[Linux-Xtensa] Re: [PATCH 0/3] xtensa: add MMUv3 and medium-level interrupts support

Pete Delaney piet.delaney at gmail.com
Tue Jan 29 20:05:31 PST 2013

On 01/14/2013 07:57 AM, Chris Zankel wrote:
> Sorry, missed them. Added to for_next.
> Thanks,
> -Chris
> On 1/12/13 10:04 PM, Max Filippov wrote:
>> On Sun, Jan 13, 2013 at 9:29 AM, Chris Zankel <chris at zankel.net> wrote:
>>> Hi Max,
>>> I have applied your patches except for the MMU v3 patch. It feels that
>>> it needs a bit more clean-up, but I need to go through it more
>>> thoroughly to be sure. For example, it's about MMU, but the description
>>> only talks about GDB. The comments in general feel overwhelming.
>>> Although, they maybe necessarily need to be. I'll get back to you on
>>> this one.
The folks in India found it very helpful while bringing up their V3 MMU
to be able to single step through the initial TLB brain surgery and for
DDD+gdb to correctly display the code, even when switching from the
initial virtual == physical, through to the 0x4000,000 mapping, and then
to the final 0xd000,0000 mapping.

Most CPU don't even support doing the TLB re-mapping in the kernel.
This TLB remapping sure makes it a lot easier when debugging early
startup with JTAG.

Try single stepping the early MMU initialisation with DDD+GDB and see
for yourself if it's not worth it. This is very very early
initialisation, having
a clear definition of what the TLB should look like is extremely valuable,
at least from my experience when I did it and with Mahavir last year when
we had a bug during this early startup phase. Marc recognised the root cause
when the TLB was out of sync a few stack frames later when initialising the
platform code. This early environment is nasty when things go haywire.
When bringing up a chip all kinds of hardware mistakes could cause bizarre
problems that are difficult to diagnose. Having a guiding hand during the
first 60 instructions of life can be god send.


>> Thank you, Chris.
>> BTW, there are two more fixes in the xtensa-fixes-for-upstream branch
>> that were posted a while ago, but somehow missed your for_next branch:
>> c6e407d xtensa: add finit_module syscall
>> 0fbadfb xtensa: avoid mmap cache aliasing
>> I have rebased them on top of the for_next, could you please take them
>> as well?
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