On 09/11/2014 12:04 PM, Doug Anderson wrote: > Christopher, > > On Thu, Sep 11, 2014 at 8:58 AM, Christopher Covington > <cov@xxxxxxxxxxxxxx> wrote: >> Hi Doug, >> >> On 09/11/2014 11:52 AM, Doug Anderson wrote: >>> Some 32-bit (ARMv7) systems are architected like this: >>> >>> * The firmware doesn't know and doesn't care about hypervisor mode and >>> we don't want to add the complexity of hypervisor there. >>> >>> * The firmware isn't involved in SMP bringup or resume. >>> >>> * The ARCH timer come up with an uninitialized offset between the >>> virtual and physical counters. Each core gets a different random >>> offset. >>> >>> On systems like the above, it doesn't make sense to use the virtual >>> counter. There's nobody managing the offset and each time a core goes >>> down and comes back up it will get reinitialized to some other random >>> value. >>> >>> Let's add a property to the device tree to say that we shouldn't use >>> the virtual timer. Firmware could potentially remove this property >>> before passing the device tree to the kernel if it really wants the >>> kernel to use a virtual timer. >>> >>> Note that it's been said that ARM64 (ARMv8) systems the firmware and >>> kernel really can't be architected as described above. That means >>> using the physical timer like this really only makes sense for ARMv7 >>> systems. >>> >>> In order for this patch to do anything useful, we also need Sonny's >>> patch at <https://patchwork.kernel.org/patch/4790921/> >>> >>> Signed-off-by: Doug Anderson <dianders@xxxxxxxxxxxx> >>> Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx> >>> --- >>> Documentation/devicetree/bindings/arm/arch_timer.txt | 6 ++++++ >>> drivers/clocksource/arm_arch_timer.c | 3 +++ >>> 2 files changed, 9 insertions(+) >>> >>> diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt b/Documentation/devicetree/bindings/arm/arch_timer.txt >>> index 37b2caf..876d32b 100644 >>> --- a/Documentation/devicetree/bindings/arm/arch_timer.txt >>> +++ b/Documentation/devicetree/bindings/arm/arch_timer.txt >>> @@ -22,6 +22,12 @@ to deliver its interrupts via SPIs. >>> - always-on : a boolean property. If present, the timer is powered through an >>> always-on power domain, therefore it never loses context. >>> >>> +** Optional properties: >>> + >>> +- arm,use-physical-timer : Don't ever use the virtual timer, just use the >>> + physical one. Not supported for ARM64. >>> + >>> + >>> Example: >>> >>> timer { >>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c >>> index 5163ec1..8ca07a9 100644 >>> --- a/drivers/clocksource/arm_arch_timer.c >>> +++ b/drivers/clocksource/arm_arch_timer.c >>> @@ -649,6 +649,9 @@ static void __init arch_timer_init(struct device_node *np) >>> arch_timer_ppi[i] = irq_of_parse_and_map(np, i); >>> arch_timer_detect_rate(NULL, np); >>> >>> + if (of_property_read_bool(np, "arm,use-physical-timer")) >>> + arch_timer_use_virtual = false; >>> + >>> /* >>> * If HYP mode is available, we know that the physical timer >>> * has been configured to be accessible from PL1. Use it, so >>> >> >> How's the VDSO supposed to deal with this? It currently does: >> >> cycle_now = arch_counter_get_cntvct() > > I don't see that line of code anywhere when I do a "git grep" on linux > or linuxnext. Can you give me a clearer pointer? As Nathan mentioned, the code on the 32 bit side isn't merged yet. http://www.spinics.net/lists/arm-kernel/msg356336.html 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@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
