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

[Julien Grall <julien.grall-5wv7dgnIgG8@public.gmane.org>]

Hi Dirk,

On 14/07/16 07:31, 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.

I think it is a good start, although I would like to see some rationale 
in the code and commit message about the behavior of the Linux with 
those clocks after this patch because it does not match the "contract".

Regards,

-- 
Julien Grall
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