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

Dirk Behme
2016-07-14T10:49:35Z

On 14.07.2016 12:38, Stefano Stabellini wrote:
> On Thu, 14 Jul 2016, Dirk Behme wrote:
>> On 13.07.2016 23:03, 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.
>>
>>
>> Ok, thanks, then I think we can forget about using flags for the issue we are
>> discussing here.
>>
>> Best regards
>>
>> Dirk
>>
>> P.S.: Would it be an option to merge the v4 patch we are discussing here,
>> then? From the discussion until here, it sounds to me that it's the best
>> option we have at the moment. Maybe improving it in the future, then.
>
> It might be a step in the right direction, but it doesn't really prevent
> clk_set_rate from changing properties of a clock owned by Xen. This
> patch is incomplete.

Let me ask then: Do we have a practical example where it's not
sufficient practically?

To my understanding, Xen people have been happy with the
"clk_ignore_unused" workaround since ~2 years, now [1]. And I think the
"clk_ignore_unused" workaround does mainly the same like the patch
discussed here. It doesn't care regarding clk_set_rate from changing
properties, too?

While I agree that the patch theoretically incomplete, if nobody has a
real world example I would think that from practical point of view it's
sufficient in a first step.

If this is the case, I'd propose to fix the practical issue in a first
step with a patch (this one) which is sufficient to fix the issues the
Xen users have. And update the code for theoretical future issues in a
second step.

Best regards

Dirk

[1] http://bugs.xenproject.org/xen/bug/45

> We need to understand at least what it would take
> to have a complete solution.
>
> Michael, do you have any suggestions on how it would be possible to set
> CLK_SET_RATE_GATE and CLK_SET_PARENT_GATE for those clocks in a proper
> way?
>
> Like you wrote, I would imagine it needs to be done by the clock
> provider driver. Maybe to do that, it would be easier to have a new
> device tree property on the clock node, rather than listing phandle and
> clock-specifier pairs under the Xen node?