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+= cpufreq Device Driver Design
+
+This is a quick note on changes to cpufreq drivers.
+
+== Motivation
+
+Currently, all cpu frequency control drivers are added using identify routines.
+These routines check different characteristics of the CPU directly or using the device tree, or the ACPI properties of the CPU.
+When they can arrange for exactly one node to be added, things work well.
+x86, however, has several different cpu frequency controls, and it can be a challenge to ensure only one.
+esp and p4tcc can 'race' to see which one attaches based on a detail of how the loader sorts data sets.
+A number of hacks are in place with the different drivers to 'arbitrate' which driver to attach, many of which can be fragile due to ordering issues.
+It can also be hard to add new drivers, especially ones that may conflict with others.
+
+== High Level Design
+
+Turn the current cpufreq device which is added by other drivers into being the base-class for those drivers.
+With this, then the cpu drivers in the tree can add a "cpufreq" device, and then probe and attach that.
+The identify routines would be eliminated.
+When the "cpufreq" device is probed, all the drivers will "bid" on the device.
+When multiple drivers could handle the CPU, then one that returns the highest priority will get the device.
+Drivers would need to be modified to be a subclass, and the registration routines would need to change a bit.
+Add explicit disabling checks.
+
+== Notes
+
+We have to explicitly check in the probe routine if the device is disabled.
+Normally, we probe the device, allow that to succed, then disable the device just before we attach it.
+This works well because usually there's only one driver that will attach to a device.
+However, for devices that are subclasses, multiple drivers are bidding for that one device.
+To get disable to mean 'don't use it' as it normally would, the probe routines would need to do it.
+Perhaps we need to fix the probe and attach stuff to not 'accept' a bid from a disablfed device that's disabled, but maybe we need to accept it if it's the only choice.
+
+It's unclear if we need both `acpi_throttle` and `esp`, but it's quite likely that we don't.
+
+We have to have a design that allows other kinds of devices attached to cpu.
+We attach hw performance, temperature monitoring, etc nodes to the CPU, so we either have to work around it, or have a design that creates another psuedo bus for these devices.
+Subclassing seems simpler than an extra layer.
+
+Currently, `est_probe` does this crazy thing:
+[source]
+----
+	/*
+	 * If the ACPI perf driver has attached and is not just offering
+	 * info, let it manage things.
+	 */
+	perf_dev = device_find_child(device_get_parent(dev), "acpi_perf", -1);
+	if (perf_dev && device_is_attached(perf_dev)) {
+		error = CPUFREQ_DRV_TYPE(perf_dev, &type);
+		if (error == 0 && (type & CPUFREQ_FLAG_INFO_ONLY) == 0)
+			return (ENXIO);
+	}
+----
+So we need to move the info only stuff up into `acpi_cpu` and have `acpi_perf`
+only attach if it isn't info only.
+
+Also, `est_probe` tries to enable SpeedStep and fails to probe if it can't.
+It's unclear if this belongs in the probe, or if we should defer that to attach.
+In the `identify` world we check to see if `acpi_perf` is attached before we try.
+In the newbus priority world, though, I think we should defer this to attach and then we'll only do it if we're selected.
+This is safer, and I think should just work.
+
+`hwpstate_amd` has a comment `Only hwpstate0. It goes well with acpi_throttle.` and then enforces only one instance. what does this mean?
+Also, hwpstate_amd has interlock with acpi_perf in strange ways.
+This needs to be disentabgled.
+[source]
+----
+	/*
+	 * Check if acpi_perf has INFO only flag.
+	 */
+	perf_dev = device_find_child(device_get_parent(dev), "acpi_perf", -1);
+	error = TRUE;
+	if (perf_dev && device_is_attached(perf_dev)) {
+		error = CPUFREQ_DRV_TYPE(perf_dev, &type);
+		if (error == 0) {
+			if ((type & CPUFREQ_FLAG_INFO_ONLY) == 0) {
+				/*
+				 * If acpi_perf doesn't have INFO_ONLY flag,
+				 * it will take care of pstate transitions.
+				 */
+				HWPSTATE_DEBUG(dev, "acpi_perf will take care of pstate transitions.\n");
+				return (ENXIO);
+			} else {
+				/*
+				 * If acpi_perf has INFO_ONLY flag, (_PCT has FFixedHW)
+				 * we can get _PSS info from acpi_perf
+				 * without going into ACPI.
+				 */
+				HWPSTATE_DEBUG(dev, "going to fetch info from acpi_perf\n");
+				error = hwpstate_get_info_from_acpi_perf(dev, perf_dev);
+			}
+		}
+	}
+
+	if (error == 0) {
+		/*
+		 * Now we get _PSS info from acpi_perf without error.
+		 * Let's check it.
+		 */
+		msr = rdmsr(MSR_AMD_10H_11H_LIMIT);
+		if (sc->cfnum != 1 + AMD_10H_11H_GET_PSTATE_MAX_VAL(msr)) {
+			HWPSTATE_DEBUG(dev, "MSR (%jd) and ACPI _PSS (%d)"
+			    " count mismatch\n", (intmax_t)msr, sc->cfnum);
+			error = TRUE;
+		}
+	}
+----
+
+smist has familiar coupling with `acpi_perf`, but also `ichss`
+[source]
+----
+	/*
+	 * If the ACPI perf or ICH SpeedStep drivers have attached and not
+	 * just offering info, let them manage things.
+	 */
+	perf_dev = device_find_child(device_get_parent(dev), "acpi_perf", -1);
+	if (perf_dev && device_is_attached(perf_dev)) {
+		rv = CPUFREQ_DRV_TYPE(perf_dev, &type);
+		if (rv == 0 && (type & CPUFREQ_FLAG_INFO_ONLY) == 0)
+			return (ENXIO);
+	}
+	ichss_dev = device_find_child(device_get_parent(dev), "ichss", -1);
+	if (ichss_dev && device_is_attached(ichss_dev))
+		return (ENXIO);
+----
+
+`ichss` does a lot of filtering for very old devices, released 2002.
+Maybe we need to thin this one.
+
+`acpi_perf` probes and attaches itself in `identify`.
+Thankfully, this is disappearing, as it rightly should.
+
+So we have the following partial ordering in the code:
+* `est` > `ichss`
+* `acpi_perf` > `ichss`
+* `ichss` > `smist`
+* `acpi_perf` > `smist`
+* `acpi_perf` > `hwpstate_amd`
+* `acpi_perf` > `est`
+* `hwpstate_intel` > `est`
+* `est` > `p4tcc`
+* `p4tcc` > `acpi_throttle`
+
+.Partial Ordering Table
+|================================
+|hwpstate_intel | > est          | > p4tcc | > acpi_throttle |
+|acpi_perf      | > est          | > ichss | > smist |
+|acpi_perf      |                | > ichss | > smist |
+|acpi_perf      | > hwpstate_amd | |
+|================================
+
+.Priorities
+|================================
+|smist		| BUS_PROBE_DEFAULT - 15 |
+|acpi_throttle	| BUS_PROBE_DEFAULT - 15 |
+|ichss		| BUS_PROBE_DEFAULT - 10 |
+|p4tcc		| BUS_PROBE_DEFAULT - 10 |
+|est		| BUS_PROBE_DEFAULT - 5 |
+|hwpstate_amd	| BUS_PROBE_DEFAULT - 5 |
+|acpi_perf	| BUS_PROBE_DEFAULT |
+|hwpstate_intel	| BUS_PROBE_DEFAULT |
+|powernow       | BUS_PROBE_DEFAULT |
+|================================
+
+Should acpi_perf be higher or lower than hwpstate_intel?
+Or the same?
+Normally, when we have acpi and non-acpi we have a tie-breaker.
+
+Note:
+Changed hwpstate_intel from BUS_PROBE_NOWILDCARD, which has magic, to BUS_PROBE_DEFAULT.
+Changed smist from -1500 to BUS_PROBE_DEFAULT - 15.
+Changed ichss from -1000 to BUS_PROBE_DEFAULT - 10.
+
+cpufreq_dt returns BUS_PROBE_GENERIC.
+Should it do something else like BUS_PROBE_DEFAULT?
+Right now, with the identify will ensure it always attaches, so it's hard to know what it should defer to.
+
+acpi_perf claims a device when there's a _PCT method, but PkgGas fails...
+Should we fail the probe of the device instead?
+
+== Tasks
+Many of the checklist items are nops, but we need to audit each driver for each of these things.
+There are 18 drivers in the tree that register as cpufreq drivers.
+
+* [x] Declare cpufreq to be a subclass
+* [x] Add explicit checks to all current cpu frequency control devices for being disable
+* [*] Make all the `probe` routines accept only the conditions that `identify` accepts.
+** [*] mpc85xx_jog
+** [*] dfs
+** [*] pcr
+** [*] pmufreq
+** [*] pmcr
+** [*] tegra210_cpufreq
+** [*] acpi_throttle
+** [*] acpi_perf
+** [*] cpufreq_dt
+** [*] ichss
+** [*] smist
+** [*] hwpstate_intel
+** [*] hwpstate_amd
+** [*] powernow
+** [*] p4tcc
+** [*] est
+** [*] bcm2835_cpufreq
+** [*] tegra124_cpufreq
+* [*] Make all the `probe` routines return PROBE_BUS_DEFAULT +/- bias.
+** [*] mpc85xx_jog
+** [*] dfs
+** [*] pcr
+** [*] pmufreq
+** [*] pmcr
+** [*] tegra210_cpufreq
+** [*] acpi_throttle
+** [*] acpi_perf
+** [*] cpufreq_dt
+** [*] ichss
+** [*] smist
+** [*] hwpstate_intel
+** [*] hwpstate_amd
+** [*] powernow
+** [*] p4tcc
+** [*] est
+** [*] bcm2835_cpufreq
+** [*] tegra124_cpufreq
+* [ ] Create `cpufreq_ctor` to replace `cpufreq_register`
+* [ ] Create `cpufreq_dtor` to replace `cpufreq_unregister`
+* [ ] Migrate to subclass with `cpufreq_ctor` and `cpufreq_dtor`
+** [ ] mpc85xx_jog
+** [ ] dfs
+** [ ] pcr
+** [ ] pmufreq
+** [ ] pmcr
+** [ ] tegra210_cpufreq
+** [ ] acpi_throttle
+** [ ] acpi_perf
+** [ ] cpufreq_dt
+** [ ] ichss
+** [ ] smist
+** [ ] hwpstate_intel
+** [ ] hwpstate_amd
+** [ ] powernow
+** [ ] p4tcc
+** [ ] est
+** [ ] bcm2835_cpufreq
+** [ ] tegra124_cpufreq
+* [ ] Split out the CPUFREQ_FLAG_INFO_ONLY stuff from acpi_perf and move it into acpi_cpu.
+* [ ] Remove now-useless `identify` routine
+** [ ] mpc85xx_jog
+** [ ] dfs
+** [ ] pcr
+** [ ] pmufreq
+** [ ] pmcr
+** [ ] tegra210_cpufreq
+** [ ] acpi_throttle
+** [ ] acpi_perf
+** [ ] cpufreq_dt
+** [ ] ichss
+** [ ] smist
+** [ ] hwpstate_intel
+** [ ] hwpstate_amd
+** [ ] powernow
+** [ ] p4tcc
+** [ ] est
+** [ ] bcm2835_cpufreq
+** [ ] tegra124_cpufreq
+* [ ] Update cpu drivers to explicitly add "cpufreq" child
+** [ ] ofw_cpu
+** [ ] legacy
+** [ ] acpi
+* [ ] Remove `cpufreq_register` and `cpufreq_unregister`