Kukjin,
Sorry for the delay in responding--this thread started just as I was
walking out the door for vacation...
On Wed, May 21, 2014 at 5:47 AM, Kukjin Kim <kgene.kim@xxxxxxxxxxx> wrote:
> Chirantan Ekbote wrote:
>>
>> >>> Anyway, I'm by no means opposed to switching to arch timers. They
>> >>> provide a well designed, generic interface and drivers shared by
>> >>> multiple platforms, which means more code sharing and possibly more eyes
>> >>> looking at the code, which is always good. However if they don't support
>> >>> low power states correctly, we can't just remove MCT.
>> >>
>> >> I think low power states aren't in mainline (right?).
>> >>
>> >> One solution that might work could be to leave the device tree entry
>> >> alone but change the MCT init code to simply act as a no-op if it sees
>> >> an arch timer is in the device tree and enabled. Then when/if someone
>> >> got the low power states enabled we could just change source code
>> >> rather than dts files.
>> >>
>> Doug and I were talking about this and we think we may have a way to
>> have the mct and arch timers co-exist. The main issue is that the mct
>> (and therefore arch timer) gets cleared once during boot and every
>> time we do a suspend / resume. This happens in
>> exynos4_mct_frc_start() but it's not immediately clear to us why the
>> counter needs to be reset at all. If we remove the lines that clear
>> the counter then there is no longer an issue with having both the mct
>> and the arch timers on at the same time.
>>
> Yeah, actually we don't need to reset the count value after suspend/resume.
> So, how about following? I think, it should be fine to you.
>
> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
> index 8d64200..d24db6f 100644
> --- a/drivers/clocksource/exynos_mct.c
> +++ b/drivers/clocksource/exynos_mct.c
> @@ -157,12 +157,15 @@ static void exynos4_mct_frc_start(u32 hi, u32 lo)
> {
> u32 reg;
>
> - exynos4_mct_write(lo, EXYNOS4_MCT_G_CNT_L);
> - exynos4_mct_write(hi, EXYNOS4_MCT_G_CNT_U);
> -
> reg = __raw_readl(reg_base + EXYNOS4_MCT_G_TCON);
> - reg |= MCT_G_TCON_START;
> - exynos4_mct_write(reg, EXYNOS4_MCT_G_TCON);
> +
> + if (!(reg & MCT_G_TCON_START)) {
> + exynos4_mct_write(lo, EXYNOS4_MCT_G_CNT_L);
> + exynos4_mct_write(hi, EXYNOS4_MCT_G_CNT_U);
> +
> + reg |= MCT_G_TCON_START;
> + exynos4_mct_write(reg, EXYNOS4_MCT_G_TCON);
> + }
I guess this is OK, but personally I'd vote to remove the init
altogether unless there is a hardware bug on some versions of exynos
that requires that we init this. The kernel couldn't care less about
what value the timer starts at, so even on systems where the MCT isn't
started by the bootloader this is just a waste of code.
Chirantan: can you post your patch
<https://chromium-review.googlesource.com/#/c/201143/> up?
>> Alternately, if there is some code that depends on the mct being reset
>> we could store an offset instead of clearing the counter and then
>> subtract that offset every time something reads it. Doug has a patch
>> that does this at
>> https://chromium-review.googlesource.com/#/c/200298/. Effectively the
>> visible behavior will not change. Would either of these options work?
>>
> Hmm...I cannot open the webpage :(
Can you try again? I think there may have been a glitch. It's also
possible that your browser grabbed the "." at the end of the URL,
which would confuse things.
-Doug
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