Date: Fri, 12 Jul 2013 11:24:37 +0100
From: Dave Martin <Dave.Martin@arm.com>
To: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: =?utf-8?B?0JjQstCw0LnQu9C+INCU0LjQvNC40YLRgNC+0LI=?= 
	<freemangordon@abv.bg>,
        nm@ti.com, linux@arm.linux.org.uk, aaro.koskinen@iki.fi,
        tony@atomide.com, pdeschrijver@nvidia.com,
        linux-kernel@vger.kernel.org, pavel@ucw.cz, pali.rohar@gmail.com,
        linux-omap@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 1/2] ARM: OMAP: Add secure function omap_smc3() which
 calling instruction smc #1
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 <51DF0D7A.4090008@ti.com>
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On Thu, Jul 11, 2013 at 03:54:34PM -0400, Santosh Shilimkar wrote:
> On Thursday 11 July 2013 03:43 PM, Ивайло Димитров wrote:
> >  
> > 
> > 
> > 
> > 
> >  >-------- Оригинално писмо --------
> >  >От:  Dave Martin 
> >  >Относно: Re: [PATCH v2 1/2] ARM: OMAP: Add secure function omap_smc3() which
> >  calling instruction smc #1
> >  >До: Pali Rohár 
> >  >Изпратено на: Сряда, 2013, Юли 10 20:45:26 EEST
> >  >
> >  >
> >  >On Wed, Jul 10, 2013 at 02:59:04PM +0200, Pali Rohár wrote:
> >  >> Other secure functions omap_smc1() and omap_smc2() calling instruction smc #0
> >  >> but Nokia RX-51 board needs to call smc #1 for PPA access.
> >  >> 
> >  >> Signed-off-by: Ivaylo Dimitrov 
> >  >> Signed-off-by: Pali Rohár 
> >  >> ---
> >  >>  arch/arm/mach-omap2/omap-secure.h |    1 +
> >  >>  arch/arm/mach-omap2/omap-smc.S    |   22 +++++++++++++++++++++-
> >  >>  2 files changed, 22 insertions(+), 1 deletion(-)
> >  >> 
> >  >> diff --git a/arch/arm/mach-omap2/omap-secure.h b/arch/arm/mach-omap2/omap-secure.h
> >  >> index 0e72917..c4586f4 100644
> >  >> --- a/arch/arm/mach-omap2/omap-secure.h
> >  >> +++ b/arch/arm/mach-omap2/omap-secure.h
> >  >> @@ -51,6 +51,7 @@
> >  >>  extern u32 omap_secure_dispatcher(u32 idx, u32 flag, u32 nargs,
> >  >>  				u32 arg1, u32 arg2, u32 arg3, u32 arg4);
> >  >>  extern u32 omap_smc2(u32 id, u32 falg, u32 pargs);
> >  >> +extern u32 omap_smc3(u32 id, u32 process, u32 flag, u32 pargs);
> >  >>  extern phys_addr_t omap_secure_ram_mempool_base(void);
> >  >>  extern int omap_secure_ram_reserve_memblock(void);
> >  >>  
> >  >> diff --git a/arch/arm/mach-omap2/omap-smc.S b/arch/arm/mach-omap2/omap-smc.S
> >  >> index f6441c1..5c02b8d 100644
> >  >> --- a/arch/arm/mach-omap2/omap-smc.S
> >  >> +++ b/arch/arm/mach-omap2/omap-smc.S
> >  >> @@ -1,9 +1,11 @@
> >  >>  /*
> >  >> - * OMAP44xx secure APIs file.
> >  >> + * OMAP34xx and OMAP44xx secure APIs file.
> >  >>   *
> >  >>   * Copyright (C) 2010 Texas Instruments, Inc.
> >  >>   * Written by Santosh Shilimkar 
> >  >>   *
> >  >> + * Copyright (C) 2012 Ivaylo Dimitrov 
> >  >> + * Copyright (C) 2013 Pali Rohár 
> >  >>   *
> >  >>   * This program is free software,you can redistribute it and/or modify
> >  >>   * it under the terms of the GNU General Public License version 2 as
> >  >> @@ -54,6 +56,24 @@ ENTRY(omap_smc2)
> >  >>  	ldmfd   sp!, {r4-r12, pc}
> >  >>  ENDPROC(omap_smc2)
> >  >>  
> >  >> +/**
> >  >> + * u32 omap_smc3(u32 service_id, u32 process_id, u32 flag, u32 pargs)
> >  >> + * Low level common routine for secure HAL and PPA APIs via smc #1
> >  >> + * r0 - @service_id: Secure Service ID
> >  >> + * r1 - @process_id: Process ID
> >  >> + * r2 - @flag: Flag to indicate the criticality of operation
> >  >> + * r3 - @pargs: Physical address of parameter list
> >  >> + */
> >  >> +ENTRY(omap_smc3)
> >  >> +	stmfd	sp!, {r4-r12, lr}
> >  >
> >  >You don't need to save/restore r12.  The ABI allows it to be clobbered
> >  >across function calls.
> >  >
> >  >> +	mov	r12, r0		@ Copy the secure service ID
> >  >> +	mov	r6, #0xff	@ Indicate new Task call
> >  >> +	dsb
> >  >> +	dmb
> >  >
> >  >dsb synchronises a superset of what dmb synchronises, so the dmb here is
> >  >not useful.
> >  >
> >  >In any case, any code calling this must flush the region addressed by
> >  >r3 beforehand anyway, which will include a dsb as part of its semantics
> >  >-- this is how you call it from rx51_secure_dispatcher().
> >  >
> >  >So I think the dsb may not be needed here (?)
> >  >
> >  >Cheers
> >  >---Dave
> >  >
> >  >
> > 
> > Could be, but I wonder why almost all the kernel code(I am aware of) that uses SMC and is written by TI, is storing r12 and is using both DSB and DMB. See arch/arm/mach-omap2/sleep34xx.S or arch/arm/mach-omap2/omap-smc.S for examples. I'd rather play it safe and leave it that way, unless someone confirms the other code using SMC has extra DSB/DMB instructions too. I wouldn't risk passing invalid/stale data to the Secure Monitor to just save 8 bytes and barriers in a performance non-critical code which will be called only a couple of times during the boot-up process. r12 save/restore is a legacy from omap_smc2 in omap-smc.S, so I guess it can go away without much of a trouble.
> > 
> Dave pointed out about the dsb and r12 to me in another thread. R12 can be easily removed
> but the DSB's were needed on OMAP for power sequencing. Without those, we have seen
> many issues. So you can leave the dsb's to be consistent with rest of the code.

Consistency is a perfectly good reason, especially in code like this
where a certain code sequence has been proven, but it's preferable to
include brief comments to explain nonetheless.

Difficulty in explaining precisely why something is needed should be a
warning flag that a comment is needed.

Cheers
---Dave

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