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

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Julien Grall <julien.grall@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	Dirk Behme <dirk.behme@de.bosch.com>,
	Michael Turquette <mturquette@baylibre.com>,
	Stephen Boyd <sboyd@codeaurora.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	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 14:46:43 +0200	[thread overview]
Message-ID: <CAMuHMdWdqdT4jnae8h9tDDzzh8ZAbJJwiMW-scPBNVwmjYSiWQ__10199.7069119991$1469018875$gmane$org@mail.gmail.com> (raw)
In-Reply-To: <04d9dbfb-196d-a775-7fbb-526aba8085f4@arm.com>

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?

What to do on SoCs where the serial device is part of a power area (e.g.
Renesas SH/R-Mobile)? Who will make sure it is not powered down?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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