Re: [PATCH 12/12] arm64: dts: exynos: Add Exynos850 SoC support

[Sam Protsenko]

On Wed, 4 Aug 2021 at 18:01, Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Wed, 04 Aug 2021 15:39:38 +0100,
> Sam Protsenko <semen.protsenko@xxxxxxxxxx> wrote:
>
> > > You are also missing the hypervisor virtual timer interrupt.
> > >
> >
> > Checked SoC TRM, there is no PPI for hypervisor virtual timer
> > interrupt, and no mentioning of it at all. Likewise, I checked ARMv8
> > ARM and TRM, almost no description of it. Also, I checked other
> > platforms, and seems like everyone does the same (having only 4
> > interrupts). And I wasn't able to find any documentation on that, so I
> > guess I'll leave it as is, if you don't mind.
>
> I *do* mind, and other DTs being wrong isn't a good enough excuse! ;-)
>
> From the ARMv8 ARM (ARM DDI 0487G.b)
> <quote>
> D11.2.4 Timers
>
> In an implementation of the Generic Timer that includes EL3, if EL3
> can use AArch64, the following timers are implemented:
>
> * An EL1 physical timer, that:
>   - In Secure state, can be accessed from EL1.
>   - In Non-secure state, can be accessed from EL1 unless those
>     accesses are trapped to EL2.
>     When this timer can be accessed from EL1, an EL1 control
>     determines whether it can be accessed from EL0.
> * A Non-secure EL2 physical timer.
> * A Secure EL3 physical timer. An EL3 control determines whether this
>   register is accessible from Secure EL1.
> * An EL1 virtual timer.
> * When FEAT_VHE is implemented, a Non-secure EL2 virtual timer.
> * When FEAT_SEL2 is implemented, a Secure EL2 physical timer.
> * When FEAT_SEL2 is implemented, a Secure EL2 virtual timer.
> </quote>
>
> Cortex-A55 being an ARMv8.2 implementation, it has FEAT_VHE, and thus
> it does have a NS-EL2 virtual timer. This is further confirmed by the
> TRM which documents CNTHV*_EL2 as valid system registers[1].
>
> So the timer exists, the signal is routed out of the core, and it
> is likely that it is connected to the GIC.
>
> If the designers have omitted it, then it needs to be documented as
> such.
>

Ok, I've checked thoroughly all docs again, and it seems like there is
no dedicated PPI number for this "EL2 Hypervisor Virtual Timer" in
Exynos850 SoC. The timer instance itself might exist of course, but
interrupt line is probably wasn't connected to GIC by SoC designers,
at least it's not documented.

Moreover, from [1,2] it looks like if it were existing it would have
been PPI=12 (INTID=28). But in GIC-400 TRM this PPI is assigned to
"Legacy FIQ signal", and all there is no PPI for Hypervisor Virtual
Timer documented there as well. In Exynos850 TRM the source for this
PPI's interrupt source is marked as "-", which means it's not used.

So if you know something that I don't know -- please point me out the
doc where this PPI line is documented. Otherwise I can add the comment
to device tree, stating that this interrupt line is not present in
SoC's GIC, i.e. something like this:

8<------------------------------------------------------------------------------->8
    timer {
        compatible = "arm,armv8-timer";
        interrupts = <GIC_PPI 13 (GIC_CPU_MASK_SIMPLE(8) |
                      IRQ_TYPE_LEVEL_LOW)>,
                 <GIC_PPI 14 (GIC_CPU_MASK_SIMPLE(8) |
                      IRQ_TYPE_LEVEL_LOW)>,
                 <GIC_PPI 11 (GIC_CPU_MASK_SIMPLE(8) |
                      IRQ_TYPE_LEVEL_LOW)>,
                 <GIC_PPI 10 (GIC_CPU_MASK_SIMPLE(8) |
                      IRQ_TYPE_LEVEL_LOW)>;
        /* Hypervisor Virtual Timer PPI is not present in this SoC GIC */
    };
8<------------------------------------------------------------------------------->8

Is that ok with you?

[1] https://developer.arm.com/documentation/102379/0000/The-processor-timers?lang=en
[2] https://gem5.googlesource.com/public/gem5/+/refs/heads/master/src/arch/arm/fastmodel/CortexA76/FastModelCortexA76.py#150

> Thanks,
>
>         M.
>
> [1] https://developer.arm.com/documentation/100442/0100/register-descriptions/aarch64-system-registers/aarch64-architectural-system-register-summary
>
> --
> Without deviation from the norm, progress is not possible.