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

From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Dirk Behme <dirk.behme@de.bosch.com>,
	linux-arm-kernel@lists.infradead.org,
	Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, xen-devel@lists.xenproject.org,
	linux-clk@vger.kernel.org,
	Michael Turquette <mturquette@baylibre.com>,
	Stephen Boyd <sboyd@codeaurora.org>
Subject: Re: [PATCH v2] xen/arm: register clocks used by the hypervisor
Date: Tue, 5 Jul 2016 15:37:48 +0100 (BST)	[thread overview]
Message-ID: <alpine.DEB.2.10.1607051534411.2575@sstabellini-ThinkPad-X260> (raw)
In-Reply-To: <577BBF69.8010809@arm.com>

On Tue, 5 Jul 2016, Julien Grall wrote:
> 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.

Yes, but it is only done once and we only need to do the search for
clocks of "disabled" devices.

> > 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]).

I am not sure what's the best way to achieve that for clocks shared
across multiple devices. Let's see what the clock maintainers suggest.