Messages in this thread |  | | Date | Thu, 19 Jun 2014 09:01:38 -0700 | | Subject | Re: [PATCH v2] clocksource: exynos-mct: Register the timer for stable udelay | | From | Doug Anderson <> |
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Hi,
On Thu, Jun 19, 2014 at 3:21 AM, Tomasz Figa <tomasz.figa@gmail.com> wrote: >> +static struct delay_timer exynos4_delay_timer; >> + >> +static unsigned long exynos4_read_current_timer(void)
Note: I think this should return a cycles_t, not an unsigned long. They're the same (right now), but probably shouldn't be (see below).
>> +{ >> +#ifdef ARM >> + return __raw_readl(reg_base + EXYNOS4_MCT_G_CNT_L); >> +#else /* ARM64, etc */ >> + return exynos4_frc_read(&mct_frc); >> +#endif >> +} >> + > > No need for anything like this. Even if running on ARM64, the delay > timer code should be able to cope with different timer widths. For > delays, 32 bits are enough, so just always read the lower part.
I agree that the timer code should cope but it doesn't appear to. I see:
cycles_t start = get_cycles(); while ((get_cycles() - start) < cycles) cpu_relax();
Right now cycles_t is defined as "unsigned long". If that's 64-bits on ARM64 then this function will have problems with wraparound.
My personal vote would be to submit a patch to change "cycles_t" to always be 32-bits. Given that 32-bits was fine for udelay() for ARM that seems sane and simple. If someone later comes up with a super compelling reason why we need 64-bit timers for udelay (really??) then they can later add all the complexity needed.
Amit: can you code up such a patch and add it to the series? I know it changes code that touches all ARM devices but I still think it's the right thing to do and actually only really changes behavior on ARM64.
> Also use of raw accessors in drivers is discouraged - please use > readl_relaxed().
It doesn't seem like that should happen in the same patch. Perhaps Amit can do a cleanup patch first that changes all instances of __raw_readl / __raw_writel in this file, then submit his patch atop that.
> Btw. I don't even see support for this on ARM64 in mainline, where arch > timer is always used for delays and AFAIK this is a platform requirement.
Yeah, I'd vote for not using MCT on ARM64, but it I suppose it doesn't hurt to keep it working.
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