Re: QSD8250 illegal instruction on WinCE devices

[Martijn Stolk <martijn.stolk@gmail.com>]

On 06/29/2010 10:38 PM, Daniel Walker wrote:
> On Tue, 2010-06-29 at 21:03 +0200, Martijn Stolk wrote:
>> No worries. I wanted to isolate what specifically fixes the problem
>> myself in order to make a minimal patch. I'm also hoping for some help
>> regarding information, as I don't have access to official QSD8250
>> documentation.
>>
>> I've found the key register that differs between between how it is
>> initialized for Windows CE and for Linux, and solves the problem for us.
>> It is the Auxilliary Control Register (cp15, 0, c1, c0, 1).
>>
>> For Windows CE it is initialized to 0x002C0077
>> (=0b00000000001011000000000001110111)
>> For Linux it is initialized to 0x000C0037
>> (=0b00000000000011000000000000110111)
>>
>> The following page explains this register for the Cortex-A8, the ARM
>> core on which the QSD8250 is based:
>> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0344k/Bgbffjhh.html
>>
>> The 7th bit (bit 6 on that page) controls the behaviour of the
>> Invalidate All&  by MVA instructions. It needs to be disabled for Linux.
>>
>> The 22nd bit, however, is in an area marked as "reserved" on that page.
>> I am very curious what this bit does. Maybe this reserved area is
>> documented in QSD8250 documentation? Could anyone provide clearity about
>> this bit?
>
> You found that you needed both of these set? One or the other wasn't
> enough.
>
> I'm not sure that we can open talk about what this 22nd bit does, so I'd
> just set it like you have it and not worry about it.
>
> Daniel
>
Here's a table on what I noticed during my tests:

bit: 7 | 22 | remark
      1 |  1 | Linux crashes due to segmentation faults
      0 |  1 | Linux works fine, no crashes, no segfaults
      0 |  0 | Same as above
      1 |  0 | Linux doesn't segfault immediately, but seems to only 
cause problems
               when the device is idling for a second or less (maybe 
power management
               or something).

Disabling bit 7 solves the segfault issues already. Bit 22 just causes 
some mysterious behaviour which I am curious about. We're able to 
continue without knowing this of course, but I was hoping you were 
allowed to answer a specific question like this. Don't worry if you can't.

Thanks either way for your help.

Regards,
Martijn