Re: [Xen-devel] [PATCH v4] xen/arm: Add a clock property

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 12 Jul 2016, Dirk Behme wrote:
> Clocks described by this property are reserved for use by Xen, and the OS
> must not alter their state any way, such as disabling or gating a clock,
> or modifying its rate. Ensuring this may impose constraints on parent
> clocks or other resources used by the clock tree.
> 
> This property is used to proxy clocks for devices Xen has taken ownership
> of, such as UARTs, for which the associated clock controller(s) remain
> under the control of Dom0.
> 
> 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.
> 
> Signed-off-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>
> ---
> Changes in v4: Switch to the xen.txt description proposed by Mark:
>                https://www.spinics.net/lists/arm-kernel/msg516158.html
> 
> 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 | 12 +++++++
>  arch/arm/xen/enlighten.c                      | 47 
> +++++++++++++++++++++++++++
>  2 files changed, 59 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/arm/xen.txt 
> b/Documentation/devicetree/bindings/arm/xen.txt
> index c9b9321..437e50b 100644
> --- a/Documentation/devicetree/bindings/arm/xen.txt
> +++ b/Documentation/devicetree/bindings/arm/xen.txt
> @@ -17,6 +17,18 @@ the following properties:
>    A GIC node is also required.
>    This property is unnecessary when booting Dom0 using ACPI.
>  
> +Optional properties:
> +
> +- clocks: a list of phandle + clock-specifier pairs
> +  Clocks described by this property are reserved for use by Xen, and the
> +  OS must not alter their state any way, such as disabling or gating a
> +  clock, or modifying its rate. Ensuring this may impose constraints on
> +  parent clocks or other resources used by the clock tree.
> +
> +  Note: this property is used to proxy clocks for devices Xen has taken
> +  ownership of, such as UARTs, for which the associated clock
> +  controller(s) remain under the control of Dom0.
> +
>  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>
>  #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;
> +     }

Given that this is a late initcall, the following should work:

    if (!xen_domain())
        return -ENODEV;

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