[U-Boot] scripts: RPi 2: only 1 out of 4 CPUs brought up

[Eric Anholt <eric@anholt.net>]

Stephen Warren <swarren@wwwdotorg.org> writes:

> On 06/30/2015 05:56 AM, Jonas Jensen wrote:
>> Hello,
>> 
>> I have found the following issue with RPi 2:
>> 
>> Only 1 CPU is brought up when the kernel is started from script (see [1]).
>> 
>> All 4 CPUs are brought up if started "manually" typing in environment
>> variables from said script (see [2]).
>
> I suspect it's a fluke that it works when you run things manually/slowly.
>
> I /think/ that before the binary FW starts U-Boot (or whatever it
> boots), it releases all 4 CPUs from reset, and CPU1..n all end up
> running a tiny piece of code ("SMP pen") that just loops doing nothing
> until some flag is set. All code that runs on CPU0 must be careful not
> to corrupt that code or the flag it uses. However, U-Boot (and I expect
> the upstream kernel) don't yet know about this, and hence
> sometimes/often corrupt that SMP pen, resulting in strange behaviour.
> I'm pretty sure I've seen all 4 CPUs start booting the Linux kernel in
> parallel before.
>
> In summary, I know there are issues in this area when using U-Boot. I
> don't know of any fix at present. Either U-Boot must hard-code some
> reserved memory regions, or perhaps the binary FW has some way of
> informing SW where the SMP pen region is, and U-Boot needs to learn how
> to find/interpret that information (and pass it to the kernel via
> /memreserve/ or similar in DT).
>
> Eric, did you find out any more details on the SMP pen mechanism since I
> last asked you about it?

Oh, I thought we were settled on this back in May -- the CPUs are
spinning in the low 8kb.  They are in that state by the time the
firmware has handed off to us.  I think it's modifying a /memreserve/
node for you to know where the pen is.

Hey, do you know what's going on with merging code?  I feel really stuck
here -- I've got a graphics driver, and a Pi2 implementation, and Andrea
has cpufreq, and we're still stuck on getting the firmware driver
merged.
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