[PATCH v2] xen/arm: register clocks used by the hypervisor

[Michael Turquette]

Hi!

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.

> 
> 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 at de.bosch.com>
> ---
> Changes in v2:
>  - Rebase against git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git for-linus-4.8
>  - Add changes to Documentation/devicetree/bindings/arm/xen.txt
> 
>  Documentation/devicetree/bindings/arm/xen.txt | 11 +++++++
>  arch/arm/xen/enlighten.c                      | 47 +++++++++++++++++++++++++++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/arm/xen.txt b/Documentation/devicetree/bindings/arm/xen.txt
> index c9b9321..55dfd3b 100644
> --- a/Documentation/devicetree/bindings/arm/xen.txt
> +++ b/Documentation/devicetree/bindings/arm/xen.txt
> @@ -17,6 +17,17 @@ 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.

s/registered/consumed/

For appropriate DT binding script to steal I picked one at random:

Documentation/devicetree/bindings/clock/ti/clockdomain.txt

> +  Xen hypervisor drivers might replace native drivers, resulting in
> +  clocks not registered by these native drivers. To avoid that these
> +  unregistered clocks are disabled, then, e.g. by clk_disable_unused(),
> +  register them in the hypervisor node.
> +  An example for this are the clocks of the serial driver. If the clocks
> +  used by the serial hardware interface are not registered by the serial
> +  driver the serial output might stop once clk_disable_unused() is called.
> +
>  To support UEFI on Xen ARM virtual platforms, Xen populates the FDT "uefi" node
>  under /hypervisor with following parameters:
>  
> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> index 47acb36..5c546d0 100644
> --- a/arch/arm/xen/enlighten.c
> +++ b/arch/arm/xen/enlighten.c
> @@ -24,6 +24,7 @@
>  #include <linux/of_fdt.h>
>  #include <linux/of_irq.h>
>  #include <linux/of_address.h>
> +#include <linux/clk-provider.h>

s/clk-provider.h/clk.h/

clk-provider.h is only used for providers and this bit of code is a
consumer.

>  #include <linux/cpuidle.h>
>  #include <linux/cpufreq.h>
>  #include <linux/cpu.h>
> @@ -444,6 +445,52 @@ static int __init xen_pm_init(void)
>  }
>  late_initcall(xen_pm_init);
>  
> +/*
> + * Check if we want to register some clocks, that they
> + * are not freed because unused by clk_disable_unused().
> + * E.g. the serial console clock.
> + */
> +static int __init xen_arm_register_clks(void)
> +{
> +       struct clk *clk;
> +       struct device_node *xen_node;
> +       unsigned int i, count;
> +       int ret = 0;
> +
> +       xen_node = of_find_compatible_node(NULL, NULL, "xen,xen");
> +       if (!xen_node) {
> +               pr_err("Xen support was detected before, but it has disappeared\n");
> +               return -EINVAL;
> +       }
> +
> +       count = of_clk_get_parent_count(xen_node);
> +       if (!count)
> +               goto out;
> +
> +       for (i = 0; i < count; i++) {
> +               clk = of_clk_get(xen_node, i);

Is there a struct device we can use here? It would be better to use
devm_clk_get if possible.

Regards,
Mike

> +               if (IS_ERR(clk)) {
> +                       pr_err("Xen failed to register clock %i. Error: %li\n",
> +                              i, PTR_ERR(clk));
> +                       ret = PTR_ERR(clk);
> +                       goto out;
> +               }
> +
> +               ret = clk_prepare_enable(clk);
> +               if (ret < 0) {
> +                       pr_err("Xen failed to enable clock %i. Error: %i\n",
> +                              i, ret);
> +                       goto out;
> +               }
> +       }
> +
> +       ret = 0;
> +
> +out:
> +       of_node_put(xen_node);
> +       return ret;
> +}
> +late_initcall(xen_arm_register_clks);
>  
>  /* empty stubs */
>  void xen_arch_pre_suspend(void) { }
> -- 
> 2.8.0
>