[U-Boot] [PATCH 3/9] arm: Move CP15 init out of cpu_init_crit()

[Albert ARIBAUD]

Le 27/10/2011 22:17, Simon Glass a écrit :
> Hi Albert,
>
> On Thu, Oct 27, 2011 at 1:05 PM, Albert ARIBAUD
> <albert.u.boot at aribaud.net>  wrote:
>> Hi Simon,
>>
>> Le 25/10/2011 23:16, Simon Glass a écrit :
>>
>>>> But please think about what you actually want in start.S. Do you
>>>> really want an #ifdef TEGRA2 with a check of a SOC register, followed
>>>> by a branch around the cp15_init code?
>>>
>>> ^^^^
>>> This is the bit we need to sort out.
>>
>> Indeed, but I dont want a '#ifdef TEGRA2'. In the worst case I want a 'if
>> (!core_is_avp())' by way of an identification register, and in the best
>> case, I just want cp15 to be set unconditionally.
>
> You already have #ifdef TEGRA2 (look at the top of start.S). That is
> what I am trying to remove.
>
> How can I call core_is_avp() on an SOC that isn't Tegra? Can you
> please write a little code fragment that I should use in start.S, so
> that I can understand what is required here.
>
>>
>>>>>>> According to ARM, coprocessors 14 and 15 should always be supported
>>>>>>> from
>>>>>>> ARMv6 up. Are you sure the AVP does not have it? If there is a Tegra
>>>>>>> specification available, I'd like to have a look at it.
>>>>>>
>>>>>> Sorry I meant caches/MMU, etc. I should try doing the CP15 cache/MMU
>>>>>> init on the AVP and see what happens.
>>>>>
>>>>> Please do check this. It may be that you can actually write to it from
>>>>> AVP
>>>>> even though it has no or not all expected effect; and if that is
>>>>> confirmed,
>>>>> then you don't have to separate the boot paths any more,
>>>>
>>>> Yes I'm tempted to go with this even if the ARM ARM is silent on the
>>>> matter. I will try it out today.
>>>
>>> Silly me of course it doesn't work - since if the CP15 doesn't have
>>> those features then you get undef instruction exception. Please see
>>> ^^^ above!
>>
>> Are you stating a reasoning here or the results of an actual test? What
>> causes an undefined instruction exception is the absence of a coprocessor to
>> handle the instruction, not the presence of a coprocessor which incidentally
>> cannot perform all of what"s being asked.
>
> Yes I did test it - it hangs. Would you like me to spend some time to
> debug exactly why? I suspect it is an undef instruction, since
> executing cache instructions on an ARM7TDMI is dodgy, but I could
> check if it would help. But let's assume for now we can't do that!

What was the test exactly? Depending on which register of CP15 you try 
to access, things could be different. Can you test MRC 
p15,0,<Rt>,c0,c0,0 ? That's the MIDR and it should be always accessible, 
plus it should have a unique value for each type of core, so it could be 
used to not write cp15 for caches etc from the AVP.

> Regards,
> Simon

Amicalement,
-- 
Albert.