Re: BeagleBoard using GCC 4.6.0

[Darren Hart <dvhart@linux.intel.com>]

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?

--
Darren

> 
> 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
> 

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel