Re: [PATCH v3 2/2] arm64: dts: Fix broken architected timer interrupt trigger

[Marc Zyngier]

On Fri, 10 Jun 2016 10:32:24 -0700
David Daney <ddaney@xxxxxxxxxxxxxxxxxx> wrote:

> On 06/10/2016 09:56 AM, Marc Zyngier wrote:
> > On 10/06/16 17:50, David Daney wrote:
> >> On 06/10/2016 12:23 AM, Marc Zyngier wrote:
> >>> On Thu, 09 Jun 2016 14:06:02 -0700
> >>> David Daney <ddaney.cavm@xxxxxxxxx> wrote:
> >>>
> >>>> I spoke too soon...
> >>>>
> >>>> On 06/09/2016 11:11 AM, David Daney wrote:
> >>>>> On 06/06/2016 10:56 AM, Marc Zyngier wrote:
> >>>>>> The ARM architected timer specification mandates that the interrupt
> >>>>>> associated with each timer is level triggered (which corresponds to
> >>>>>> the "counter >= comparator" condition).
> >>>>>>
> >>>>>> A number of DTs are being remarkably creative, declaring the interrupt
> >>>>>> to be edge triggered. A quick look at the TRM for the corresponding ARM
> >>>>>> CPUs clearly shows that this is wrong, and I've corrected those.
> >>>>>> For non-ARM designs (and in the absence of a publicly available TRM),
> >>>>>> I've made them active low as well, which can't be completely wrong
> >>>>>> as the GIC cannot disinguish between level low and level high.
> >>>>>>
> >>>>>> The respective maintainers are of course welcome to prove me wrong.
> >>>>>>
> >>>>>> While I was at it, I took the liberty to fix a couple of related issue,
> >>>>>> such as some spurious affinity bits on ThunderX, and their complete
> >>>>>> absence on ls1043a (both of which seem to be related to copy-pasting
> >>>>>> from other DTs).
> >>>>>>
> >>>>>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> >>>>>> ---
> >>>>>>     arch/arm64/boot/dts/altera/socfpga_stratix10.dtsi    | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/amlogic/meson-gxbb.dtsi          | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/apm/apm-storm.dtsi               | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/broadcom/ns2.dtsi                | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/cavium/thunder-88xx.dtsi         | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/exynos/exynos7.dtsi              | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/freescale/fsl-ls1043a.dtsi       | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/marvell/armada-ap806.dtsi        | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/socionext/uniphier-ph1-ld20.dtsi | 8 ++++----
> >>>>>>     arch/arm64/boot/dts/xilinx/zynqmp.dtsi               | 8 ++++----
> >>>>>>     10 files changed, 40 insertions(+), 40 deletions(-)
> >>>>>>
> >>>>> [...]
> >>>>>> diff --git a/arch/arm64/boot/dts/cavium/thunder-88xx.dtsi
> >>>>>> b/arch/arm64/boot/dts/cavium/thunder-88xx.dtsi
> >>>>>> index 2eb9b22..382d86f 100644
> >>>>>> --- a/arch/arm64/boot/dts/cavium/thunder-88xx.dtsi
> >>>>>> +++ b/arch/arm64/boot/dts/cavium/thunder-88xx.dtsi
> >>>>>> @@ -354,10 +354,10 @@
> >>>>>>
> >>>>>>         timer {
> >>>>>>             compatible = "arm,armv8-timer";
> >>>>>> -        interrupts = <1 13 0xff01>,
> >>>>>> -                     <1 14 0xff01>,
> >>>>>> -                     <1 11 0xff01>,
> >>>>>> -                     <1 10 0xff01>;
> >>>>>> +        interrupts = <1 13 8>,
> >>>>>> +                     <1 14 8>,
> >>>>>> +                     <1 11 8>,
> >>>>>> +                     <1 10 8>;
> >>>>
> >>>>
> >>>> NAK!
> >>>>
> >>>> According to arm,gic-v3.txt the trigger value must be either 1 or 4:
> >>>>
> >>>>      The 3rd cell is the flags, encoded as follows:
> >>>>            bits[3:0] trigger type and level flags.
> >>>>                    1 = edge triggered
> >>>>                    4 = level triggered
> >>>
> >>> Which is a bug in the binding description. PPIs can be any trigger
> >>> (just look at the TRM for CPUs that have devices connected to a PPI to
> >>> be convinced - most of them are level low).
> >>>
> >>> This doesn't mean that you can distinguish level-high from level-low
> >>> in a programmatic way. But the HW definitely can handle it.
> >>>
> >>> I'll update the GICv3 binding to reflect this.
> >>>
> >>> Now, coming back to your NAK: is level-low the right or wrong trigger
> >>> for your implementation of the architected timers?
> >>>
> >>
> >> For the Cavium Thunder implementation of GIC-v3, there is no concept of
> >> high and low.  All we have is asserted and not-asserted, we have chosen
> >> to map the concept of an asserted level-triggered source to
> >> IRQ_TYPE_LEVEL_HIGH, and the transition from not-asserted to asserted on
> >> an edge triggered source to IRQ_TYPE_EDGE_RISING.
> >
> > The GIC, no matter if it is from Cavium or not, doesn't have a notion of
> > high or low indeed *from a programming interface point of view*. What
> > matters here is the *device*, and how it is connected to the interrupt
> > controller. And I'm pretty sure your timers are *physically* one or the
> > other (unless they are simply floating? ;-)
> >
> 
> There is no wire.  So the concept of measuring the voltage doesn't 
> exist.  Everything is message based (with either architecturally defined 
> or implementation defined protocols).

That's your implementation, which cannot be generalized. PPIs can
perfectly be wired (and definitely *are* wires in ARM implementations)
from the device all the way to the redistributor, where it effectively
becomes message-based in an architecturally defined way.

> Let's turn the question around.
> 
> What bits in the GIC-v3 registers and data structures do we need to 
> program differently to account for the difference between 
> IRQ_TYPE_LEVEL_HIGH and IRQ_TYPE_LEVEL_LOW?
> 
> If the answer is that there are none, then allowing both to be 
> specified, falsely implies that there is a difference and causes mental 
> effort to be expended trying to decide which to use.

They do exist at the physical level, at least in a number of non-Cavium
implementations.

> 
> Unless you can tell me that there is a bit in the GIC-v3 that depends on 
> HIGH vs. LOW, I remain strongly opposed to changing the binding document.

That's fine by me. I'll drop the ThunderX changes from this patch.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.