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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] cpufreq implementation for OMAP under xen hypervisor.
On Thu, Sep 25, 2014 at 1:14 PM, Stefano Stabellini
<stefano.stabellini@xxxxxxxxxxxxx> wrote:
> On Thu, 25 Sep 2014, Oleksandr Dmytryshyn wrote:
>> On Wed, Sep 24, 2014 at 5:36 PM, Stefano Stabellini
>> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> > On Tue, 16 Sep 2014, Oleksandr Dmytryshyn wrote:
>> >> On Wed, Sep 10, 2014 at 10:31 PM, Konrad Rzeszutek Wilk
>> >> <konrad.wilk@xxxxxxxxxx> wrote:
>> >> > On Wed, Sep 10, 2014 at 07:35:47PM +0100, Stefano Stabellini wrote:
>> >> >> On Wed, 10 Sep 2014, Andrii Tseglytskyi wrote:
>> >> >> > Hi,
>> >> >> >
>> >> >> > On Wed, Sep 10, 2014 at 12:42 PM, Ian Campbell
>> >> >> > <Ian.Campbell@xxxxxxxxxx> wrote:
>> >> >> > >
>> >> >> > > On Tue, 2014-09-09 at 22:41 +0100, Stefano Stabellini wrote:
>> >> >> > > > On Tue, 9 Sep 2014, Ian Campbell wrote:
>> >> >> > > > > On Thu, 2014-09-04 at 22:56 +0100, Stefano Stabellini wrote:
>> >> >> > > > > > I am trying to think of an alternative, such as passing the
>> >> >> > > > > > real cpu
>> >> >> > > > > > nodes to dom0 but then adding status = "disabled", but I am
>> >> >> > > > > > not sure
>> >> >> > > > > > whether Linux checks the status for cpu nodes.
>> >> >> > > > >
>> >> >> > > > > status = "disabled" is defined to have a specific (i.e.
>> >> >> > > > > non-default)
>> >> >> > > > > meaning for cpu nodes, Julien mentioned this when I tried to
>> >> >> > > > > add a
>> >> >> > > > > similar patch to Xen to ignore them. I think it basically
>> >> >> > > > > means "present
>> >> >> > > > > but not running, you should start them!".
>> >> >> > > > >
>> >> >> > > > > > In addition this scheme
>> >> >> > > > > > wouldn't support the case where dom0 has more vcpus than
>> >> >> > > > > > pcpus on the
>> >> >> > > > > > system. Granted it is not very common and might even be
>> >> >> > > > > > detrimental for
>> >> >> > > > > > performances, but we should be able to support it.
>> >> >> > > > >
>> >> >> > > > > It's a bit of an edge case, for sure. I guess it wouldn't be
>> >> >> > > > > totally
>> >> >> > > > > unreasonable to say that if you use this sort of configuration
>> >> >> > > > > you may
>> >> >> > > > > not get cpufreq support.
>> >> >> > > > >
>> >> >> > > > > > Ian, what do you think about this?
>> >> >> > > > >
>> >> >> > > > > All the options suck in one way or another AFAICT. I think we
>> >> >> > > > > are going
>> >> >> > > > > to be looking for the least bad solution not necessarily a
>> >> >> > > > > good one.
>> >> >> > > > >
>> >> >> > > > > Fundamentally are we trying to avoid having to have a i2c
>> >> >> > > > > subsystem etc
>> >> >> > > > > in the hypervisor to be be able to change the voltages
>> >> >> > > > > before/after
>> >> >> > > > > changing the frequency?
>> >> >> > > > >
>> >> >> > > > > We can't just say "that's part of the cpufreq driver" since
>> >> >> > > > > different
>> >> >> > > > > boards using the same SoC might use different voltage
>> >> >> > > > > regulators, over
>> >> >> > > > > i2c or some other bus etc, so we end up with a matrix.
>> >> >> > > > >
>> >> >> > > > > It's arguable that we should be letting dom0 poke at that
>> >> >> > > > > regulator
>> >> >> > > > > functionality anyway, at least not all of it. Taking that
>> >> >> > > > > ability away
>> >> >> > > > > would necessarily imply more platform specific functionality
>> >> >> > > > > in the
>> >> >> > > > > hypervisor.
>> >> >> > > >
>> >> >> > > > Right.
>> >> >> > > > I am afraid that in order to avoid more code in Xen, we end up
>> >> >> > > > with an
>> >> >> > > > unmaintainable interface and unupstreamable hacks in dom0.
>> >> >> > >
>> >> >> > > That's what I'm worried about to. Hence I'm wondering if we should
>> >> >> > > just
>> >> >> > > do this in the hypervisor.
>> >> >> > >
>> >> >> > > Although there are a myriad of them the parts used to do voltage
>> >> >> > > control
>> >> >> > > tend to be fairly simple.
>> >> >> > >
>> >> >> > > One concern I have is that i2c busses also tend to have other
>> >> >> > > things on
>> >> >> > > them which dom0 might legitimately access (e.g. rtc), I'm not sure
>> >> >> > > what
>> >> >> > > to suggest here.
>> >> >> >
>> >> >> > I would try to avoid i2c transactions in Xen. I2C driver is quite
>> >> >> > complicated in Linux kernel. It consists of several parts - common
>> >> >> > core + platform specific. I'm pretty sure Xen should not handle this.
>> >> >> > I think that establishing of event channel for frequency changing is
>> >> >> > a
>> >> >> > good idea. It would be good to try to implement this. In process of
>> >> >> > implementation we will see what is need to be resolved.
>> >> >>
>> >> >> OK, that's reasonable.
>> >> >>
>> >> >>
>> >> >> > The only question here is how to pass physical cpu to dom0.
>> >> >>
>> >> >> We can use a device tree based interface to pass the information to
>> >> >> dom0, but requiring a number of dom0 vcpus equal to the number of
>> >> >> physical cpus and in addition to that having to pin the vcpus each to a
>> >> >> different pcpu is quite a stringent limitation. However I don't know
>> >> >> the
>> >> >> frequency changing interfaces in Linux well enough to know how hard
>> >> >> would be to lift it.
>> >> >>
>> >> >>
>> >> >> > Regarding x86.
>> >> >> > I'm not sure but maybe ACPI interface encapsulate voltage changing
>> >> >> > as well?
>> >> >>
>> >> >> I think so (but I am not an expert on that).
>> >> >
>> >> > The usual states are P and C states. The P states is the closes to what
>> >> > you
>> >> > are looking at:
>> >> >
>> >> > struct acpi_processor_px {
>> >> > u64 core_frequency; /* megahertz */
>> >> > u64 power; /* milliWatts */
>> >> > u64 transition_latency; /* microseconds */
>> >> > u64 bus_master_latency; /* microseconds */
>> >> > u64 control; /* control value */
>> >> > u64 status; /* success indicator */
>> >> > };
>> >> >
>> >> >>
>> >> >>
>> >> >>
>> >> >> > Regards,
>> >> >> > Andrii
>> >> >> >
>> >> >> >
>> >> >> > --
>> >> >> >
>> >> >> > Andrii Tseglytskyi | Embedded Dev
>> >> >> > GlobalLogic
>> >> >> > www.globallogic.com
>> >> >> >
>> >> >>
>> >> >> _______________________________________________
>> >> >> Xen-devel mailing list
>> >> >> Xen-devel@xxxxxxxxxxxxx
>> >> >> http://lists.xen.org/xen-devel
>> >>
>> >>
>> >> Cpufreq driver implementation.
>> >> ____________
>> >> / \
>> >> | xenpm tool |
>> >> \____________/
>> >> Dom0 kernel user-space
>> >> ---------------------------------------------------------------------------
>> >>
>> >> ________________ _____
>> >> / \ / \ CPU
>> >> | DevTree Parser | /->| ARM | driver
>> >> \________________/ | \_____/
>> >> Dom0 kernel | |
>> >> -----------------------------------------------------|-----|---------------
>> >> | |
>> >> _____________________________________ | |
>> >> | __________ ___________ | | |
>> >> | / \ / \ | | |
>> >> | | ondemand | | userspace | | | |
>> >> Registered | \__________/ \___________/ | | |
>> >> cpufreq | _____________ ___________ | | |
>> >> governor | / \ / \ | | |
>> >> | | performance | | powersave | | | |
>> >> | \_____________/ \___________/ | | |
>> >> |_____________________________________| | |
>> >> ^ | |
>> >> | | |
>> >> ______|_______ | |
>> >> / \ | | Change
>> >> | cpufreq core |-------------/ | frequency
>> >> \______________/ set/get freq |
>> >> commands |
>> >> Xen |
>> >> -----------------------------------------------------------|--------------
>> >> Hardware __V__
>> >> | |
>> >> | CPU |
>> >> |_____|
>> >>
>> >>
>> >> Description of the implementation:
>> >> Cpufreq core and registered cpufreq governors are located in xen. Dom0
>> >> has CPU driver
>> >> which can only change frequency of the physical CPUs. In addition this
>> >> driver
>> >> can change CPUs regulator voltage. I'll reuse some ACPI-specific
>> >> variables for ARM.
>> >> Thus I can make minimum modification in the xen cpufreq driver and all
>> >> utilities
>> >> (as xenpm) will be working without modification if the xen code. In first
>> >> implementation xenpm tool won't show information about C-states, but it
>> >> can show
>> >> information about P-states and can change cpufreq parameters and
>> >> change governor.
>> >> DevTree parser is a part of the CPU driver in Dom0 and it will read
>> >> information
>> >> from /cpus/cpu@0/private_data path instead of the original /cpus path.
>> >>
>> >> Steps of the initialization:
>> >> 1. Xen copies all cpu@xxxxxx@N nodes (from input device tree) with
>> >> properties to
>> >> /cpus/cpu@0/private_data node (device tree for Dom0). Thus we can have
>> >> any number
>> >> of VCPUs in Dom0 and we give all information about all physical CPUs in
>> >> the private_data node.
>> >>
>> >> 2. Driver in Dom0 will parse /cpus/cpu@0/private_data path instead of the
>> >> /cpus
>> >> path and give the information about CPUs parameters to the hypervisor via
>> >> XENPF_set_processor_pminfo hypercall. (Some parameters are calculated in
>> >> the
>> >> Dom0 driver and can not be calculated in the hypervisor).
>> >>
>> >> 3. Cpufreq core driver in the hypervisor will communicate via some
>> >> interface
>> >> with Dom0 (event channel can be used to notify Dom0) and give some
>> >> commands
>> >> to the CPU driver in Dom0. Those command are set/get frequency, etc.
>> >>
>> >> Can I implement cpufreq driver in this way?
>> >
>> > The architecture looks sane to me. As Konrad pointed out, the difficulty
>> > here is to be able to upstream the changes to the Linux driver in 2),
>> > that you later in the thread identified as
>> > drivers/cpufreq/cpufreq-cpu0.c.
>> I'll write driver drivers/xen/xen-cpufreq.c and it replace original
>> drivers/cpufreq/cpufreq.c
>> And in the original cpufreq-cpu0 driver I'll chande only one string -
>> path in the device tree
>> with the settings for the CPUs opp:
>> string
>> np = of_find_node_by_path("/cpus/cpu@0");
>> will changed to:
>> np = of_find_node_by_path("/cpus/cpu@0/private_data/cpu@0");
>
> There is no way that Linux upstream is going to accept code copied
> like that. However if you refactor the code from
> drivers/cpufreq/cpufreq-cpu0.c like Konrad suggested, moving the
> functions we need to a separate file, then you can call into these
> function from both cpufreq-cpu0.c and xen-cpufreq.c.
I'll see what can I do. At first I'll write and check the cpufreq
driver and then
I'll try to upstream patches for Linux and Xen hypervisor.
>
>> > If the changes are not invasive and you manage to upstream them in
>> > Linux, I am all for this solution.
>> In Linux kernel I should make few changes:
>> 1. Enable CONFIG_CPU_FREQ_TABLE
>> with disabled CONFIG_CPU_FREQ
>> 2. Enable CONFIG_GENERIC_CPUFREQ_CPU0
>> with disabled CONFIG_CPU_FREQ
>>
>> I mean make those configs dependent on
>> CONFIG_CPU_FREQ or CONFIX_XEN_DOM0
>> instead of
>> CONFIG_CPU_FREQ
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