Hi Mark,
On 05.07.2016 12:39, Mark Rutland wrote:
Hi,
On Tue, Jul 05, 2016 at 08:50:23AM +0200, Dirk Behme wrote:
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.
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
too.
To fix this, we will add the "unused" clocks in Xen to the hypervisor
node. The Linux kernel has to register the clocks from the hypervisor
node, then.
Therefore, check if there is a "clocks" entry in the hypervisor node
and if so register the given clocks to the Linux kernel clock
framework and with this mark them as used. This prevents the clocks
from being disabled.
Signed-off-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>
---
Changes in v2: Drop the Linux implementation details like clk_disable_unused
in xen.txt.
Thanks for doing this.
Documentation/devicetree/bindings/arm/xen.txt | 13 ++++++++
arch/arm/xen/enlighten.c | 47 +++++++++++++++++++++++++++
2 files changed, 60 insertions(+)
diff --git a/Documentation/devicetree/bindings/arm/xen.txt b/Documentation/devicetree/bindings/arm/xen.txt
index c9b9321..21fd469 100644
--- a/Documentation/devicetree/bindings/arm/xen.txt
+++ b/Documentation/devicetree/bindings/arm/xen.txt
@@ -17,6 +17,19 @@ the following properties:
A GIC node is also required.
This property is unnecessary when booting Dom0 using ACPI.
+Optional properties:
+
+- clocks: one or more clocks to be registered.
+ Xen hypervisor drivers might replace native drivers, resulting in
+ clocks not registered by these native drivers. To avoid that these
+ unregistered clocks are disabled by the Linux kernel initialization
+ register them in the hypervisor node.
+ An example for this are the clocks of a serial driver already enabled
+ by the firmware. If the clocks used by the serial hardware interface
+ are not registered by the serial driver itself the serial output
+ might stop once the Linux kernel initialization disables the 'unused'
+ clocks.
The above describes the set of problems, but doesn't set out the actual
contract. It also covers a number of Linux implementation details in
abstract.
Could you kindly be a little more specific which 'implementation
details' you don't like?
E.g. to my understanding, the 'implementation detail' that Linux
disables unregistered clocks is needed for the description.
If you have a different wording in mind, could you kindly share that?
As I commented previously [1], the binding should describe the set of
guarantees that you rewquire (e.g. that the clocks must be left as-is,
not gated, and their rates left unchanged).
Please describe the specific set of guarantees that you require.
To my understanding this is done, already: "avoid that these ... clocks
are disabled"
Best regards
Dirk
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