linux-kernel - Re: [PATCH soc] ARM: use ARM_SINGLE

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Date:	Fri, 22 May 2015 19:06:47 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Stefan Agner <stefan@...er.ch>
Cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org, manabian@...il.com,
	linux-kernel@...r.kernel.org,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, olof@...om.net
Subject: Re: [PATCH soc] ARM: use ARM_SINGLE_ARMV7M for ARMv7-M platforms

On Fri, May 22, 2015 at 06:28:16PM +0200, Stefan Agner wrote:
> On 2015-05-22 17:56, Daniel Thompson wrote:
> > On 22/05/15 16:29, Maxime Coquelin wrote:
> >> 2015-05-22 16:50 GMT+02:00 Arnd Bergmann <arnd@...db.de>:
> >>> [one small request as I have four armv7-m folks on Cc already:
> >>>   could one of you try to fix the warning that I get with every
> >>>   single build: "/git/arm-soc/arch/arm/kernel/head-nommu.S: Assembler
> >>> messages: /git/arm-soc/arch/arm/kernel/head-nommu.S:167: Warning:
> >>> Use of r13 as a source register is deprecated when r15 is the
> >>> destination register."]
> >>
> >> Moving r13 to r12 and returning r12 seems to do the job (see below).
> >> But I don't know if there is a more elegant way, and if it is also
> >> valid for other architectures than armv7-m.
> > 
> > Why not just s/r13/r11/?
> > 
> > (works for me but I'm only working on single core system)
> 
> For ARMv7-M this works, since r11 is not used in the processors
> PROCINFO_INITFUNC function (__cpu_flush in struct proc_info_list, which
> is __v7m_setup in proc-v7m.S).
> 
> However, afaik, head-nommu.S can be used by different processors too,
> hence that register needs to be free to use for all possible __cpu_flush
> implementations.
> 
> That said, proc-v7.S stores r11 on the stack, so it really seems that
> r11 is ok to use?

Please use r3 (as I just said).  We don't need random deviations between
MMU and noMMU stuff - that just makes maintanence of other code more
difficult.

You can also avoid the issues of having it passed through the processor
specific init function (which isn't guaranteed to preserve r13) by
doing this:

-	ldr	r13, =__mmap_switched		@ address to jump to after
-						@ initialising sctlr
	badr	lr, 1f				@ return (PIC) address
	ldr	r12, [r10, #PROCINFO_INITFUNC]
	add	r12, r12, r10
	ret	r12
- 1:	b	__after_proc_init
+1:	ldr	r13, =__mmap_switched		@ address to jump to after
+						@ initialising sctlr
	b	__after_proc_init

However, because you have no MMU to turn on, and no address switch,
you actually don't need any of this.  __after_proc_init can become
a "function" which returns via the link register.

You can then do:

1:	bl	__after_proc_init
	b	__mmap_switched

You'll need to fix secondary_startup in there as well:

-	adr	r4, __secondary_data
-	ldmia	r4, {r7, r12}
	ldr	r7, __secondary_data

#ifdef CONFIG_ARM_MPU
	/* Use MPU region info supplied by __cpu_up */
	ldr	r6, [r7]			@ get secondary_data.mpu_szr
	bl	__setup_mpu			@ Initialize the MPU
#endif

-	badr	lr, __after_proc_init		@ return address
-	mov	r13, r12			@ __secondary_switched address
+	badr	lr, 1f				@ return (PIC) address
	ldr	r12, [r10, #PROCINFO_INITFUNC]
	add	r12, r12, r10
	ret	r12
-ENDPROC(secondary_startup)
-
-ENTRY(__secondary_switched)
+1:	bl	__after_proc_init
	ldr	sp, [r7, #12]			@ set up the stack pointer
	mov	fp, #0
	b	secondary_start_kernel
-ENDPROC(__secondary_switched)
+ENDPROC(secondary_startup)


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