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

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Date:   Tue, 19 Nov 2019 14:12:18 -0500
From:   "Andrew F. Davis" <afd@...com>
To:     Tony Lindgren <tony@...mide.com>
CC:     Mark Rutland <mark.rutland@....com>, <linux-omap@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: OMAP: Use ARM SMC Calling Convention when OP-TEE is
 available

On 11/19/19 2:07 PM, Tony Lindgren wrote:
> * Andrew F. Davis <afd@...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?


> 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?
> 
>> 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.

Andrew


> Regards,
> 
> Tony
>