On Wed, Jan 31, 2024 at 10:23:03AM -0800, Saravana Kannan wrote:
> On Wed, Jan 31, 2024 at 9:06 AM Rob Herring <robh@xxxxxxxxxx> wrote:
> >
> > On Fri, Jan 26, 2024 at 04:43:15PM -0800, David Dai wrote:
> > > Adding bindings to represent a virtual cpufreq device.
> > >
> > > Virtual machines may expose MMIO regions for a virtual cpufreq device
> > > for guests to read frequency information or to request frequency
> > > selection. The virtual cpufreq device has an individual controller for
> > > each frequency domain. Performance points for a given domain can be
> > > normalized across all domains for ease of allowing for virtual machines
> > > to migrate between hosts.
> > >
> > > Co-developed-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> > > Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
> > > Signed-off-by: David Dai <davidai@xxxxxxxxxx>
> > > ---
> > > .../cpufreq/qemu,cpufreq-virtual.yaml | 110 ++++++++++++++++++
> >
> > > + const: qemu,virtual-cpufreq
> >
> > Well, the filename almost matches the compatible.
> >
> > > +
> > > + reg:
> > > + maxItems: 1
> > > + description:
> > > + Address and size of region containing frequency controls for each of the
> > > + frequency domains. Regions for each frequency domain is placed
> > > + contiguously and contain registers for controlling DVFS(Dynamic Frequency
> > > + and Voltage) characteristics. The size of the region is proportional to
> > > + total number of frequency domains. This device also needs the CPUs to
> > > + list their OPPs using operating-points-v2 tables. The OPP tables for the
> > > + CPUs should use normalized "frequency" values where the OPP with the
> > > + highest performance among all the vCPUs is listed as 1024 KHz. The rest
> > > + of the frequencies of all the vCPUs should be normalized based on their
> > > + performance relative to that 1024 KHz OPP. This makes it much easier to
> > > + migrate the VM across systems which might have different physical CPU
> > > + OPPs.
> > > +
> > > +required:
> > > + - compatible
> > > + - reg
> > > +
> > > +additionalProperties: false
> > > +
> > > +examples:
> > > + - |
> > > + // This example shows a two CPU configuration with a frequency domain
> > > + // for each CPU showing normalized performance points.
> > > + cpus {
> > > + #address-cells = <1>;
> > > + #size-cells = <0>;
> > > +
> > > + cpu@0 {
> > > + compatible = "arm,armv8";
> > > + device_type = "cpu";
> > > + reg = <0x0>;
> > > + operating-points-v2 = <&opp_table0>;
> > > + };
> > > +
> > > + cpu@1 {
> > > + compatible = "arm,armv8";
> > > + device_type = "cpu";
> > > + reg = <0x0>;
> > > + operating-points-v2 = <&opp_table1>;
> > > + };
> > > + };
> > > +
> > > + opp_table0: opp-table-0 {
> > > + compatible = "operating-points-v2";
> > > +
> > > + opp64000 { opp-hz = /bits/ 64 <64000>; };
> >
> > opp-64000 is the preferred form.
> >
> > > + opp128000 { opp-hz = /bits/ 64 <128000>; };
> > > + opp192000 { opp-hz = /bits/ 64 <192000>; };
> > > + opp256000 { opp-hz = /bits/ 64 <256000>; };
> > > + opp320000 { opp-hz = /bits/ 64 <320000>; };
> > > + opp384000 { opp-hz = /bits/ 64 <384000>; };
> > > + opp425000 { opp-hz = /bits/ 64 <425000>; };
> > > + };
> > > +
> > > + opp_table1: opp-table-1 {
> > > + compatible = "operating-points-v2";
> > > +
> > > + opp64000 { opp-hz = /bits/ 64 <64000>; };
> > > + opp128000 { opp-hz = /bits/ 64 <128000>; };
> > > + opp192000 { opp-hz = /bits/ 64 <192000>; };
> > > + opp256000 { opp-hz = /bits/ 64 <256000>; };
> > > + opp320000 { opp-hz = /bits/ 64 <320000>; };
> > > + opp384000 { opp-hz = /bits/ 64 <384000>; };
> > > + opp448000 { opp-hz = /bits/ 64 <448000>; };
> > > + opp512000 { opp-hz = /bits/ 64 <512000>; };
> > > + opp576000 { opp-hz = /bits/ 64 <576000>; };
> > > + opp640000 { opp-hz = /bits/ 64 <640000>; };
> > > + opp704000 { opp-hz = /bits/ 64 <704000>; };
> > > + opp768000 { opp-hz = /bits/ 64 <768000>; };
> > > + opp832000 { opp-hz = /bits/ 64 <832000>; };
> > > + opp896000 { opp-hz = /bits/ 64 <896000>; };
> > > + opp960000 { opp-hz = /bits/ 64 <960000>; };
> > > + opp1024000 { opp-hz = /bits/ 64 <1024000>; };
> > > +
> > > + };
> >
> > I don't recall your prior versions having an OPP table. Maybe it was
> > incomplete. You are designing the "h/w" interface. Why don't you make it
> > discoverable or implicit (fixed for the h/w)?
>
> We also need the OPP tables to indicate which CPUs are part of the
> same cluster, etc. Don't want to invent a new "protocol" and just use
> existing DT bindings.
Topology binding is for that.
What about when x86 and other ACPI systems need to do this too? You
define a discoverable interface, then it works regardless of firmware.
KVM, Virtio, VFIO, etc. are all their own protocols.
> > Do you really need it if the frequency is normalized?
>
> Yeah, we can have little and big CPUs and want to emulate different
> performance levels. So while the Fmax on big is 1024, we still want to
> be able to say little is 425. So we definitely need frequency tables.
You need per CPU Fmax, sure. But all the frequencies? I don't follow why
you don't just have a max available capacity and then request the
desired capacity. Then the host maps that to an underlying OPP. Why have
an intermediate set of fake frequencies?
As these are normalized, I guess you are normalizing for capacity as
well? Or you are using "capacity-dmips-mhz"?
I'm also lost how this would work when you migrate and the underlying
CPU changes. The DT is fixed.
> > Also, we have "opp-level" for opaque values that aren't Hz.
>
> Still want to keep it Hz to be compatible with arch_freq_scale and
> when virtualized CPU perf counters are available.
Seems like no one would want "opp-level" then. Shrug.
Anyway, if Viresh and Marc are fine with all this, I'll shut up.
Rob
