Re: [Xen-devel] [PATCH] xen/arm: register clocks used by the hypervisor

[Julien Grall <julien.grall@xxxxxxx>]

Hello Dirk,

On 21/06/16 11:16, 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.
This new property would need to be documented in:
   - linux/Documentation/devicetree/bindings/arm/xen.txt
   - xen/docs/misc/arm/device-tree/guest.txt

> Signed-off-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>
> ---
>   arch/arm/xen/enlighten.c | 35 +++++++++++++++++++++++++++++++++++
>   1 file changed, 35 insertions(+)
>
> diff --git a/arch/arm/xen/enlighten.c b/arch/arm/xen/enlighten.c
> index 75cd734..ee6e81f 100644
> --- a/arch/arm/xen/enlighten.c
> +++ b/arch/arm/xen/enlighten.c
> @@ -22,6 +22,7 @@
>   #include <linux/of.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>
> @@ -381,6 +382,40 @@ 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;
> +       unsigned int i, count;
> +       int ret;
> +
> +       count = of_clk_get_parent_count(xen_node);
The code in this file has changed quite a lot with the support of ACPI. 
For instance "xen_node" is not anymore a global variable. Can you rebase 
your rework on top of the branch for-linus-4.8?
You can find the branch in:

git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git

> +       if (!count)
> +               return 0;
> +
> +       for (i = 0; i < count; i++) {
> +               clk = of_clk_get(xen_node, i);
> +               if (IS_ERR(clk)) {
> +                       pr_err("Xen failed to register clock %i. Error: %li\n",
> +                              i, PTR_ERR(clk));
> +                       return PTR_ERR(clk);
> +               }
> +
> +               ret = clk_prepare_enable(clk);
I am not sure why we would want Linux to enable the clock. Should not it 
be already done either by the firmware or Xen?
Would it be possible to use the flags CLK_IGNORE_UNUSED instead? So the 
clock will be ignored by clk_disable_unused_subtree.
> +               if (ret < 0) {
> +                       pr_err("Xen failed to enable clock %i. Error: %i\n",
> +                              i, ret);
> +                       return ret;
> +               }
> +       }
> +
> +       return 0;
> +}
> +late_initcall(xen_arm_register_clks);
>
>   /* empty stubs */
>   void xen_arch_pre_suspend(void) { }
Regards,

--
Julien Grall

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