linux-kernel - Re: [PATCH 0/4] clocksource: Add standalone MMIO ARM arch timer driver

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Message-ID: <aJx4g8z3l438Qgnv@raptor>
Date: Wed, 13 Aug 2025 12:35:31 +0100
From: Alexandru Elisei <alexandru.elisei@....com>
To: Sudeep Holla <sudeep.holla@....com>
Cc: Marc Zyngier <maz@...nel.org>, linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Hanjun Guo <guohanjun@...wei.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mark Rutland <mark.rutland@....com>, alexandru.elisei@....com
Subject: Re: [PATCH 0/4] clocksource: Add standalone MMIO ARM arch timer
 driver

Hello,

On Wed, Aug 13, 2025 at 11:55:48AM +0100, Sudeep Holla wrote:
> +Alexandru
> 
> On Thu, Aug 07, 2025 at 05:02:39PM +0100, Marc Zyngier wrote:
> > For the past 10 years, both Mark and I have been lamenting about the
> > sorry state of the badly named "arch_timer" driver, and about the way
> > the MMIO part is intricately weaved into the system-register part.
> > 
> > The time has finally come to have a stab at it.
> > 
> > This small series simply creates a new timer driver for the MMIO arch
> > timer, and only that. It is an actual driver, and not some kludge that
> > has to run super early (that's what the per-CPU timers are for). This
> > allows, in turn, a pretty large cleanup of the per-CPU driver, though
> > there is more to come -- one thing at a time.
> > 
> > As an added bonus, we get a clocksource, which the original code
> > didn't provide. Just in case it might be useful. The end-result is far
> > more readable, and about 100 lines smaller.
> > 
> 
> (Tested it on Juno R2 and FVP in both DT and ACPI boot)
> 
> Tested-by: Sudeep Holla <sudeep.holla@....com>
> Reviewed-by: Sudeep Holla <sudeep.holla@....com>
> 
> Alexandru found it useful(avoids some unexpected hang IIUC) in his setup
> based on bootwrapper which doesn't initialise MMIO timers.

Just FYI, this is the testing that I did.

Without this series, if firmware (boot-wrapper-aarch64 in my testing) doesn't
configure access to the memory-mapped timer:

[    0.000000] arch_timer: Unable to find a suitable frame in timer @ 0x000000002a810000
[    0.000000] Failed to initialize '/timer@...10000': -22
..
[    0.528000] kvm [1]: kvm_arch_timer: uninitialized timecounter
..
# ls /dev/kvm
ls: cannot access '/dev/kvm': No such file or directory

With this series, if firmware doesn't configure access to the memory-mapped
timer:

[    0.549399] kvm [1]: Hyp nVHE mode initialized successfully
..
[    2.018050] arch-timer-mmio 2a810000.timer: Unable to find a suitable frame in timer @ 0x000000002a810000
[    2.018123] arch-timer-mmio 2a810000.timer: probe with driver arch-timer-mmio failed with error -22
..
# ls /dev/kvm
/dev/kvm

Thanks,
Alex