| Message ID | 20180308042110.30538-1-yasushi.shoji@gmail.com |
|---|---|
| State | Accepted |
| Commit | 314d9f7e3eff41873011d9a0687f469680a00074 |
| Headers | show |
| Series | [U-Boot] imx: syscounter: make sure asm is volatile | expand |
[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(). > > [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> 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 >
On 09/03/2018 11:46, Fabio Estevam wrote: > [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(). >> >> [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> > > Hi Stefano, > > Do you think this one could be applied for the upcoming release? > Applied as fix as already discussed in other thread, thanks ! Best regards, Stefano
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);
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> --- arch/arm/mach-imx/syscounter.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)