Re: [PATCH v2] xen/arm: register clocks used by the hypervisor

From: Julien Grall <julien.grall-5wv7dgnIgG8@public.gmane.org>
To: Stefano Stabellini
	<sstabellini-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Dirk Behme <dirk.behme-V5te9oGctAVWk0Htik3J/w@public.gmane.org>
Cc: linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org,
	Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>,
	devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	xen-devel-GuqFBffKawtpuQazS67q72D2FQJk+8+b@public.gmane.org,
	linux-clk-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Michael Turquette
	<mturquette-rdvid1DuHRBWk0Htik3J/w@public.gmane.org>,
	Stephen Boyd <sboyd-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
Subject: Re: [PATCH v2] xen/arm: register clocks used by the hypervisor
Date: Tue, 5 Jul 2016 15:02:19 +0100	[thread overview]
Message-ID: <577BBDEB.7000305@arm.com> (raw)
In-Reply-To: <577BBC1A.8020209-5wv7dgnIgG8@public.gmane.org>

On 05/07/16 14:54, Julien Grall wrote:
>
>
> On 05/07/16 14:53, Stefano Stabellini wrote:
>> On Thu, 30 Jun 2016, Dirk Behme wrote:
>>> +- clocks: one or more clocks to be registered.
>>> +  Xen hypervisor drivers might replace native drivers, resulting in
>>> +  clocks not registered by these native drivers. To avoid that these
>>> +  unregistered clocks are disabled, then, e.g. by clk_disable_unused(),
>>> +  register them in the hypervisor node.
>>> +  An example for this are the clocks of the serial driver. If the
>>> clocks
>>> +  used by the serial hardware interface are not registered by the
>>> serial
>>> +  driver the serial output might stop once clk_disable_unused() is
>>> called.
>>
>> What if we use the "status" property of the clocks? Could we set it to
>> "disabled" in Xen? Would that be enough for Linux to leave them alone?
>
> clocks could be shared between multiple devices. So it is not possible
> to disable the clock.

To give an example, below the UART node for juno 
(arch/arm64/boot/dts/arm/juno-base.dtsi):

         soc_uart0: uart@7ff80000 {
                 compatible = "arm,pl011", "arm,primecell";
                 reg = <0x0 0x7ff80000 0x0 0x1000>;
                 interrupts = <GIC_SPI 83 IRQ_TYPE_LEVEL_HIGH>;
                 clocks = <&soc_uartclk>, <&soc_refclk100mhz>;
                 clock-names = "uartclk", "apb_pclk";
         };

The clock soc_refclk100mhz is shared with the mailbox.

Regards,

-- 
Julien Grall
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