Hi, On Wed, Aug 27, 2014 at 7:58 PM, Olof Johansson <o...@lixom.net> wrote: > On Wed, Aug 27, 2014 at 5:56 PM, Stephen Boyd <sb...@codeaurora.org> wrote: >> On 08/27/14 15:33, Olof Johansson wrote: >>> On Wed, Aug 27, 2014 at 3:26 PM, Stephen Boyd <sb...@codeaurora.org> wrote: >>> >>>> Is there any reason why the virtual counter can't be read? Maybe we're >>>> the hyp and we need to make sure we don't use the virtual timer so that >>>> the guest can use it, but that doesn't have any effect on the usage of >>>> the virtual counter for the clocksource. >>> There are several cases where virtual is unusable -- in particular it >>> might not have been configured properly (i.e. the phys/virt offset is >>> at a bad value). >>> >> >> Any specifics? It would be nice to say so in the commit text so that >> others using such devices know they need this patch. I'm guessing the >> firmware can't be fixed?
Even if we could change things to use a virtual timer in some cases, Sonny's patch still fixes a bug. The code as written right now makes pretenses about supporting the physical timer, but it doesn't work. That should be fixed. > Yeah, there are a few. The big.LITTLE on the Chromebook 2 models have > this issue, due to the A7 cluster having an incorrect offset > programmed. However, arch timers aren't supported on that SoC in the > first place, so it's not a problem in reality. > > The other known platform is rk3288. It has products out in the wild > where firmware updates are unlikely. One other reason is that (I'm told) that the virtual offset is lost in certain power down conditions (powering down a core, going into S3, etc). When we power back up the offset is effectively reset to a random value. That means we need something to reprogram the virtual timer offset whenever we power things back up. If we've got a hypervisor then the hypervisor will definitely be involved in powering things back up and it can reset the virtual offset. ...but forcing systems to implement a hypervisor (or somehow adding an interface for the kernel to call back into firmware) is a huge effort and it means more hard-to-update code sitting in firmware. Note: having the virtual offset initted to a random value seems like an unfortunate design choice for the virtual timer offset (guaranteeing it was initted to 0 would have avoided the problem), but that's what we seem to have. -Doug -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
