Hi, On Tue, Jul 28, 2015 at 9:20 AM, Alexey Klimov <klimov.linux@xxxxxxxxx> wrote: > Hi Doug, > > On Tue, Jul 28, 2015 at 6:24 PM, Doug Anderson <dianders@xxxxxxxxxxxx> wrote: >> Alexey, >> >> On Mon, Jul 27, 2015 at 2:28 PM, Alexey Klimov <klimov.linux@xxxxxxxxx> wrote: >>> Hi all, >>> >>> year(s) ago it was discovered that MCT timer and ARM architectured >>> timer >>> are the same hardware with different interface. Here [1]. >>> >>> I followed mail-list discussions about removing MCT and using arch >>> timer for Exynos5-based SoCs but things aren't moving at least latest >>> upstream kernel on odroid-xu3 will use MCT as default timers. >>> Maybe the reason are some power-management related things that very >>> specific to Samsung. I don't know. >>> >>> >>> Idea of this draft patchset comes from Doug patches when he tried to >>> optimize read of 64-bit counter located in mmio. [2] >>> Why not using cp15 counter instead if possible? >> >> I hate to burst your bubble here, but... >> >> ...I think it would be a bad idea to use the cp15 counter on exynos >> 5422. According to Samsung, there are issues where using cp15 could >> sometimes return the wrong value, especially if you happen to read it >> while on an A7 instead of an A15. It will tend to work pretty well, >> but Samsung claimed that it might not work right on some CPUs or at >> some temperatures. > > I think this is right time when I can say "If only I had known about > it before!". Yeah, we spent a whole lot of time on this too before giving up after we found out it was not possible to use. > That's quite interesting especially about temperature issues. > Maybe it's worth to insert comments in driver/docs about this. Maybe not a bad idea. Depends if you think someone else will try to do this... ...and if they do, will they check mailing list traffic. >> Generally I'd say that if the arch timer works for you: use the arch >> timer. If the arch timer doesn't work for you, use the MCT. >> >> -Doug > > I possess no knowledge if there're any different hw revisions of > odroid-xu3 on the field > but looks like arch timer works fine for me on odroid-xu3 Exynos5422 > SoC with upstream kernel. Well, it looked like it worked for us too. In fact, I never saw a failure. All I know is that Samsung said: don't do this, it's not a good idea. The performance gain wasn't enough to go against that... That's when I started trying to optimize the MCT a little bit. ;) -Doug -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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