Author: nwhitehorn
Date: Tue Jun 23 04:28:32 2009
New Revision: 194679
URL: http://svn.freebsd.org/changeset/base/194679

Log:
  Add cpufreq support on the PowerPC G5, along with a skeleton SMU driver
  in order to slew CPU voltage during frequency changes. The OpenBSD SMU
  driver was an extremely helpful reference for this.

Added:
  head/sys/powerpc/cpufreq/pcr.c   (contents, props changed)
  head/sys/powerpc/powermac/smu.c   (contents, props changed)
Modified:
  head/sys/conf/files.powerpc
  head/sys/powerpc/conf/GENERIC
  head/sys/powerpc/conf/NOTES

Modified: head/sys/conf/files.powerpc
==============================================================================
--- head/sys/conf/files.powerpc Tue Jun 23 04:02:36 2009        (r194678)
+++ head/sys/conf/files.powerpc Tue Jun 23 04:28:32 2009        (r194679)
@@ -99,6 +99,7 @@ powerpc/booke/swtch.S         optional        e500
 powerpc/booke/trap.c           optional        e500
 powerpc/booke/vm_machdep.c     optional        e500
 powerpc/cpufreq/dfs.c          optional        cpufreq
+powerpc/cpufreq/pcr.c          optional        cpufreq aim
 powerpc/fpu/fpu_add.c          optional        fpu_emu
 powerpc/fpu/fpu_compare.c      optional        fpu_emu
 powerpc/fpu/fpu_div.c          optional        fpu_emu
@@ -127,18 +128,19 @@ powerpc/ofw/ofw_syscons.c optional        sc ai
 powerpc/powermac/ata_kauai.c   optional        powermac ata | powermac atamacio
 powerpc/powermac/ata_macio.c   optional        powermac ata | powermac atamacio
 powerpc/powermac/ata_dbdma.c   optional        powermac ata | powermac atamacio
+powerpc/powermac/cuda.c                optional        powermac cuda
+powerpc/powermac/cpcht.c       optional        powermac pci
 powerpc/powermac/dbdma.c       optional        powermac pci
 powerpc/powermac/grackle.c     optional        powermac pci
 powerpc/powermac/hrowpic.c     optional        powermac pci
 powerpc/powermac/kiic.c                optional        powermac kiic
+powerpc/powermac/macgpio.c     optional        powermac pci 
 powerpc/powermac/macio.c       optional        powermac pci
 powerpc/powermac/openpic_macio.c optional      powermac pci
 powerpc/powermac/pswitch.c     optional        powermac pswitch
-powerpc/powermac/uninorth.c    optional        powermac pci
-powerpc/powermac/cuda.c                optional        powermac cuda
 powerpc/powermac/pmu.c         optional        powermac pmu 
-powerpc/powermac/macgpio.c     optional        powermac pci 
-powerpc/powermac/cpcht.c       optional        powermac pci
+powerpc/powermac/smu.c         optional        powermac smu 
+powerpc/powermac/uninorth.c    optional        powermac pci
 powerpc/powermac/vcoregpio.c   optional        powermac 
 powerpc/powerpc/altivec.c      optional        aim
 powerpc/powerpc/atomic.S       standard

Modified: head/sys/powerpc/conf/GENERIC
==============================================================================
--- head/sys/powerpc/conf/GENERIC       Tue Jun 23 04:02:36 2009        
(r194678)
+++ head/sys/powerpc/conf/GENERIC       Tue Jun 23 04:28:32 2009        
(r194679)
@@ -158,6 +158,7 @@ device              fwe             # Ethernet over 
FireWire (n
 
 # Misc
 device         powermac_nvram  # Open Firmware configuration NVRAM
+device         smu             # Apple System Management Unit
 
 # ADB support
 device         adb

Modified: head/sys/powerpc/conf/NOTES
==============================================================================
--- head/sys/powerpc/conf/NOTES Tue Jun 23 04:02:36 2009        (r194678)
+++ head/sys/powerpc/conf/NOTES Tue Jun 23 04:28:32 2009        (r194679)
@@ -33,6 +33,7 @@ device                ofwd            # Open Firmware disks
 device         adb             # Apple Desktop Bus
 device         cuda            # VIA-CUDA ADB interface
 device         pmu             # Apple Power Management Unit
+device         smu             # Apple System Management Unit
 device         snd_ai2s        # Apple I2S Audio
 device         snd_davbus      # Apple Davbus Audio
 

Added: head/sys/powerpc/cpufreq/pcr.c
==============================================================================
--- /dev/null   00:00:00 1970   (empty, because file is newly added)
+++ head/sys/powerpc/cpufreq/pcr.c      Tue Jun 23 04:28:32 2009        
(r194679)
@@ -0,0 +1,333 @@
+/*-
+ * Copyright (c) 2009 Nathan Whitehorn
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/param.h>
+#include <sys/systm.h>
+#include <sys/bus.h>
+#include <sys/cpu.h>
+#include <sys/kernel.h>
+#include <sys/module.h>
+
+#include <dev/ofw/ofw_bus.h>
+
+#include "cpufreq_if.h"
+
+struct pcr_softc {
+       device_t dev;
+       uint32_t pcr_vals[3];
+       int nmodes;
+};
+
+static void    pcr_identify(driver_t *driver, device_t parent);
+static int     pcr_probe(device_t dev);
+static int     pcr_attach(device_t dev);
+static int     pcr_settings(device_t dev, struct cf_setting *sets, int *count);
+static int     pcr_set(device_t dev, const struct cf_setting *set);
+static int     pcr_get(device_t dev, struct cf_setting *set);
+static int     pcr_type(device_t dev, int *type);
+
+static device_method_t pcr_methods[] = {
+       /* Device interface */
+       DEVMETHOD(device_identify,      pcr_identify),
+       DEVMETHOD(device_probe,         pcr_probe),
+       DEVMETHOD(device_attach,        pcr_attach),
+
+       /* cpufreq interface */
+       DEVMETHOD(cpufreq_drv_set,      pcr_set),
+       DEVMETHOD(cpufreq_drv_get,      pcr_get),
+       DEVMETHOD(cpufreq_drv_type,     pcr_type),
+       DEVMETHOD(cpufreq_drv_settings, pcr_settings),
+
+       {0, 0}
+};
+
+static driver_t pcr_driver = {
+       "pcr",
+       pcr_methods,
+       sizeof(struct pcr_softc)
+};
+
+static devclass_t pcr_devclass;
+DRIVER_MODULE(pcr, cpu, pcr_driver, pcr_devclass, 0, 0);
+
+/*
+ * States
+ */
+
+#define PCR_TO_FREQ(a) ((a >> 17) & 3)
+
+#define        PCR_FULL        0
+#define PCR_HALF       1
+#define PCR_QUARTER    2               /* Only on 970MP */
+
+#define PSR_RECEIVED   (1ULL << 61)
+#define PSR_COMPLETED  (1ULL << 61)
+
+/*
+ * SCOM addresses
+ */
+
+#define        SCOM_PCR        0x0aa00100      /* Power Control Register */
+#define SCOM_PCR_BIT   0x80000000      /* Data bit for PCR */
+#define SCOM_PSR       0x40800100      /* Power Status Register */
+
+/*
+ * SCOM Glue
+ */
+
+#define SCOMC_READ     0x00008000
+#define SCOMC_WRITE    0x00000000 
+
+static void
+write_scom(register_t address, uint64_t value)
+{
+       register_t msr;
+       register_t hi, lo, scratch;
+
+       hi = (value >> 32) & 0xffffffff;
+       lo = value & 0xffffffff;
+
+       msr = mfmsr();
+       mtmsr(msr & ~PSL_EE); isync();
+
+       mtspr64(SPR_SCOMD, hi, lo, scratch); 
+       isync();
+       mtspr(SPR_SCOMC, address | SCOMC_WRITE);
+       isync();
+
+       mtmsr(msr); isync();
+}
+
+static uint64_t
+read_scom(register_t address)
+{
+       register_t msr;
+       uint64_t ret;
+
+       msr = mfmsr();
+       mtmsr(msr & ~PSL_EE); isync();
+
+       mtspr(SPR_SCOMC, address | SCOMC_READ);
+       isync();
+
+       __asm __volatile ("mfspr %0,%1;"
+            " mr %0+1, %0; srdi %0,%0,32" : "=r" (ret) : "K" (SPR_SCOMD));
+
+       (void)mfspr(SPR_SCOMC); /* Complete transcation */
+
+       mtmsr(msr); isync();
+
+       return (ret);
+}
+
+static void
+pcr_identify(driver_t *driver, device_t parent)
+{
+       uint16_t vers;
+       vers = mfpvr() >> 16;
+
+       /* Check for an IBM 970-class CPU */
+       switch (vers) {
+               case IBM970FX:
+               case IBM970GX:
+               case IBM970MP:
+                       break;
+               default:
+                       return;
+       }
+
+       /* Make sure we're not being doubly invoked. */
+       if (device_find_child(parent, "pcr", -1) != NULL)
+               return;
+
+       /*
+        * We attach a child for every CPU since settings need to
+        * be performed on every CPU in the SMP case.
+        */
+       if (BUS_ADD_CHILD(parent, 10, "pcr", -1) == NULL)
+               device_printf(parent, "add pcr child failed\n");
+}
+
+static int
+pcr_probe(device_t dev)
+{
+       if (resource_disabled("pcr", 0))
+               return (ENXIO);
+
+       device_set_desc(dev, "PPC 970 Power Control Register");
+       return (0);
+}
+
+static int
+pcr_attach(device_t dev)
+{
+       struct pcr_softc *sc;
+       phandle_t cpu;
+       uint32_t modes[3];
+       int i;
+
+       sc = device_get_softc(dev);
+       sc->dev = dev;
+
+       cpu = ofw_bus_get_node(device_get_parent(dev));
+
+       if (cpu <= 0) {
+               device_printf(dev,"No CPU device tree node!\n");
+               return (ENXIO);
+       }
+
+       /*
+        * Collect the PCR values for each mode from the device tree.
+        * These include bus timing information, and so cannot be
+        * directly computed.
+        */
+       sc->nmodes = OF_getproplen(cpu, "power-mode-data");
+       if (sc->nmodes <= 0 || sc->nmodes > sizeof(sc->pcr_vals)) {
+               device_printf(dev,"No power mode data in device tree!\n");
+               return (ENXIO);
+       }
+       OF_getprop(cpu, "power-mode-data", modes, sc->nmodes);
+       sc->nmodes /= sizeof(modes[0]);
+
+       /* Sort the modes */
+       for (i = 0; i < sc->nmodes; i++)
+               sc->pcr_vals[PCR_TO_FREQ(modes[i])] = modes[i];
+
+       cpufreq_register(dev);
+       return (0);
+}
+
+static int
+pcr_settings(device_t dev, struct cf_setting *sets, int *count)
+{
+       struct pcr_softc *sc;
+
+       sc = device_get_softc(dev);
+       if (sets == NULL || count == NULL)
+               return (EINVAL);
+       if (*count < sc->nmodes)
+               return (E2BIG);
+
+       /* Return a list of valid settings for this driver. */
+       memset(sets, CPUFREQ_VAL_UNKNOWN, sizeof(*sets) * sc->nmodes);
+
+       sets[0].freq = 10000; sets[0].dev = dev;
+       sets[1].freq = 5000; sets[1].dev = dev;
+       if (sc->nmodes > 2)
+               sets[2].freq = 2500; sets[2].dev = dev;
+       *count = sc->nmodes;
+
+       return (0);
+}
+
+static int
+pcr_set(device_t dev, const struct cf_setting *set)
+{
+       struct pcr_softc *sc;
+       register_t pcr, msr;
+       uint64_t psr;
+       
+       if (set == NULL)
+               return (EINVAL);
+       sc = device_get_softc(dev);
+
+       /* Construct the new PCR */
+
+       pcr = SCOM_PCR_BIT;
+
+       if (set->freq == 10000)
+               pcr |= sc->pcr_vals[0];
+       else if (set->freq == 5000)
+               pcr |= sc->pcr_vals[1];
+       else if (set->freq == 2500)
+               pcr |= sc->pcr_vals[2];
+
+       msr = mfmsr(); 
+       mtmsr(msr & ~PSL_EE); isync();
+
+       /* 970MP requires PCR and PCRH to be cleared first */
+
+       write_scom(SCOM_PCR,0);                 /* Clear PCRH */
+       write_scom(SCOM_PCR,SCOM_PCR_BIT);      /* Clear PCR */
+
+       /* Set PCR */
+
+       write_scom(SCOM_PCR, pcr);
+
+       /* Wait for completion */
+
+       do {
+               DELAY(100);
+               psr = read_scom(SCOM_PSR);
+       } while ((psr & PSR_RECEIVED) && !(psr & PSR_COMPLETED));
+
+       mtmsr(msr); isync();
+
+       return (0);
+}
+
+static int
+pcr_get(device_t dev, struct cf_setting *set)
+{
+       struct pcr_softc *sc;
+       uint64_t psr;
+
+       if (set == NULL)
+               return (EINVAL);
+       sc = device_get_softc(dev);
+
+       memset(set, CPUFREQ_VAL_UNKNOWN, sizeof(*set));
+
+       psr = read_scom(SCOM_PSR);
+
+       /* We want bits 6 and 7 */
+       psr = (psr >> 56) & 3;
+
+       set->freq = 10000;
+       if (psr == PCR_HALF)
+               set->freq = 5000;
+       else if (psr == PCR_QUARTER)
+               set->freq = 2500;
+
+       set->dev = dev;
+
+       return (0);
+}
+
+static int
+pcr_type(device_t dev, int *type)
+{
+
+       if (type == NULL)
+               return (EINVAL);
+
+       *type = CPUFREQ_TYPE_RELATIVE;
+       return (0);
+}
+

Added: head/sys/powerpc/powermac/smu.c
==============================================================================
--- /dev/null   00:00:00 1970   (empty, because file is newly added)
+++ head/sys/powerpc/powermac/smu.c     Tue Jun 23 04:28:32 2009        
(r194679)
@@ -0,0 +1,288 @@
+/*-
+ * Copyright (c) 2009 Nathan Whitehorn
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/param.h>
+#include <sys/bus.h>
+#include <sys/systm.h>
+#include <sys/module.h>
+#include <sys/conf.h>
+#include <sys/cpu.h>
+#include <sys/kernel.h>
+#include <sys/rman.h>
+#include <sys/sysctl.h>
+
+#include <machine/bus.h>
+#include <machine/md_var.h>
+
+#include <dev/ofw/openfirm.h>
+#include <dev/ofw/ofw_bus.h>
+#include <powerpc/powermac/macgpiovar.h>
+
+struct smu_cmd {
+       uint8_t         cmd;
+       uint8_t         len;
+       uint8_t         data[254];
+};
+
+struct smu_softc {
+       device_t        sc_dev;
+       struct mtx      sc_mtx;
+
+       struct resource *sc_memr;
+       int             sc_memrid;
+
+       bus_dma_tag_t   sc_dmatag;
+       bus_space_tag_t sc_bt;
+       bus_space_handle_t sc_mailbox;
+
+       struct smu_cmd  *sc_cmd;
+       bus_addr_t      sc_cmd_phys;
+       bus_dmamap_t    sc_cmd_dmamap;
+};
+
+/* regular bus attachment functions */
+
+static int     smu_probe(device_t);
+static int     smu_attach(device_t);
+
+/* cpufreq notification hooks */
+
+static void    smu_cpufreq_pre_change(device_t, const struct cf_level *level);
+static void    smu_cpufreq_post_change(device_t, const struct cf_level *level);
+
+/* where to find the doorbell GPIO */
+
+static device_t        smu_doorbell = NULL;
+
+static device_method_t  smu_methods[] = {
+       /* Device interface */
+       DEVMETHOD(device_probe,         smu_probe),
+       DEVMETHOD(device_attach,        smu_attach),
+       { 0, 0 },
+};
+
+static driver_t smu_driver = {
+       "smu",
+       smu_methods,
+       sizeof(struct smu_softc)
+};
+
+static devclass_t smu_devclass;
+
+DRIVER_MODULE(smu, nexus, smu_driver, smu_devclass, 0, 0);
+
+#define SMU_MAILBOX    0x860c
+
+/* Command types */
+#define SMU_POWER      0xaa
+
+static int
+smu_probe(device_t dev)
+{
+       const char *name = ofw_bus_get_name(dev);
+
+       if (strcmp(name, "smu") != 0)
+               return (ENXIO);
+
+       device_set_desc(dev, "Apple System Management Unit");
+       return (0);
+}
+
+static void
+smu_phys_callback(void *xsc, bus_dma_segment_t *segs, int nsegs, int error)
+{
+       struct smu_softc *sc = xsc;
+
+       sc->sc_cmd_phys = segs[0].ds_addr;
+}
+
+static int
+smu_attach(device_t dev)
+{
+       struct smu_softc *sc;
+
+       sc = device_get_softc(dev);
+
+       mtx_init(&sc->sc_mtx, "smu", NULL, MTX_DEF);
+
+       /*
+        * Map the mailbox area. This should be determined from firmware,
+        * but I have not found a simple way to do that.
+        */
+       bus_dma_tag_create(NULL, 16, 0, BUS_SPACE_MAXADDR_32BIT,
+           BUS_SPACE_MAXADDR, NULL, NULL, PAGE_SIZE, 1, PAGE_SIZE, 0, NULL,
+           NULL, &(sc->sc_dmatag));
+       sc->sc_bt = &bs_be_tag;
+       bus_space_map(sc->sc_bt, SMU_MAILBOX, 4, 0, &sc->sc_mailbox);
+
+       /*
+        * Allocate the command buffer. This can be anywhere in the low 4 GB
+        * of memory.
+        */
+       bus_dmamem_alloc(sc->sc_dmatag, (void **)&sc->sc_cmd, BUS_DMA_WAITOK | 
+           BUS_DMA_ZERO, &sc->sc_cmd_dmamap);
+       bus_dmamap_load(sc->sc_dmatag, sc->sc_cmd_dmamap,
+           sc->sc_cmd, PAGE_SIZE, smu_phys_callback, sc, 0);
+
+       /*
+        * Set up handlers to change CPU voltage when CPU frequency is changed.
+        */
+       EVENTHANDLER_REGISTER(cpufreq_pre_change, smu_cpufreq_pre_change, dev,
+           EVENTHANDLER_PRI_ANY);
+       EVENTHANDLER_REGISTER(cpufreq_post_change, smu_cpufreq_post_change, dev,
+           EVENTHANDLER_PRI_ANY);
+
+       return (0);
+}
+
+static int
+smu_run_cmd(device_t dev, struct smu_cmd *cmd)
+{
+       struct smu_softc *sc;
+       int doorbell_ack, result;
+
+       sc = device_get_softc(dev);
+
+       mtx_lock(&sc->sc_mtx);
+
+       /* Copy the command to the mailbox */
+       memcpy(sc->sc_cmd, cmd, sizeof(*cmd));
+       bus_dmamap_sync(sc->sc_dmatag, sc->sc_cmd_dmamap, BUS_DMASYNC_PREWRITE);
+       bus_space_write_4(sc->sc_bt, sc->sc_mailbox, 0, sc->sc_cmd_phys);
+
+       /* Invalidate the cacheline it is in -- SMU bypasses the cache */
+       __asm __volatile("dcbst 0,%0; sync" :: "r"(sc->sc_cmd): "memory");
+
+       /* Ring SMU doorbell */
+       macgpio_write(smu_doorbell, GPIO_DDR_OUTPUT);
+
+       /* Wait for the doorbell GPIO to go high, signaling completion */
+       do {
+               /* XXX: timeout */
+               DELAY(50);
+               doorbell_ack = macgpio_read(smu_doorbell);
+       } while (!doorbell_ack);
+
+       /* Check result. First invalidate the cache again... */
+       __asm __volatile("dcbf 0,%0; sync" :: "r"(sc->sc_cmd) : "memory");
+       
+       bus_dmamap_sync(sc->sc_dmatag, sc->sc_cmd_dmamap, BUS_DMASYNC_POSTREAD);
+
+       /* SMU acks the command by inverting the command bits */
+       if (sc->sc_cmd->cmd == ~cmd->cmd)
+               result = 0;
+       else
+               result = EIO;
+
+       mtx_unlock(&sc->sc_mtx);
+
+       return (result);
+}
+
+static void
+smu_slew_cpu_voltage(device_t dev, int to)
+{
+       struct smu_cmd cmd;
+
+       cmd.cmd = SMU_POWER;
+       cmd.len = 8;
+       cmd.data[0] = 'V';
+       cmd.data[1] = 'S'; 
+       cmd.data[2] = 'L'; 
+       cmd.data[3] = 'E'; 
+       cmd.data[4] = 'W'; 
+       cmd.data[5] = 0xff;
+       cmd.data[6] = 1;
+       cmd.data[7] = to;
+
+       smu_run_cmd(dev, &cmd);
+}
+
+static void
+smu_cpufreq_pre_change(device_t dev, const struct cf_level *level)
+{
+       /*
+        * Make sure the CPU voltage is raised before we raise
+        * the clock.
+        */
+               
+       if (level->rel_set[0].freq == 10000 /* max */)
+               smu_slew_cpu_voltage(dev, 0);
+}
+
+static void
+smu_cpufreq_post_change(device_t dev, const struct cf_level *level)
+{
+       /* We are safe to reduce CPU voltage after a downward transition */
+
+       if (level->rel_set[0].freq < 10000 /* max */)
+               smu_slew_cpu_voltage(dev, 1); /* XXX: 1/4 voltage for 970MP? */
+}
+
+/* Routines for probing the SMU doorbell GPIO */
+static int doorbell_probe(device_t dev);
+static int doorbell_attach(device_t dev);
+
+static device_method_t  doorbell_methods[] = {
+       /* Device interface */
+       DEVMETHOD(device_probe,         doorbell_probe),
+       DEVMETHOD(device_attach,        doorbell_attach),
+       { 0, 0 },
+};
+
+static driver_t doorbell_driver = {
+       "smudoorbell",
+       doorbell_methods,
+       0
+};
+
+static devclass_t doorbell_devclass;
+
+DRIVER_MODULE(smudoorbell, macgpio, doorbell_driver, doorbell_devclass, 0, 0);
+
+static int
+doorbell_probe(device_t dev)
+{
+       const char *name = ofw_bus_get_name(dev);
+
+       if (strcmp(name, "smu-doorbell") != 0)
+               return (ENXIO);
+
+       device_set_desc(dev, "SMU Doorbell GPIO");
+       device_quiet(dev);
+       return (0);
+}
+
+static int
+doorbell_attach(device_t dev)
+{
+       smu_doorbell = dev;
+       return (0);
+}
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