[ARM] OMAP3 pwrdm: add hardware save-and-restore (SAR) support

OMAP3430ES2+ introduces a new feature: optional powerdomain context
hardware save-and-restore (SAR).  Currently, this feature only applies
to USBHOST and USBTLL module context when the USBHOST or CORE
powerdomains enter a low-power sleep state[1].  This feature avoids
re-enumeration of USB devices when the powerdomains return from idle,
which is potentially time-consuming.

This patch adds support for enabling and disabling hardware
save-and-restore to the powerdomain code.  Three new functions are
added, pwrdm_enable_hdwr_sar(), pwrdm_disable_hdwr_sar(), and
pwrdm_can_hdwr_sar().  A new struct powerdomain "flags" field is
added, with a PWRDM_HAS_HDWR_SAR flag to indicate powerdomains with
SAR support.

Thanks to Jouni Högander <jouni.hogander@nokia.com> for reviewing an
earlier version of these patches, and Richard Woodruff <r-woodruff2@ti.com>
for clarifying the purpose of these bits.

1.  For the USBHOST controller module, context loss occurs when the
    USBHOST powerdomain enters off-idle.  For USBTLL, context loss
    occurs either if CORE enters off-idle, or if the CORE logic is
    configured to turn off when CORE enters retention-idle (OSWR).
    34xx ES2 TRM 4.8.6.1.1, 4.8.6.1.2

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mach-omap2/powerdomain.c

@@ -1002,6 +1002,74 @@ int pwrdm_clear_all_prev_pwrst(struct powerdomain *pwrdm)
 	return 0;
 }
 
+/**
+ * pwrdm_enable_hdwr_sar - enable automatic hardware SAR for a pwrdm
+ * @pwrdm: struct powerdomain *
+ *
+ * Enable automatic context save-and-restore upon power state change
+ * for some devices in a powerdomain.  Warning: this only affects a
+ * subset of devices in a powerdomain; check the TRM closely.  Returns
+ * -EINVAL if the powerdomain pointer is null or if the powerdomain
+ * does not support automatic save-and-restore, or returns 0 upon
+ * success.
+ */
+int pwrdm_enable_hdwr_sar(struct powerdomain *pwrdm)
+{
+	if (!pwrdm)
+		return -EINVAL;
+
+	if (!(pwrdm->flags & PWRDM_HAS_HDWR_SAR))
+		return -EINVAL;
+
+	pr_debug("powerdomain: %s: setting SAVEANDRESTORE bit\n",
+		 pwrdm->name);
+
+	prm_rmw_mod_reg_bits(0, 1 << OMAP3430ES2_SAVEANDRESTORE_SHIFT,
+			     pwrdm->prcm_offs, PM_PWSTCTRL);
+
+	return 0;
+}
+
+/**
+ * pwrdm_disable_hdwr_sar - disable automatic hardware SAR for a pwrdm
+ * @pwrdm: struct powerdomain *
+ *
+ * Disable automatic context save-and-restore upon power state change
+ * for some devices in a powerdomain.  Warning: this only affects a
+ * subset of devices in a powerdomain; check the TRM closely.  Returns
+ * -EINVAL if the powerdomain pointer is null or if the powerdomain
+ * does not support automatic save-and-restore, or returns 0 upon
+ * success.
+ */
+int pwrdm_disable_hdwr_sar(struct powerdomain *pwrdm)
+{
+	if (!pwrdm)
+		return -EINVAL;
+
+	if (!(pwrdm->flags & PWRDM_HAS_HDWR_SAR))
+		return -EINVAL;
+
+	pr_debug("powerdomain: %s: clearing SAVEANDRESTORE bit\n",
+		 pwrdm->name);
+
+	prm_rmw_mod_reg_bits(1 << OMAP3430ES2_SAVEANDRESTORE_SHIFT, 0,
+			     pwrdm->prcm_offs, PM_PWSTCTRL);
+
+	return 0;
+}
+
+/**
+ * pwrdm_has_hdwr_sar - test whether powerdomain supports hardware SAR
+ * @pwrdm: struct powerdomain *
+ *
+ * Returns 1 if powerdomain 'pwrdm' supports hardware save-and-restore
+ * for some devices, or 0 if it does not.
+ */
+bool pwrdm_has_hdwr_sar(struct powerdomain *pwrdm)
+{
+	return (pwrdm && pwrdm->flags & PWRDM_HAS_HDWR_SAR) ? 1 : 0;
+}
+
 /**
  * pwrdm_wait_transition - wait for powerdomain power transition to finish
  * @pwrdm: struct powerdomain * to wait for