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

From: Julien Grall <julien.grall@arm.com>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, Dirk Behme <dirk.behme@de.bosch.com>,
	Michael Turquette <mturquette@baylibre.com>,
	Stephen Boyd <sboyd@codeaurora.org>,
	xen-devel@lists.xenproject.org, linux-clk@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2] xen/arm: register clocks used by the hypervisor
Date: Tue, 5 Jul 2016 15:08:41 +0100	[thread overview]
Message-ID: <577BBF69.8010809__3841.03270848772$1467727799$gmane$org@arm.com> (raw)
In-Reply-To: <alpine.DEB.2.10.1607051456410.2575@sstabellini-ThinkPad-X260>

On 05/07/16 15:04, Stefano Stabellini wrote:
> On Tue, 5 Jul 2016, 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 clarify my suggestion: I am not saying we should disable the clock, I
> am saying we should set the "status" property to "disabled" in Xen for
> the clock used by the serial or passthrough devices (for which the
> "status" property is already set to "disabled"). That should work for
> cases where the clock is not shared among multiple devices.

How would you be able to differentiate in Xen between a clock shared and 
a non-shared one? The only way I can think it going through all the 
device tree which sounds really expensive.

> If the clock is shared, then I don't think we would run into the issue
> described by Dirk because I wouldn't imagine clk_disable_unused would
> try to disable the clock anymore, because it would actually be in use
> from Linux POV.

We also want to prevent Linux changing the rate of the clock (see Mark's 
mail [1]).

Regards,

[1] 
https://lists.xenproject.org/archives/html/xen-devel/2016-07/msg00335.html

-- 
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel