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[1] to put volatile on all asm(), so this commit also adds it
to the asm() in timer_init().
[1]: https://lists.denx.de/pipermail/u-boot/2018-March/322062.html
Signed-off-by: Yasushi SHOJI <yasushi.shoji@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
diff --git a/arch/arm/mach-imx/syscounter.c b/arch/arm/mach-imx/syscounter.c
index 9290918..1d4ebfe 100644
--- a/arch/arm/mach-imx/syscounter.c
+++ b/arch/arm/mach-imx/syscounter.c
@@ -62,7 +62,7 @@
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 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);