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

Julien Grall
2016-07-14T17:14:50Z

Hello,

On 14/07/16 17:30, Dirk Behme wrote:
> On 14.07.2016 17:55, Stefano Stabellini wrote:
>> On Thu, 14 Jul 2016, Dirk Behme wrote:
>>> 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?
>>
>> Let me premise that I appreciate what you are trying to achieve with
>> this patch and I don't want to feature-creep it.
>>
>> However we are defining a new Device Tree binding,
>
>
> I don't think so.
>
> We are just using the existing one
>
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/clock/clock-bindings.txt#n66
>
>
> , pick it from other device tree nodes (e.g. serial, timer etc) and add
> it to the hypervisor node. And then use this existing one with the
> existing well defined clock API.
>
>
>> one which will have
>> to be supported for a long time by both Xen and Linux, so at the very
>> least we need to have the full picture. We need to understand if the
>> binding if sufficient
>
>
> Even if it's not sufficient, you can't change it.

I think you misunderstood Stefano's comment. Whilst the clock bindings
is set in stone, the binding you are adding in this patch is not yet
fixed. There is no requirement to follow what was defined in
clock-bindings.txt. I agree it would be convenient, but as mentioned by
Stefano this will need to be supported for a long time by Xen, Linux,
and any other consumer (i.e BSD kernels). So we have to be careful on
how it has been defined.

I would wait the answer of Michael on Stefano's question before taking
any decision here.

>
>
>> or if we need something different to solve the
>> problem completely.
>
>
> You might need anything additionally. E.g. an extension of the Linux
> kernel clock API to be able to modify the flags was proposed.

The Linux kernel is not the only consumer of the device tree bindings.
This is also used by other OS such as FreeBSD where you might already be
able (I have not actually checked) to forbid a user to change the clock
rate.

>
> Best regards
>
> Dirk
>
> P.S.: I still would be interested if we do have a practical example
> where it's not sufficient practically?

Very easy. What does prevent a driver to change the clock rate? Nothing
but the flags mentioned by Michael. There are already drivers which
modify the clock rate, thankfully those clocks are not shared with the
UART for now.

But we cannot rule out that it will not be possible in the future. Think
about a clock that would be used by another guest (I know it is still
theoretical as we have not yet solved the problem).

Regards,

--
Julien Grall