Re: [PATCH] ARM: OMAP: Use ARM SMC Calling Convention when OP-TEE is available

From: Andrew F. Davis
Date: Tue Nov 19 2019 - 14:35:40 EST

On 11/19/19 2:20 PM, Tony Lindgren wrote:
> * Andrew F. Davis <afd@xxxxxx> [191119 19:13]:
>> On 11/19/19 2:07 PM, Tony Lindgren wrote:
>>> * Andrew F. Davis <afd@xxxxxx> [191119 18:51]:
>>>> On 11/19/19 1:32 PM, Tony Lindgren wrote:
>>>>> It would allow us to completely change over to using
>>>>> arm_smccc_smc() and forget the custom calls.
>>>> We would need more than just the r12 quirk to replace all our custom SMC
>>>> handlers, we would need quirks for omap_smc2 which puts process ID in r1
>>>> and puts #0xff in r6, and omap_smc3 that uses smc #1. All of our legacy
>>>> SMC calls also trash r4-r11, that is very non SMCCC complaint as only
>>>> r4-r7 need be caller saved. I don't see arm_smccc_smc() working with
>>>> legacy ROM no matter how much we hack at it :(
>>> We would just have omap_smc2() call arm_smccc_smc() and in that
>>> case. And omap_smc2() would still deal with saving and restoring
>>> the registers.
>> Then why call arm_smccc_smc()? omap_smc2() is already an assembly
>> function, all it needs to do after loading the registers and saving the
>> right ones is issue an "smc #0" instruction, why would we want to
>> instead call into some other function to re-save registers and issue the
>> exact same instruction?
> To use Linux generic API for smc calls where possible.

But we are not using generic API calls, we are using omap_smcx() which
cannot call into arm_smccc_smc(). For all the above reasons plus
arm_smccc_smc() uses r12 to save the stack pointer, our ROM expects r12
to store the function ID.

>>> Certainly the wrapper functions calling arm_smccc_smc() can deal
>>> with r12 too if the r12-quirk version and the plain version are
>>> never needed the same time on a booted SoC.
>>> Are they ever needed the same time on a booted SoC or not?
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

They should not be needed at the same time, either OP-TEE is on the
secure side or ROM is there.


> Sorry but maybe check the font size on your screen. I'm trying to
> get your attention again for the second time above to answer a
> question I asked.
>>>> I can make OP-TEE also compatible with the r12 quirk, which is what I
>>>> used to do. That way we didn't need to do any detection. The issue was
>>>> that non-standard SMC calls should not go through the common SMCCC
>>>> handler (unless you are QCOM for some reason..).
>>> Sounds like for optee nothing must be done for r12 :)
>> Unless all our calls use the r12 hack, then we would need to fixup
>> OP-TEE to accept that also.
> No idea about that that part, but sounds like r12 use is up to
> the caller in the optee case.
> Regards,
> Tony