Hi Michael and Julien,
On 08.07.2016 11:34, Julien Grall wrote:
Hi Dirk,
On 08/07/16 08:44, Dirk Behme wrote:
Xen hypervisor drivers might replace native OS drivers. The result is
that some important clocks that are enabled by the OS in the non-Xen
case are not properly enabled in the presence of Xen. The clocks
property enumerates the clocks that must be enabled by the Xen clock
consumer.
An example is a serial driver enabled by the hypervisor. Xen must
I would say "An example is the UART used by the hypervisor."
consume and enable these clocks in the OS to ensure behavior continues
after firmware configures the UART hardware and corresponding clock
harder.
What do you mean by "harder"?
Also, relying on DOM0 to enable the clock looks very wrong to me and you
give an example which prove that. The UART will be used before hand by
Xen, however it will not be possible to use it if you expect DOM0 to
enable the clock (or even modify the clock frequency).
The clock should be enabled either by the firmware or Xen. But not DOM0.
DOM0 should not touch this clock at all.
Furthermore, this property could be used for clock associated to device
that will be passthrough-ed to a guest. In this case, the clock would be
enabled even if the device is not in use which will result more power
consumption.
I took the description directly from Michael's proposal
http://www.spinics.net/lists/arm-kernel/msg516576.html
Would it be possible that you two experts agree on the exact wording you
like to see?
Best regards
Dirk
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 OS has to consume and enable the clocks from the hypervisor
node, then.
Therefore, check if there is a "clocks" entry in the hypervisor node
and if so consume and enable the given clocks. This prevents the clocks
from being disabled by the OS.
Signed-off-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>
---
Changes in v3: Use the xen.txt description proposed by Michael. Thanks!
Changes in v2: Drop the Linux implementation details like
clk_disable_unused
in xen.txt.
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..00f2165 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 enabled
+ Xen hypervisor drivers might replace native OS drivers. The result is
"native OS" has no meaning on Xen. This seems to be a cumbersome way to
say that the device will be used by the hypervisor and hidden to DOM0
(aka hardware domain).
+ that some important clocks that are enabled by the OS in the non-Xen
+ case are not properly enabled in the presence of Xen. The clocks
+ property enumerates the clocks that must be enabled by the Xen clock
+ consumer.
+ An example is a serial driver enabled by the hypervisor. Xen must
+ consume and enable these clocks in the OS to ensure behavior continues
+ after firmware configures the UART hardware and corresponding clock
+ harder.
+
To support UEFI on Xen ARM virtual platforms, Xen populates the FDT
"uefi" node
under /hypervisor with following parameters:
Regards,
--
To unsubscribe from this list: send the line "unsubscribe devicetree" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
