Data Abort MMU exception on mapped IO register read

Stefan Kalkowski stefan.kalkowski at ...1...
Tue Apr 14 09:27:03 CEST 2015

Hi Bob,

On 04/13/2015 02:41 PM, Bob Stewart wrote:
> Hi,
> I'm running with 15.02, base-hw on a TI AM3359 processor. I've 
> successfully created device drivers for most of the devices on the 
> peripheral bus without issue. Accessing the the final device i need a 
> driver for, is causing me fits.
> I attached the IO device register space with the usual creation of a 
> Attached_io_mem_dataspace  based object which also uses the very useful 
> MMIO utility object. I also tried it without MMIO and just accessed the 
> space through local_addr<void>.
> When I try to read an address in the mapped space I get an "unknown MMU 
> exception". When I write to the same address no error occurs (I'm 
> assuming for now it's failing silently). Putting PDBG's in the arm 
> thread code shows that the error on the read is is being caused by a 
> DATAT_ABORT. Peeking at the data fault status register (dfsr) shows 
> 0x805 which according the the the ARM7 technical reference manual 
> decodes to a translation table error.

What make me wonder is that a DFSR value of 0x805, which means as you
already stated a first-level translation fault, should never end in a
"unknown MM exception" message. Getting this kind of translation fault
is pretty normal, and normally leads to the translation tables being

When you look at the corresponding lines of code:


Actually when following the execution path you should never get the
message with such a DFSR. Are you sure you examined the fault correctly?
Do you print 'cpu_exception', and DFSR value directly inside the
"unknown MMU exception" message scope?

> The virtual address created when the io device is mapped is 0x81B0_0000 
> (the load_addr<void>), but I see no section table entry being created 
> for 0x8100_0000. Only one is created at 0x8000_0000. If I'm remembering 
> correctly sections are 16 MB is size for this architecture. I'm 
> instrumenting short_translation_table.h to see what translation table 
> entries are being created. I also don't seen any short translation table 
> entries being created in the 0x81bxx_xxxx region.

As being said, this is normal. You should not see any translation table
entries before it is accessed the first time. It is populated lazily.
Normally the exception you describe is the correct time to fill in the
entries. The question is why the exception is not treated as an ordinary
page-fault exception, but as an unusual type of MMU exception instead.

I assume, you have examined the ordinary exception, but not the actual
problematic MMU exception (which is probably an external data-abort -
means bus error). Please try out the attached patch, and post the
message results.


> My platform support code does include the physical address and size for 
> the region containing the device I'm trying to access.
> The only physical difference with this device and the others I've 
> successfully created drivers for, is that is runs of the L4 
> interconnection whereas the others were on the L3 interconnect. In this 
> SoC the L4 is bridged to the L3 interconnect and runs at half the speed 
> of the L3 bus. But I doubt that this would make any difference to the 
> mapping of the device. This device is also much higher in memory that 
> the other devices.
> Any hints at further debugging of this issue would be greatly appreciated.
> Thanks,
>              Bob Stewart
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Stefan Kalkowski
Genode Labs ยท
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