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

Julien Grall julien.grall at arm.com
Wed Jul 6 06:26:32 PDT 2016

On 06/07/16 14:16, Stefano Stabellini wrote:
> On Wed, 6 Jul 2016, Julien Grall wrote:
>> On 06/07/16 02:34, Michael Turquette wrote:
>>> Hi!
>> Hello Michael,
>>> Quoting Dirk Behme (2016-06-30 03:32:32)
>>>> Some clocks might be used by the Xen hypervisor and not by the Linux
>>>> kernel. If these are not registered by the Linux kernel, they might be
>>>> disabled by clk_disable_unused() as the kernel doesn't know that they
>>>> are used. The clock of the serial console handled by Xen is one
>>>> example for this. It might be disabled by clk_disable_unused() which
>>>> stops the whole serial output, even from Xen, then.
>>> This whole thread had me confused until I realized that it all boiled
>>> down to some nomenclature issues (for me).
>>> This code does not _register_ any clocks. It simply gets them and
>>> enables them, which is what every other clk consumer in the Linux kernel
>>> does. More details below.
>>>> Up to now, the workaround for this has been to use the Linux kernel
>>>> command line parameter 'clk_ignore_unused'. See Xen bug
>>>> http://bugs.xenproject.org/xen/bug/45
>>> clk_ignore_unused is a band-aid, not a proper medical solution. Setting
>>> that flag will not turn clocks on for you, nor will it guarantee that
>>> those clocks are never turned off in the future. It looks like you
>>> figured this out correctly in the patch below but it is worth repeating.
>>> Also the new CLK_IS_CRITICAL flag might be of interest to you, but that
>>> flag only exists as a way to enable clocks that must be enabled for the
>>> system to function (hence, "critical") AND when those same clocks do not
>>> have an accompanying Linux driver to consume them and enable them.
>> I don't think we want the kernel to enable the clock for the hypervisor. We
>> want to tell the kernel "don't touch at all to this clock, it does not belong
>> to you".
> Right, and that's why I was suggesting that another way to do this would
> be to set the "status" to "disabled" in Xen: so that Linux would leave
> the clock alone. But in that case Linux would not be happy to see
> disabled clocks which are actually supposed to be used by some devices.
> Is that correct?

The problem is not "whether Linux would be happy or not", but the 
meaning of this property.

Based on the ePAPR, "status = disabled" indicates that the device is not 
presently operational, but it might become operational later (for 
example, something is not plugged in, or switched off).

An operating system which read this property should interpret as "the 
clock should not be used". However, with your suggestion the OS would 
need to differentiate between "the clock may be used by the hypervisor" 
and "the clock is not wired up/present/...".


Julien Grall

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