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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 13 Jul 2016, Michael Turquette wrote:
> 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.

Just for disambiguation, I guess you are referring to the Xen code in
Linux (such as arch/arm/xen/enlighten.c), right?

We have "Xen", the hypervisor, which runs before Linux and it is a
separate component running at EL2, and we have Xen support in Linux,
such as arch/arm/xen and drivers/xen. These are drivers to communicate
with the Xen hypervisor and other components in a Xen system.

In this case, the Xen hypervisor would be the owner of the clock. Xen
support in Linux, specifically arch/arm/xen/enlighten.c, only comes into
the picture to "warn" Linux that shouldn't really be messing with the
clocks. I guess that from a Linux clock driver point of view, it is a
consumer.

I hope that helps.


> > > 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@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;
> > >>>> +       }
> > >>>> +
> > >>>> +       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) { }
> > >>>> --
> > >>
> > >
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@xxxxxxxxxxxxx
> > > https://lists.xen.org/xen-devel
> > >
> > 
> 

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