From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christopher Covington Subject: Re: [PATCH v2] clocksource: arch_timer: Allow the device tree to specify the physical timer Date: Fri, 12 Sep 2014 07:43:04 -0400 Message-ID: <5412DC48.7030708@codeaurora.org> References: <1410452204-7277-1-git-send-email-dianders@chromium.org> <20140911164710.GW6158@arm.com> <5411D528.4050605@arm.com> <5411DA67.2040402@arm.com> <5411DF5D.8060906@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <5411DF5D.8060906-5wv7dgnIgG8@public.gmane.org> Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Marc Zyngier Cc: Doug Anderson , Will Deacon , "olof-nZhT3qVonbNeoWH0uzbU5w@public.gmane.org" , Sonny Rao , Catalin Marinas , Mark Rutland , Stephen Boyd , Sudeep Holla , Lorenzo Pieralisi , Thomas Gleixner , Daniel Lezcano , Nathan Lynch , "linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org" , "robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org" , Pawel Moll , "ijc+devicetree-KcIKpvwj1kUDXYZnReoRVg@public.gmane.org" , "galak-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org" , "devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" List-Id: devicetree@vger.kernel.org Hi Marc, On 09/11/2014 01:43 PM, Marc Zyngier wrote: > On 11/09/14 18:29, Doug Anderson wrote: >> Marc, >> >> On Thu, Sep 11, 2014 at 10:22 AM, Marc Zyngier wrote: >>>> We would need to run this code potentially at processor bringup and >>>> after suspend/resume, but that seems possible too. >>> >>> Note that this would be an ARMv7 only thing (you can't do that on ARMv8, >>> at all). >> >> Yes, of course. >> >> >>>> Is the transition to monitor mode and back simple? Where would you >>>> suggest putting this code? It would definitely need to be pretty >>>> early. We'd also need to be able to detect that we're in Secure SVC >>>> and not mess up anyone else who happened to boot in Non Secure SVC. >>> >>> This would have to live in some very early platform-specific code. The >>> ugly part is that you cannot find out what world you're in (accessing >>> SCR is going to send you to UNDEF-land if accessed from NS). >> >> Yup, so the question is: would such code be accepted upstream, or are >> we going to embark on a big job for someone to figure out how to do >> this only to get NAKed? >> >> If there was some indication that folks would take this, I think we >> might be able to get it coded up. If someone else wanted to volunteer >> to code it that would make me even happier, but maybe that's pushing >> my luck. ;) > > Writing the code is a 5 minute job. Getting it accepted is another > story, and I'm not sure everyone would agree on that. > >>> If I was suicidal, I'd suggest you could pass a parameter to the command >>> line, interpreted by the timer code... But I since I'm not, let's >>> pretend I haven't said anything... ;-) >> >> I did this in the past (again, see Sonny's thread), but didn't >> consider myself knowledgeable to know if that was truly a good test: >> >> asm volatile("mrc p15, 0, %0, c1, c1, 0" : "=r" (val)); >> pr_info("DOUG: val is %#010x", val); >> val |= (1 << 2); >> asm volatile("mcr p15, 0, %0, c1, c1, 0" : : "r" (val)); >> val = 0xffffffff; >> asm volatile("mrc p15, 0, %0, c1, c1, 0" : "=r" (val)); >> pr_info("DOUG: val is %#010x", val); >> >> The idea being that if you can make modifications to the SCR register >> (and see your changes take effect) then you must be in secure mode. >> In my case the first printout was 0x0 and the second was 0x4. > > The main issue is when you're *not* in secure mode. It is likely that > this will explode badly. This is why I suggested something that is set > by the bootloader (after all. it knows which mode it is booted in), and > that the timer driver can use when the CPU comes up. What exactly does "exploding badly" look like? Causing and undefined instruction exception? That's just a branch with a mode switch. Any reason the code couldn't deal with that or even use that to its advantage? Thanks, Christopher -- Employee of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by the Linux Foundation. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html