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

[Stefano Stabellini <sstabellini@kernel.org>]

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.

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