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 11:20:29 -0800
From:   Tony Lindgren <tony@...mide.com>
To:     "Andrew F. Davis" <afd@...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

* Andrew F. Davis <afd@...com> [191119 19:13]:
> 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?

To use Linux generic API for smc calls where possible.

> > 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?

   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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