Re: [PATCH 1/3] dt-bindings: chosen: Add clocksource and clockevent selection
From: Mark Rutland <mark.rutland@arm.com>
Date: 2017-12-15 11:32:53
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linux-arm-kernel, lkml
Hi, On Wed, Dec 13, 2017 at 07:53:11PM +0100, Alexandre Belloni wrote:
The clocksource and clockevent timer are probed early in the boot process. At that time it is difficult for linux to know whether a particular timer can be used as the clocksource or the clockevent or by another driver, especially when they are all identical or have similar features.
I think that to solve this problem, we need to stop treating clocksources and clockevent devices as completely separate device types, and instead treat them as particular cases of a more general clock device. That way, a driver can register a single device, with flags saying whether it is: * a clocksource only * a clockevent device only * both a clocksource and clockevent device * both, but mutually exclusive at runtime ... and thus drivers don't have to make an impossible decision up-front as to how the device should be used. As more devices get registered, the core timekeeping code can improve on its choices and re-assign devices. That doesn't solve the case where a clock device may use resources we want for something else, but I think we can solve that separately. Thanks, Mark.
quoted hunk ↗ jump to hunk
Until now, multiple strategies have been used to solve that: - use Kconfig option as MXC_USE_EPIT or ATMEL_TCB_CLKSRC_BLOCK - use a kernel parameter as the "clocksource" early_param in mach-omap2 - registering the first seen timer as a clockevent and the second one as a clocksource as in rk_timer_init or dw_apb_timer_init Add a linux,clocksource and a linux,clockevent node in chosen with a timer property pointing to the timer to use. Other properties, like the targeted precision may be added later. Signed-off-by: Alexandre Belloni <redacted> --- Documentation/devicetree/bindings/chosen.txt | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+)diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt index e3b13ea7d2ae..c7ee3ecb5276 100644 --- a/Documentation/devicetree/bindings/chosen.txt +++ b/Documentation/devicetree/bindings/chosen.txt@@ -120,3 +120,23 @@ e.g. While this property does not represent a real hardware, the address and the size are expressed in #address-cells and #size-cells, respectively, of the root node. + +linux,clocksource and linux,clockevent +-------------------------------------- + +Those nodes have a timer property. This property is a phandle to the timer to be +chosen as the clocksource or clockevent. This is only useful when the platform +has multiple identical timers and it is not possible to let linux make the +correct choice. + +/ { + chosen { + linux,clocksource { + timer = <&timer0>; + }; + + linux,clockevent { + timer = <&timer1>; + }; + }; +};-- 2.15.1 -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html