From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dirk Behme Subject: Re: [Xen-devel] [PATCH v4] xen/arm: Add a clock property Date: Wed, 27 Jul 2016 07:05:37 +0200 Message-ID: References: <1468309605-19522-1-git-send-email-dirk.behme@de.bosch.com> <146836236759.73491.11707389619985827497@resonance> <57868EDE.6060406@gmail.com> <146844380895.73491.4867379517577413421@resonance> <7df784ab-d0c0-939b-393e-214535c4b191@de.bosch.com> <146914607316.8780.7961396342647226841@resonance> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-clk-owner@vger.kernel.org To: Stefano Stabellini , Michael Turquette , Julien Grall Cc: Dirk Behme , Mark Rutland , devicetree@vger.kernel.org, Stephen Boyd , xen-devel@lists.xenproject.org, linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org List-Id: devicetree@vger.kernel.org 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? Best regards Dirk