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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