PERFORCE change 135517 for review

[Rafal Jaworowski]

Marcel Moolenaar wrote:
>>>>>>>
>>>>>>>   Save U-Boot's registers at startup and restore them when
>>>>>>>   performing a syscall. This way we don't have to compile
>>>>>>>   code specially to avoid using those registers. Otherwise
>>>>>>>   we have to encode knowledge of those registers in at least
>>>>>>>   4 makefiles and introduce a build knob to enable it all.
>>>>>>>   This does not allow us to build everything with a single
>>>>>>>   build world.
>>>>>>>
>>>>>>
>>>>>> Hi Marcel,
>>>>>>
>>>>>> I'm not quite sure this is sufficient... I already had a similar
>>>>>> save/restore
>>>>>> in place, but there is some general problem with U-Boot that leads to
>>>>>> hangs
>>>>>> (experienced):
>>>>>
>>>>> Interesting, I didn't see any such problems with 1.3.2-rc1.
>>>>>
>>>>
>>>> Chances are it might not surface, depending on the regs usage pattern,
>>>> compiler etc., for example a -O0 build would usually hide this issue,
>>>> but in
>>>> principle those regs are not exception/interrupt safe.
>>>
>>> Agreed. I didn't see it at -O2, BTW.
>>>
>>
>> When I was looking at this it also did not always pop up (with -O2 too),
>> although typically when loader(8) was executed and left idle for some
>> time it
>> would hang sooner or later.
>>
>>> We can disable interrupts when not running in U-Boot, right?
>>> The impact should be marginal...
>>>
>>
>> Yes, this could be worked around like this, but with degraded
>> functionality:
>> all timer-related calls from the API would not make sense, and things
>> like
>> autoboot count down will not work. Even worse, the [polled] networking
>> might
>> not work at all if decrementer was shut, as we'd not be able to time
>> out while
>> waiting on the packet, status registers' change and so on. I think
>> this issue
>> needs some further investigation and proper resolution.
> 
> We always call into U-Boot when we're waiting, so interrupts should be
> enabled when it's important. This is the case when we wait for time to
> pass, a key press or a network packet. In all those cases we shouldn't
> be running for extensive amounts of time with interrupts disabled, so
> we shouldn't lose too much precision, keys or packets.
> 

Well, the point is we would need to run with interrupts disabled during all
loader's code paths, so that interrups could be allowed only for the duration
of the syscall; not sure how this would serve. Anyway, let me know your findings.

> I'll play with this and see if that statement is true...
> 

>From the other end: I'll take a look at U-Boot exceptions handlers macros for
the prologue/epilogue and try to fix this down there. It shouldn't be
difficult, but propagating the changes to their main line code will take time...

Rafal