Problems booting exynos5420 with >1 CPU

[Abhilash Kesavan]

Hi Olof,

On Sat, Jun 7, 2014 at 2:07 AM, Olof Johansson <olof at lixom.net> wrote:
> [Adding Nico since he was involved in the original reviews]
>
> Hi,
>
> On Fri, Jun 06, 2014 at 11:20:56AM -0700, Doug Anderson wrote:
>> Abhilash,
>>
>>
>>
>> On Fri, Jun 6, 2014 at 11:12 AM, Abhilash Kesavan
>> <kesavan.abhilash at gmail.com> wrote:
>> > Hi Doug,
>> >
>> > On Fri, Jun 6, 2014 at 11:32 PM, Doug Anderson <dianders at google.com> wrote:
>> >> Abhilash,
>> >>
>> >> On Fri, Jun 6, 2014 at 10:36 AM, Abhilash Kesavan
>> >> <kesavan.abhilash at gmail.com> wrote:
>> >>> Hi Doug,
>> >>>
>> >>> The first change in the kernel (clearing an iRAM location) is needed
>> >>> because of an unnecessary change that we are carrying in the Chrome
>> >>> U-boot. There is no reason for us to have the workaround in the
>> >>> mainline kernel. Rather, we should remove the check from our u-boot.
>> >>> However AFAIR a clean-up patch that I had posted internally was not
>> >>> accepted as we had frozen the SPL at the time.
>> >>
>> >> Ah, is that this one, or a different one?
>> >>
>> >> https://chromium-review.googlesource.com/#/c/66049/
>> > Yes, this along with a kernel side change.
>>
>> Can we safely take this one without the kernel-side one?
>>
>>
>> >> If we land that patch now it won't help since nobody is going to be
>> >> updating their read-only firmware.  We'll need to put code somewhere
>> >> that fixes it.
>> > We just carry the workaround fix locally until we migrate to mainline
>> > u-boot for 5420 where the unnecessay check will not be present.
>>
>> I think there are people out there who want to run a mainline kernel
>> on existing Chromebook 2 hardware and don't want to rewrite their RO
>> firmware.  We need a solution for those people.
>
> Agree. The answer to this is most definitely _not_ "install mainline u-boot".
> The upstream kernel needs to be able to boot with the firmware that was shipped
> on the device.
My answer is not "use mainline u-boot" primarily because I am not sure
mainline u-boot actually works on 5420 :). My answer is keep a patch
locally (or make a trivial change to the bootcmd) for people who would
like to use an upstream kernel with the firmware on the device. Once
we do have a working mainline u-boot, that can then be used by the
interested parties.
>
> In this case it shouldn't be controversial to add this. What we need is
> a one-time boot-time setup, not runtime so cpuidle shouldn't be a factor
> at that time. The earlier reservations were about runtime changes and this is
> quite different.
I think there is some confusion here, the clearing of the iRAM
location is what I have been pushing against. It has got nothing to do
cpuidle. If it were to be done then  it would be a one time setup and
I could quite easily do it in mcpm_init.

Regards,
Abhilash
>
>> >>> The second change is to enable snoops for boot cluster. Internally, we
>> >>> were disabling the snoops for both the clusters at power off and
>> >>> enabling it in power_up_setup and power_up. However, I dropped the
>> >>> approach due to problems pointed out by Nicolas here
>> >>> http://www.spinics.net/lists/arm-kernel/msg324091.html related to
>> >>> cpuidle. Hence, we turn it on at the u-boot.
>> >>
>> >> I think I followed all that.  What you're saying is that our kernel
>> >> dynamically enables and disables snoops as needed, but Nicolas pointed
>> >> out that it was unsafe (though apparently we're not seeing problems in
>> >> our usage).
>> > We did not see any problems as CPUIdle was one of the problematic
>> > scenarios which we have not got enabled.
>>
>> Ah, makes sense!
>>
>>
>>
>> I'm still trying to figure out all of this code, but we'll also need
>> to make sure whatever solution we come up with handles suspend/resume
>> properly.  I know SRAM is lost across suspend/resume so someone
>> (either the SPL from read-only memory or the kernel) must be
>> recreating the SRAM structures after S2R...
>
>
> -Olof