dt-bindings: wkup-m3-ipc: Add ti,set-io-isolation property

Add documentation for the ti,set-io-isolation DT property on the wkup_m3_ipc
node which tells the wkup_m3_ipc driver to use the wkup_m3 to enable
IO Isolation during low power mode transitions on am43xx platforms.
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
[dfustini: convert to YAML, make DTS example that passes check]
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220414192722.2978837-2-dfustini@baylibre.com

Documentation/devicetree/bindings/soc/ti/wkup-m3-ipc.yaml

@@ -24,14 +24,22 @@ description: |+
   A wkup_m3_ipc device node is used to represent the IPC registers within an
   SoC.
 
-  Support for VTT Toggle
-  ==================================
+  Support for VTT Toggle with GPIO pin
+  ====================================
   On some boards like the AM335x EVM-SK and the AM437x GP EVM, a GPIO pin is
   connected to the enable pin on the DDR VTT regulator. This allows the
   regulator to be disabled upon suspend and enabled upon resume. Please note
   that the GPIO pin must be part of the GPIO0 module as only this GPIO module
   is in the wakeup power domain.
 
+  Support for IO Isolation
+  ========================
+  On AM437x SoCs, certain pins can be forced into an alternate state when IO
+  isolation is activated. Those pins have pad control registers prefixed by
+  'CTRL_CONF_' that contain DS0 (e.g. deep sleep) configuration bits that can
+  override the pin's existing bias (pull-up/pull-down) and value (high/low) when
+  IO isolation is active.
+
 properties:
   compatible:
     enum:
@@ -63,6 +71,25 @@ properties:
     $ref: /schemas/types.yaml#/definitions/uint32
     description: GPIO pin connected to enable pin on VTT regulator
 
+  ti,set-io-isolation:
+    type: boolean
+    description:
+      If this property is present, then the wkup_m3_ipc driver will instruct
+      the CM3 firmware to activate IO isolation when suspending to deep sleep.
+      This can be leveraged by a board design to put other devices on the board
+      into a low power state.
+
+allOf:
+  - if:
+      properties:
+        compatible:
+          not:
+            contains:
+              const: ti,am4372-wkup-m3-ipc
+    then:
+      properties:
+        ti,set-io-isolation: false
+
 required:
   - compatible
   - reg
@@ -74,21 +101,63 @@ additionalProperties: false
 
 examples:
   - |
+    /* Example for AM335x SoC */
     soc {
         #address-cells = <1>;
         #size-cells = <1>;
 
-        mailbox: mailbox {
+        am335x_mailbox: mailbox {
             #mbox-cells = <1>;
         };
 
-        wkup_m3_ipc: wkup_m3_ipc@1324 {
+        wkup_m3_ipc@1324 {
            compatible = "ti,am3352-wkup-m3-ipc";
            reg = <0x1324 0x24>;
            interrupts = <78>;
            ti,rproc = <&wkup_m3>;
-           mboxes = <&mailbox &mbox_wkupm3>;
+           mboxes = <&am335x_mailbox &mbox_wkupm3>;
            ti,vtt-gpio-pin = <7>;
         };
     };
+
+  - |
+    /*
+     * Example for AM473x SoC:
+     * On the AM437x-GP-EVM board, gpio5_7 is wired to enable pin of the DDR VTT
+     * regulator. The 'ddr_vtt_toggle_default' pinmux node configures gpio5_7
+     * for pull-up during normal system operation. However, the DS0 (deep sleep)
+     * state of the pin is configured for pull-down and thus the VTT regulator
+     * will be disabled to save power when IO isolation is active. Note that
+     * this method is an alternative to using the 'ti,vtt-gpio-pin' property.
+     */
+    #include <dt-bindings/pinctrl/am43xx.h>
+    soc {
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        am437x_mailbox: mailbox {
+            #mbox-cells = <1>;
+        };
+
+        am43xx_pinmux {
+            pinctrl-names = "default";
+            pinctrl-0 = <&ddr3_vtt_toggle_default>;
+
+            ddr3_vtt_toggle_default: ddr_vtt_toggle_default {
+                 pinctrl-single,pins = <
+                    0x25C (DS0_PULL_UP_DOWN_EN | PIN_OUTPUT_PULLUP | DS0_FORCE_OFF_MODE | MUX_MODE7)
+                 >;
+            };
+        };
+
+        wkup_m3_ipc@1324 {
+           compatible = "ti,am4372-wkup-m3-ipc";
+           reg = <0x1324 0x24>;
+           interrupts = <78>;
+           ti,rproc = <&wkup_m3>;
+           mboxes = <&am437x_mailbox &mbox_wkupm3>;
+           ti,set-io-isolation;
+        };
+    };
+
 ...