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

[Michael Turquette <mturquette@baylibre.com>]

Quoting Dirk Behme (2016-07-13 11:56:30)
> On 13.07.2016 20:43, Stefano Stabellini wrote:
> > On Wed, 13 Jul 2016, Dirk Behme wrote:
> >> On 13.07.2016 00:26, Michael Turquette wrote:
> >>> Quoting Dirk Behme (2016-07-12 00:46:45)
> >>>> 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 that clk_prepare_enable will not prevent the rate from changing
> >>> (clk_set_rate) or a parent from changing (clk_set_parent). The only way
> >>> to do this currently would be to set the following flags on the effected
> >>> clocks:
> >>>
> >>>     CLK_SET_RATE_GATE
> >>>     CLK_SET_PARENT_GATE
> >>
> >>
> >>
> >> Regarding setting flags, I think we already talked about that. I think the
> >> conclusion was that in our case its not possible to manipulate the flags in
> >> the OS as this isn't intended to be done in cases like ours. Therefore no API
> >> is exported for this.
> >>
> >> I.e. if we need to set these flags, we have to do that in Xen where we add the
> >> clocks to the hypervisor node in the device tree. And not in the kernel patch
> >> discussed here.
> >
> > These are internal Linux flags, aren't they?
> 
> 
> I've been under the impression that you can set clock "flags" via the 
> device tree. Seems I need to re-check that ;)

Right, you cannot set flags from the device tree. Also, setting these
flags is done by the clock provider driver, not a consumer. Xen is the
consumer.

Regards,
Mike

> 
> Best regards
> 
> Dirk
> 
> 
> > They cannot be set in Xen.
> >
> > If the only way to make sure that clk_set_rate and clk_set_parent fail
> > on one of the clocks "owned" by Xen is to set CLK_SET_RATE_GATE and
> > CLK_SET_PARENT_GATE, we'll have to do that. Even if it means introducing
> > a new internal Linux API.
> >
> >
> >>> And then enable the clocks. All calls to clk_set_parent and clk_set_rate
> >>> with those clocks in the path will fail, so long as they are prepared
> >>> and enabled. This implementation detail is specific to Linux and
> >>> definitely should not go into the binding.
> >>>
> >>>>
> >>>> 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.
> >>>
> >>> I'm still not completely sure about this type of layering and whether or
> >>> not it is sane. If you want Xen to manage the clock controller, AND you
> >>> want Linux guests or dom0 to consume those clocks and manipulate them in
> >>> other drivers, then this solution won't work.
> >>>
> >>> Regards,
> >>> Mike
> >>>
> >>>>
> >>>> 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@de.bosch.com>
> >>>> ---
> >>>> 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;
> >>>> +       }
> >>>> +
> >>>> +       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);
> >>>> +               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) { }
> >>>> --
> >>
> >
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> >
>