Re: [PATCH v4] xen/arm: Add a clock property

From: Dirk Behme <dirk.behme@de.bosch.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Michael Turquette <mturquette@baylibre.com>,
	Stephen Boyd <sboyd@codeaurora.org>,
	Julien Grall <julien.grall@arm.com>,
	xen-devel@lists.xenproject.org,
	linux-clk <linux-clk@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v4] xen/arm: Add a clock property
Date: Wed, 20 Jul 2016 14:53:44 +0200	[thread overview]
Message-ID: <847d3d22-985d-cbe9-3cd9-7c85080fae37__43080.5587130182$1469019291$gmane$org@de.bosch.com> (raw)
In-Reply-To: <CAMuHMdWdqdT4jnae8h9tDDzzh8ZAbJJwiMW-scPBNVwmjYSiWQ@mail.gmail.com>

On 20.07.2016 14:46, Geert Uytterhoeven wrote:
> Hi Julien,
>
> On Wed, Jul 20, 2016 at 2:10 PM, Julien Grall <julien.grall@arm.com> wrote:
>> On 20/07/16 12:49, Geert Uytterhoeven wrote:
>>> On Wed, Jul 20, 2016 at 1:01 PM, Julien Grall <julien.grall@arm.com>
>>> wrote:
>>>> On 20/07/16 10:43, Geert Uytterhoeven wrote:
>>>>> On Tue, Jul 12, 2016 at 9:46 AM, Dirk Behme <dirk.behme@de.bosch.com>
>>>>> wrote:
>>>>>> Clocks described by this property are reserved for use by Xen, and the
>>>>>> OS
>>>>>> must not alter their state any way, such as disabling or gating a
>>>>>> clock,
>>>>>> or modifying its rate. Ensuring this may impose constraints on parent
>>>>>> clocks or other resources used by the clock tree.
>>>>>>
>>>>>> This property is used to proxy clocks for devices Xen has taken
>>>>>> ownership
>>>>>> of, such as UARTs, for which the associated clock controller(s) remain
>>>>>> under the control of Dom0.
>>>>>
>>>>> I'm not familiar with using XEN at all, but I'm a bit puzzled...
>>>>>
>>>>> Can't you just add a clocks property to the (virtual) serial device node
>>>>> in DT?
>>>>> Then the (virtual) serial device driver can get and enable the clock?
>>>>
>>>> There is no DT node for the Xen console (hvc). The UART used by Xen will
>>>> be
>>>> completely removed from the Device tree.
>>>
>>> Why is it removed?
>>
>> Because the device is used exclusively by Xen and DOM0 should not touch it
>> at all (IRQs and MMIOs are not mapped).
>
> IMHO then it's Xen's responsability to make sure not to disable the clock(s).
>
> Who removes the device node from the DT? If Xen, can't it just remember
> which clocks were present in removed device nodes?

Yes, we are trying this with "moving" the clocks of the removed nodes to 
the Xen hypervisor node:

https://lists.xen.org/archives/html/xen-devel/2016-06/msg02605.html

The question discussed here is how to deal with these clocks properly in 
Linux, then.

Best regards

Dirk

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