[U-Boot] [PATCH v7 1/2] armv8: Support loading 32-bit OS in AArch32 execution state

[Alison Wang]

> On 11/04/2016 10:12 AM, Alexander Graf wrote:
> >
> >
> > On 04/11/2016 17:08, york sun wrote:
> >> On 11/04/2016 09:53 AM, Alexander Graf wrote:
> >>>
> >>>
> >>> On 04/11/2016 16:43, york sun wrote:
> >>>> On 11/04/2016 09:32 AM, Ryan Harkin wrote:
> >>>>>>>
> >>>>>>> Yes, with the attached patch on top of your original 2 patches,
> >>>>>>> everything works again.  I tested on FVP Foundation and AEMv8
> >>>>>>> models and Juno R0, R1 and R2.
> >>>>>>>
> >>>>>>> I don't think it would be good to stack these three patches the
> >>>>>>> way they are presented in the upstream tree because it would
> not
> >>>>>>> be bisect-able.  Some re-work or re-ordering would be needed.
> >>>>>>>
> >>>>>>> Note: I haven't attempted to understand what any of this code
> is
> >>>>>>> doing, I'm just testing it with my standard boot flow to make
> >>>>>>> sure nothing is broken for me.
> >>>>>>
> >>>>>> Ryan,
> >>>>>>
> >>>>>> I support Alison's patch order for her 32-bit patch sets. This
> >>>>>> feature doesn't exist before her first set. It is functional if
> >>>>>> you run U-Boot at EL3 after the first patch.
> >>>>>
> >>>>> Which I don't do.  I follow the boot flow recommended by ARM and
> >>>>> it doesn't work for that setup, which I don't think is the right
> >>>>> thing to do.
> >>>>>
> >>>>>
> >>>>>> It gets EL2 working after the 2nd set. If there is room to
> >>>>>> clarify in the commit message, please kindly suggest.
> >>>>>>
> >>>>>
> >>>>> Well, I'm not the maintainer of the tree, but I wouldn't want to
> >>>>> have a tree that wasn't bootable at any point in the patch
> sequence.
> >>>>> That's generally unacceptable on most projects I work on.
> Keeping
> >>>>> the tree bisect-able to prove which commit caused a problem is
> >>>>> considered to be a valuable tool.
> >>>>>
> >>>>
> >>>> Ryan,
> >>>>
> >>>> Thanks for sharing your concern. I support git-bisect. It is
> >>>> valuable, no doubt. Let me try to understand the issue here.
> >>>> Without Alison's patches, everything boots OK. With her first set,
> does something break?
> >>>
> >>> Yes, with the patches booting 64bit Linux with U-Boot running in
> EL2
> >>> breaks according to Ryan.
> >>>
> >>>> My understanding is 32-bit OS can boot. If existing 64-bit OS
> >>>> fails, then she needs to fix it.
> >>>
> >>> That's his point :). And I concur.
> >>
> >> Thanks for the confirmation.
> >>
> >>>
> >>> (btw, you guys really should start thinking about following the ARM
> >>> recommended boot model. It's pretty cumbersome to do everything
> >>> different just for NXP)
> >>
> >> If you are referring the trusted firmware, we are following that
> >> direction. Just not fully up yet on some platform.
> >>
> >> It is definitely not our intention to be cumbersome. Please point
> out
> >> where it went sideway beside the trusted firmware.
> >
> > Basically it boils down to the fact that you are the only platform
> > that runs U-Boot in EL3 :).
> >
> > If you want to keep the memory initialization inside of U-Boot, I
> > think that's great. But you could either split that into SPL/EL2 or
> > degrade yourself into EL2 as soon as possible by calling into an EL3
> firmware.
> > Whether you build that firmware as part of U-Boot (the stock one
> could
> > be very trivial) or externally is not really too much of a problem.
> >
> > Things like Alison's patches could then do a simple PSCI call into
> > said
> > EL3 firmware to call into 32bit code in EL2 for example.
> >
> 
> Basically I agree with you. U-Boot will run at EL2 as soon as we have
> the trusted firmware in place.
> 
[Alison Wang] Thanks for all your comments.

For the issue about the tree would not be bisect-able, I have
a solution. Actually it is the root cause that 64-bit kernel could not boot
up when U-Boot is running in EL2. I will move these codes from the third patch
to the first patch.

ENTRY(armv8_switch_to_el2)
        switch_el x5, 1f, 0f, 0f
-0:     ret
+       /*
+        * x3 is kernel entry point or switch_to_el1
+         * if CONFIG_ARMV8_SWITCH_TO_EL1 is defined.
+         * When running in EL2 now, jump to the
+         * address saved in x3.
+        */
+0:     br x3
 1:     armv8_switch_to_el2_m x3, x4, x5
 ENDPROC(armv8_switch_to_el2)

 ENTRY(armv8_switch_to_el1)
 	  switch_el x5, 0f, 1f, 0f
-0:     ret
+
+       /*
+         * x3 is kernel entry point. When running in EL1
+         * now, jump to the address saved in x3.
+        */
+0:     br x3
 1:     armv8_switch_to_el1_m x3, x4, x5
 ENDPROC(armv8_switch_to_el1)

With this re-order, the bitsect issue will be fixed and there is not a point
that kernel could not boot up.

If you all agree with this re-order, I will send out the v8 patch includes the
first, second and third patches.


Best Regards,
Alison Wang