Quoting Julien Grall (2016-07-08 02:34:43)

Hi Dirk,

On 08/07/16 08:44, Dirk Behme wrote:

quoted

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."

quoted

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.

Is there a need to pass clock references through to guests? If so the
unmerged CLK_ENABLE_HAND_OFF[0] feature might be useful to you? If this
flag is set on a given clk, it will be enabled at the time it is
registered by the clk provider driver, and it's enable_count reference
counter will be "handed off" to the first consumer that calls clk_get()
and clk_prepare_enable() on it. This means the clock CAN be gated by
it's proper driver, but it will be enabled at boot time as well.

This is useful for use cases like splash screens where the bootloader
configures the display and plays some animation, and we want the linux
kernel to take over the display controller hardware without cutting
clocks, blanking or reseting it. Handing off the clock reference count
helps achieve this.

[0] lkml.kernel.org/g/1455225554-13267-1-git-send-email-mturquette@baylibre.com

Regards,
Mike

quoted

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 <redacted>
---
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).

quoted

+  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,

-- 
Julien Grall