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

[Dirk Behme <dirk.behme@gmail.com>]

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 ;)

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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