Re: cpufreq implementation for OMAP under xen hypervisor.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Sep 24, 2014 at 03:36:15PM +0100, Stefano Stabellini wrote:
> On Tue, 16 Sep 2014, Oleksandr Dmytryshyn wrote:
> > On Wed, Sep 10, 2014 at 10:31 PM, Konrad Rzeszutek Wilk
> > <konrad.wilk@oracle.com> 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@citrix.com> 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@lists.xen.org
> > >> 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@0..cpu@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.
> 
> If the changes are not invasive and you manage to upstream them in
> Linux, I am all for this solution.

Looking at the driver, you could make some of the drivers functionality
be a library (all of those 'voltage-tolerance', 'clock-latency', regulator,
etc).

And then disable the cpufreq API from working altogether (disable_cpufreq())
and have your own driver (drivers/xen/xen-cpufreq-cpu0.c) use the libraries
and upload the data to the hypervisor.
(or