RE: [PATCH] arm: dts: exynos5: Remove multi core timer

[Kukjin Kim <kgene.kim@samsung.com>]

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);
+	}
 }


> 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 :(

- Kukjin