linux-kernel - Re: [PATCH] clocksource: arm_arch_timer: Support reading clock rate from a driver

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Date:	Fri, 29 Jul 2016 11:31:44 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Mark Rutland <mark.rutland@....com>,
	Rafał Miłecki <zajec5@...il.com>
Cc:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rafał Miłecki <rafal@...ecki.pl>,
	"open list:CLOCKSOURCE, CLOCKEVENT DRIVERS" 
	<linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clocksource: arm_arch_timer: Support reading clock rate
 from a driver

On 29/07/16 11:18, Mark Rutland wrote:
> [adding Marc to Cc]
> 
> On Fri, Jul 29, 2016 at 11:23:11AM +0200, Rafał Miłecki wrote:
>> From: Rafał Miłecki <rafal@...ecki.pl>
>>
>> On some devices using arch code for reading clock rate doesn't work. So
>> far the only option was to specify clock-frequency in a DT. This works
>> only if a clock frequency doesn't have to be calculated on runtime.
>>
>> On BCM53573 SoC (with Cortex-A7) there is ILP clock that needs its own
>> driver. With this change we can query such clocks by using a standard:
>> clocks = <&foo>;
> 
> The clock for the architected timer(s) are supposed to be configured
> before entering Linux. The clock-frequency property is at best a dodgy
> workaround, and this is even worse.
> 
> Please fix your firmware to configure and enabled this clock before
> entering Linux, and to program CNTFRQ appropriately.

I'll add that failure to do so breaks other subsystems which do rely on
CNTFRQ to be correctly programmed *on each CPU* (like KVM and Xen).

Please follow the requirements of the architecture and don't just paper
over such a bug.

> As this path stands, NAK.

Seconded.

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