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

On Wed, 04 Aug 2021 19:37:24 +0100,
Sam Protsenko [off-list ref] wrote:

On Wed, 4 Aug 2021 at 18:01, Marc Zyngier [off-list ref] wrote:

quoted

On Wed, 04 Aug 2021 15:39:38 +0100,
Sam Protsenko [off-list ref] wrote:

quoted

quoted

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.

Can you try and check this? You can directly program the virtual timer
so that it has a pending interrupt, and then check the pending
register on the same CPU to see if there is anything appearing there.

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",

No. That's only if you set the bypass bits in GICD_CTLR, which nobody
with half a brain would consider doing.

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?

I'd rather you verify the above first. And if you can't, I'd like a
comment that is a bit more explicit:

/* The vendor couldn't be bothered to wire the EL2 Virtual Timers */

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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