Re: [PATCH] ARM: exynos_defconfig: disable CONFIG_EXYNOS5420_MCPM; not stable

[Sudeep Holla <sudeep.holla@arm.com>]

On 26/11/14 18:41, Nicolas Pitre wrote:
> On Wed, 26 Nov 2014, Kevin Hilman wrote:
>
>> Abhilash Kesavan <kesavan.abhilash@gmail.com> writes:
>>
>>> Hi Kevin,
>>>
>>> On Wed, Nov 26, 2014 at 6:30 AM, Kevin Hilman <khilman@kernel.org> wrote:
>>>> [...]
>>>>
>>>> More specifically, with only the loopback call to turn off CCI commented
>>>> out, the imprecise aborts go away.
>>>
>>> I can't see how enabling snoops for the boot cluster is causing these
>>> aborts. Perhaps as Krzysztof commented it has something to do with the
>>> secure firmware/tz software on these boards ? Other than there does
>>> not appear to be any difference between the working/non-working
>>> setups.
>>
>> Perhaps the secure firmware is preventing the CCI to be enabled by the
>> kernel, and that is causing the imprecise abort?
>
> That is well possible.
>
> Now...... if the bootloader/firmware does not let Linux deal with both
> the CCI and caches then MCPM simply has no more purpose for this board.
> The whole point of MCPM is actually to handle the CCI properly and the
> most efficient way despite all the possible races and opportunities for
> memory corruptions. And yes, this is a complex task.
>
> So there is actually two choices: the firmware let Linux take care of it
> via the MCPM layer (easy), or the firmware has to implement it all
> _properly_ (hard) behind an interface such as PSCI, at which point MCPM
> should be configured out.
>
> If the firmware does not let Linux interact with the CCI _and_ does not
> implement full MCPM-like services then the platform is broken and only a
> firmware upgrade could fix that.  It might still be possible to boot all
> CPUs through other means, but power management would then be severely
> limited.
>

Thanks Nico for the detailed description on the requirements for using
MCPM. This is the kind of issue I was worried in the other thread on
Fijitsu platform. That's the reason I was asking the information about
their secure firmware and what exactly it configures so that we won't
end up with similar situation on there too and definitely not to push
PSCI. I completely agree with you that making a some change in firmware
to give control of CCI to kernel is easy.

Probably if the vendors disagree to apply this small fix to the firmware
we should provide them with *only choice* of PSCI implementation which
is quite complex and easy to get it wrong. That might trigger them to
provide a small fix to use MCPM.

Regards,
Sudeep