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

[Dirk Behme <dirk.behme@xxxxxxxxxxxx>]

Hi Michael,

On 06.07.2016 22:42, Michael Turquette wrote:
> Hi Julien,
>
> Quoting Julien Grall (2016-07-06 06:10:52)
> > 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".
> But the patch *does* touch the clock from the kernel. It enables the
> clock via a call clk_prepare_enable. I'm utterly confused.

Maybe we need some advice here :)


I've used clk_prepare_enable() 'just' to get the enable count incremented

http://lxr.free-electrons.com/source/drivers/clk/clk.c#L751

Because it's my understanding that enable_count is needed to prevent 
clk_disable_unused() from disabling the clock.

If there is an other / better / correct way to achieve that, please let 
us know.

I've had a look to use the CLK_IGNORE_UNUSED flag, too. But couldn't 
find a function exported by the clock framework to set that flag (?)

Best regards

Dirk

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