Re: [PATCH 4/7] clocksource: arch_timer: Add support for not-fw-configured timer on ARM64

From: Marc Zyngier
Date: Mon Oct 08 2018 - 09:17:40 EST

+ Mark Rutland

Hi Marek,

On 08/10/18 13:50, Marek Szyprowski wrote:
Use common infrastructure for ARM Architected Timers erratum to enable
support for systems with broken CPU firmware (timer registers not
properly configured). This mode has been already availabled on ARM
(32bits) architecture. This enables to run Linux kernel on ARM64 boards
using physical architected timers instead of the virtual ones. Examples
of such system with broken firmware are Samsung Exynos5433 SoC based
TM2(e) boards, which is already deployed for years and updating firmware
is not possible.

Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
drivers/clocksource/Kconfig | 11 +++++++++++
drivers/clocksource/arm_arch_timer.c | 15 ++++++++++++---
2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index a11f4ba98b05..a30752579b03 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -364,6 +364,17 @@ config ARM64_ERRATUM_858921
The workaround will be dynamically enabled when an affected
core is detected.
+ bool "Workaround for arch timer registers not configured by firmware"
+ default y
+ depends on ARM_ARCH_TIMER && ARM64
+ help
+ This option enables a workaround for boards, on which arch timer
+ registers are not properly configured by the board firmware.
+ The workaround will be dynamically enabled when an affected
+ board is detected.

I'm sorry, but I'm strongly pushing back on this.

This horrible hack was accepted with the express condition that it would be limited to ARMv7 platforms (on the ground that we never really documented the arch timer boot requirements on that version of the architecture), and would never proliferate on arm64. From day 1, we established what the boot protocol was, and we mandated that either:

- kernel is entered at EL2 on all CPUs
- cntvoff_el2 is zeroed on all CPUs

and we've got most people to fix their firmware, or live with the consequences. If these machines cannot receive a non-broken firmware, what are the odds that they will receive a mainline kernel?


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