[PATCH v4 00/17] clocksource/arm_arch_timer: Add basic ARMv8.6 support

Marc Zyngier
2021-10-17T12:42:56Z

This is v4 (final?) of the series enabling ARMv8.6 support for timer
subsystem, and was prompted by a discussion with Oliver around the
fact that an ARMv8.6 implementation must have a 1GHz counter, which
leads to a number of things to break in the timer code:

- the counter rollover can come pretty quickly as we only advertise a
56bit counter,
- the maximum timer delta can be remarkably small, as we use the
countdown interface which is limited to 32bit...

Thankfully, there is a way out: we can compute the minimal width of
the counter based on the guarantees that the architecture gives us,
and we can use the 64bit comparator interface instead of the countdown
to program the timer.

Finally, we start making use of the ARMv8.6 ECV features by switching
accesses to the counters to a self-synchronising register, removing
the need for an ISB. Hopefully, implementations will *not* just stick
an invisible ISB there...

A side effect of the switch to CVAL is that XGene-1 breaks. I have
added a workaround to keep it alive.

I have added Oliver's original patch[0] to the series and tweaked a
couple of things. Blame me if I broke anything.

The whole things has been tested on Juno (sysreg + MMIO timers),
XGene-1 (broken sysreg timers), FVP (FEAT_ECV, CNT*CTSS_EL0).

To ease merging, and after discussion with Daniel, I have created two
tags from which the patches can be pulled from
git://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git :

- clocksource-timers-arm64-8.6 has the first 13 patches, which can
go into Daniel's tree, and ultimately tip

- arm64-timers-8.6 has the full series, which can be pulled into
the arm64 tree

Alternatively, Will and Daniel can synchronise independently to
build a branch of their own.

* From v3 [3]:
- Drop the %x0 qualifiers for the registers in the asm accessors
- Added an update to cpu-feature-registers.rst
- Collected Acks, with thanks.

* From v2 [2]:
- New patch adding a HWCAP (ECV) allowing userspace to probe for
the presence of CNTVSS_EL0.

* From v1 [1]:
- New patch adding a bunch of BUILD_BUG()s for register accesses we
don't expect. This makes subsequent patches much simpler.
- New patch moving the ISBs for workaround in a way that makes
more sense for the self-synchronising accessors.
- Rework the XGene-1 workaround to rely solely on MIDR.
- Split the CNTVCTSS trap handling in its own patch.
- Rebased on 5.15-rc2
- Collected RBs, with thanks.

[0] https://lore.kernel.org/r/20210807191428.3488948-1-oupton@google.com
[1] https://lore.kernel.org/r/20210809152651.2297337-2-maz@kernel.org
[2] https://lore.kernel.org/r/20210922211941.2756270-1-maz@kernel.org
[3] https://lore.kernel.org/r/20211010114306.2910453-1-maz@kernel.org

Marc Zyngier (16):
clocksource/arm_arch_timer: Add build-time guards for unhandled
register accesses
clocksource/arm_arch_timer: Drop CNT*_TVAL read accessors
clocksource/arm_arch_timer: Extend write side of timer register
accessors to u64
clocksource/arm_arch_timer: Move system register timer programming
over to CVAL
clocksource/arm_arch_timer: Move drop _tval from erratum function
names
clocksource/arm_arch_timer: Fix MMIO base address vs callback ordering
issue
clocksource/arm_arch_timer: Move MMIO timer programming over to CVAL
clocksource/arm_arch_timer: Advertise 56bit timer to the core code
clocksource/arm_arch_timer: Work around broken CVAL implementations
clocksource/arm_arch_timer: Remove any trace of the TVAL programming
interface
clocksource/arm_arch_timer: Drop unnecessary ISB on CVAL programming
clocksource/arch_arm_timer: Move workaround synchronisation around
arm64: Add a capability for FEAT_ECV
arm64: Add CNT{P,V}CTSS_EL0 alternatives to cnt{p,v}ct_el0
arm64: Add handling of CNTVCTSS traps
arm64: Add HWCAP for self-synchronising virtual counter

Oliver Upton (1):
clocksource/arm_arch_timer: Fix masking for high freq counters

Documentation/arm64/cpu-feature-registers.rst | 12 +-
Documentation/arm64/elf_hwcaps.rst | 4 +
arch/arm/include/asm/arch_timer.h | 37 +--
arch/arm64/include/asm/arch_timer.h | 78 +++---
arch/arm64/include/asm/esr.h | 6 +
arch/arm64/include/asm/hwcap.h | 1 +
arch/arm64/include/asm/sysreg.h | 3 +
arch/arm64/include/uapi/asm/hwcap.h | 1 +
arch/arm64/kernel/cpufeature.c | 13 +-
arch/arm64/kernel/cpuinfo.c | 1 +
arch/arm64/kernel/traps.c | 11 +
arch/arm64/tools/cpucaps | 1 +
drivers/clocksource/arm_arch_timer.c | 243 +++++++++++-------
include/clocksource/arm_arch_timer.h | 2 +-
14 files changed, 264 insertions(+), 149 deletions(-)

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2.30.2