lkml.org 
[lkml]   [2014]   [Sep]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] clocksource: arch_timer: Allow the device tree to specify the physical timer
    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@chromium.org>
    > Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
    > ---
    > 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()

    Christopher

    --
    Employee of Qualcomm Innovation Center, Inc.
    Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
    hosted by the Linux Foundation.


    \
     
     \ /
      Last update: 2014-09-11 18:41    [W:4.283 / U:0.020 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site