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

Andrew F. Davis
2019-11-19T19:59:24Z

On 11/19/19 2:44 PM, Tony Lindgren wrote:
> * Andrew F. Davis <afd@ti.com> [191119 19:36]:
>> On 11/19/19 2:20 PM, Tony Lindgren wrote:
>>> * Andrew F. Davis <afd@ti.com> [191119 19:13]:
>>>> On 11/19/19 2:07 PM, Tony Lindgren wrote:
>>>>> * Andrew F. Davis <afd@ti.com> [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.
>
> Saving and restoring r12 could be handled by the arm_smccc_smc() quirk
> for the non-optee case.
>
> Then we could get rid of omap_smc1() and arm_smccc_smc() should work
> for the optee case and non-optee case, right.
>

Yes, we could have both cases working if we could get the quirk in.

>>>>> 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.
>
> OK thanks. So we could just modify the code dynamically on boot
> based on if optee is found or not. The quirk could be done along
> the lines of the qcom quirk but only for the non-optee case:
>

We wouldn't have to patch anything if we could get the quirk in. One has
to state they wish to use the quirk version in a structure passed into
arm_smccc_smc_quirk(), in which case for all legacy user we just fill
out this quirk struct. OP-TEE uses the same arm_smccc_smc() but without
the quirk struct and so it uses the compliant call.

The issue is still the same, I tried adding this, I got NAKd, if you
want to convince Mark to change his mind and allow us the quirk then we
can go down this path. Otherwise this will remain a dead end.

Andrew

> $ git grep -C10 ARM_SMCCC_QUIRK_QCOM_A6
>
> Regards,
>
> Tony
>