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

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Message-ID: <20250813-opalescent-boisterous-viper-d0061d@sudeepholla>
Date: Wed, 13 Aug 2025 13:32:54 +0100
From: Sudeep Holla <sudeep.holla@....com>
To: Alexandru Elisei <alexandru.elisei@....com>
Cc: Marc Zyngier <maz@...nel.org>, <linux-kernel@...r.kernel.org>,
	<linux-acpi@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Sudeep Holla <sudeep.holla@....com>,
	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>
Subject: Re: [PATCH 0/4] clocksource: Add standalone MMIO ARM arch timer
 driver

On Wed, Aug 13, 2025 at 12:35:31PM +0100, Alexandru Elisei wrote:
> 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 for the details. I misunderstood as some VM boot hang and was wondering
why I couldn't reproduce it(was failing gracefully though no KVM and hence
VM fails to launch).

-- 
Regards,
Sudeep