging into this. Excellent timing as I was looking at
> the same thing - I hadn't looked at the compiler yet as I was in the
> middle of some kernel work and had assumed the problem lay there. I'll
> see if I can try with the older kernel to get this resolved as well.
> Have you tried with various optimization options yet?

Not yet, I've only tried the basic options, etc, to try and see where
things might be going South.

>>
>> 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.
>>
>> 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
>>
>

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