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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Dirk,

On 27/07/16 06:05, Dirk Behme wrote:
> Hi Michael, Stefano and Julien,
>
> On 22.07.2016 03:16, Stefano Stabellini wrote:
> > On Thu, 21 Jul 2016, Michael Turquette wrote:
> > > Quoting Stefano Stabellini (2016-07-14 03:38:04)
> > > > 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. 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?
> > > No, there is no way for a consumer to do that. The provider must do it.
> > All right. But could we design a new device tree binding which the Xen
> > hypervisor would use to politely ask the clock provider in Linux to set
> > CLK_SET_RATE_GATE and CLK_SET_PARENT_GATE for a given clock?
> >
> > Xen would have to modify the DTB before booting Linux with the new
> > binding.
> >
> >
> > > > 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?
> > > Upon further reflection, I think that your clock consumer can probably
> > > use clk_set_rate_range() to "lock" in a rate. This is good because it is
> > > exactly what a clock consumer should do:
> > >
> > > 1) get the clk
> > > 2) enable the clk
> > > 3) set the required rate for the clock
> > > 4) set rate range constraints, or conversely,
> > > 5) lock in an exact rate; set the min/max rate to the same value
> > >
> > > The problem with this solution is that it requires the consumer to have
> > > knowledge of the rates that it wants for that clock, which I guess is
> > > something that Linux kernels in a Xen setup do not want/need?
> > Who is usually the component with knowledge of the clock rate to set? If
> > it's a device driver, then neither the Xen hypervisor, nor the Xen core
> > drivers in Linux would know anything about it. (Unless the clock rate is
> > specified on device tree via assigned-clock-rates of course.)
> >
> >
> > > Is it correct that you would prefer some sort of never_touch_this_clk()
> > > api?
> > > From my understading, yes, never_touch_this_clk() would make things
> > > easier.
>
> Would it be somehow worth to wait for anything like this
> never_touch_this_clk() api? Or should we try to proceed with
> clk_prepare_enable() like done in this patch for the moment?
I am not sure who will write the new api never_touch_this_clk(). Could 
you suggest an implementation based on the discussion?
Regards,

> Best regards
>
> Dirk
--
Julien Grall

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