[Adding Stefano]

On Thu, Mar 8, 2018 at 1:21 AM, Yasushi SHOJI yasushi.shoji@gmail.com wrote:

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[1] to put volatile on all asm(), so this commit also adds it to the asm() in timer_init().

Signed-off-by: Yasushi SHOJI yasushi.shoji@gmail.com Reviewed-by: Fabio Estevam fabio.estevam@nxp.com

Hi Stefano,

Do you think this one could be applied for the upcoming release?

Thanks

---

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, &sctr->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 & 0xffffffff);
    gd->arch.tbu = (unsigned long)(now >> 32);
  

-- 2.16.2