Re: [U-Boot] imx: get_ticks in syscounter.c get miscompiled by GCC 6

[Yasushi SHOJI]

Hi,

On Wed, Mar 7, 2018 at 11:27 PM, Tom Rini  wrote:
> On Wed, Mar 07, 2018 at 10:42:44AM -0300, Fabio Estevam wrote:
>> Patch looks good. Make sure to add your Signed-off-by line, then you
>> can send it via git send-email.
>
> Yes please, thanks!

I've sent it to you with my signed-of-by.

thanks,
-- 
  yashi
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Re: [U-Boot] imx: get_ticks in syscounter.c get miscompiled by GCC 6

[Tom Rini]

On Wed, Mar 07, 2018 at 10:42:44AM -0300, Fabio Estevam wrote:
> Hi Yasushi ,
> 
> On Wed, Mar 7, 2018 at 2:57 AM, Yasushi SHOJI  wrote:
> 
> > Do you guys really want to put volatile on all of these now?
> > We are at rc4 and Tom is planing to cut the release
> > March 12th.
> 
> This can be done at a later step.

Yes.  And it should be a little bit manual too.  For example, using your
regex (thanks!) I see we have some powerpc code that's doing
asm("eieio") and that should be eieio() (which is in turn an asm
volatile ...), as well as some sync;isync or just sync/isync that should
be sync();isync(); or similar.  And people that know x86 might have some
useful comments there too.

> > I'm attaching a tentative patch to fix only syscounter.c.
> > If it looks good, I'l resend it by git-send-email.
> 
> Patch looks good. Make sure to add your Signed-off-by line, then you
> can send it via git send-email.

Yes please, thanks!

-- 
Tom


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Re: [U-Boot] imx: get_ticks in syscounter.c get miscompiled by GCC 6

[Fabio Estevam]

Hi Yasushi ,

On Wed, Mar 7, 2018 at 2:57 AM, Yasushi SHOJI  wrote:

> Do you guys really want to put volatile on all of these now?
> We are at rc4 and Tom is planing to cut the release
> March 12th.

This can be done at a later step.

> I'm attaching a tentative patch to fix only syscounter.c.
> If it looks good, I'l resend it by git-send-email.

Patch looks good. Make sure to add your Signed-off-by line, then you
can send it via git send-email.

Thanks
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Re: [U-Boot] imx: get_ticks in syscounter.c get miscompiled by GCC 6

[Yasushi SHOJI]

Hi,

On Tue, Mar 6, 2018 at 10:11 PM, Fabio Estevam  wrote:
> On Tue, Mar 6, 2018 at 9:31 AM, Lothar Waßmann  
> wrote:
>
>> Without the 'volatile' attribute the compiler is entitled to move the
>> asm code around or optimize it out.
>> So, your patch is the correct fix independent from the gcc version
>> used.
>
> Yes, but then it would be better to fix all the places where asm is
> used without volatile.

That'd be a quite big patch.  A quick grep shows me that
there is over 100 asm() without volatile.

git grep -e '\basm *(' | grep  -v -e volatile -e nop | wc -l
153

Do you guys really want to put volatile on all of these now?
We are at rc4 and Tom is planing to cut the release
March 12th.

I'm attaching a tentative patch to fix only syscounter.c.
If it looks good, I'l resend it by git-send-email.

Best,
-- 
yashi
From 8772a9b5b532da4c1d0656ffa21f324e72d369e8 Mon Sep 17 00:00:00 2001
From: Yasushi SHOJI 
Date: Wed, 7 Mar 2018 14:38:05 +0900
Subject: [PATCH] imx: syscounter: make sure asm is volatile

Without the volatile attribute, compilers are entitled to optimize out
the same asm().  In the case of __udelay() in syscounter.c, it calls
`get_ticks()` twice, one for the starting time and the second in the
loop to check the current time.  When compilers inline `get_ticks()`
they see the same `mrrc` instructions and optimize out the second one.
This leads to infinite loop since we don't get updated value from the
system counter.

Here is a portion of the disassembly of __udelay:

  88:	428b  	cmp	r3, r1
  8a:	f8ce 20a4 	str.w	r2, [lr, #164]	; 0xa4
  8e:	bf08  	it	eq
  90:	4282  	cmpeq	r2, r0
  92:	f8ce 30a0 	str.w	r3, [lr, #160]	; 0xa0
  96:	d3f7  	bcc.n	88 <__udelay+0x88>
  98:	e8bd 8cf0 	ldmia.w	sp!, {r4, r5, r6, r7, sl, fp, pc}

Note that final jump / loop at 96 to 88, we don't have any `mrrc`.

With a volatile attribute, the above changes to this:

  8a:	ec53 2f0e 	mrrc	15, 0, r2, r3, cr14
  8e:	42ab  	cmp	r3, r5
  90:	f8c1 20a4 	str.w	r2, [r1, #164]	; 0xa4
  94:	bf08  	it	eq
  96:	42a2  	cmpeq	r2, r4
  98:	f8c1 30a0 	str.w	r3, [r1, #160]	; 0xa0
  9c:	d3f5  	bcc.n	8a <__udelay+0x8a>
  9e:	e8bd 8cf0 	ldmia.w	sp!, {r4, r5, r6, r7, sl, fp, pc}
  a2:	bf00  	nop

I'm advised to put volatile on all asm(), so this commit also adds it
to the asm() in timer_init().
---
 arch/arm/mach-imx/syscounter.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/mach-imx/syscounter.c b/arch/arm/mach-imx/syscounter.c
index 9290918dca..1d4ebfe343 100644
--- a/arch/arm/mach-imx/syscounter.c
+++ b/arch/arm/mach-imx/syscounter.c
@@ -62,7 +62,7 @@ int timer_init(void)
 	unsigned long val, freq;
 
 	freq = CONFIG_SC_TIMER_CLK;
-	asm("mcr p15, 0, %0, c14, c0, 0" : : "r" (freq));
+	asm volatile("mcr p15, 0, %0, c14, c0, 0" : : "r" (freq));
 
 	writel(freq, >cntfid0);
 
@@ -82,7 +82,7 @@ unsigned long long get_ticks(void)
 {
 	unsigned long long now;
 
-	asm("mrrc p15, 0, %Q0, %R0, c14" : "=r" (now));
+	asm volatile("mrrc p15, 0, %Q0, %R0, c14" : "=r" (now));
 
 	gd->arch.tbl = (unsigned long)(now & 0x);
 	gd->arch.tbu = (unsigned long)(now >> 32);
-- 
2.16.2

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Re: [U-Boot] imx: get_ticks in syscounter.c get miscompiled by GCC 6

[Fabio Estevam]

On Tue, Mar 6, 2018 at 9:31 AM, Lothar Waßmann  wrote:

> Without the 'volatile' attribute the compiler is entitled to move the
> asm code around or optimize it out.
> So, your patch is the correct fix independent from the gcc version
> used.

Yes, but then it would be better to fix all the places where asm is
used without volatile.
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Re: [U-Boot] imx: get_ticks in syscounter.c get miscompiled by GCC 6

[Lothar Waßmann]

Hi,

On Tue, 6 Mar 2018 15:06:08 +0900 Yasushi SHOJI wrote:
> Hi,
> 
> It seems to me that both GCC 6.3 and 6.4 mis-compiles
>
s/mis-compiles/optimizes/

Without the 'volatile' attribute the compiler is entitled to move the
asm code around or optimize it out.
So, your patch is the correct fix independent from the gcc version
used.

> arch/arm/mach-imx/syscounter.c.
> 
> I'm attaching two files, bad.txt is the original syscounter.c and
> good.txt is the one
> with the following patch.
> 
> diff --git a/arch/arm/mach-imx/syscounter.c b/arch/arm/mach-imx/syscounter.c
> index 9290918dca..30ed0109a2 100644
> --- a/arch/arm/mach-imx/syscounter.c
> +++ b/arch/arm/mach-imx/syscounter.c
> @@ -82,7 +82,7 @@ unsigned long long get_ticks(void)
>  {
> unsigned long long now;
> 
> -   asm("mrrc p15, 0, %Q0, %R0, c14" : "=r" (now));
> +   asm volatile("mrrc p15, 0, %Q0, %R0, c14" : "=r" (now));
> 
> gd->arch.tbl = (unsigned long)(now & 0x);
> gd->arch.tbu = (unsigned long)(now >> 32);
> 


Lothar Waßmann
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