[PATCH 2/2] dt-bindings: mmc: Add vmmc/vqmmc for Cavium driver

[Wojciech Bartczak]

Octeon/OcteonTX MMC supports up to 3 card slots.
Each slot can support SD or MMC cards with various speed.
However, any level-shifting to accommodate different signal voltages
for the cards is done by external hardware, under control of an optional
vqmmc regulator object, typically controlled by gpio.
The details of device-tree control of MMC signals via GPIO at reset
is available in this file.

If any mmc-slots have a vqmmc-supply property,
take it as a warning that we must switch carefully between
slots (unless they have the same vqmmc object), tri-stating
MMC signals to avoid any transient states as level-shifters
are enabled/disabled, by zeroing MIO_EMM_CFG[bus_id].

There's no need to list vqmmc property if all the mmc-slots
on a board run at same signal voltage, and have same width.
In this case the old behavior, enabling all probed slots in
MIO_EMM_CFG, allows faster slot-switching.

Signed-off-by: Wojciech Bartczak <wbartczak@xxxxxxxxxxx>
---
 .../devicetree/bindings/mmc/cavium-mmc.txt    | 47 ++++++++++++++++++-
 1 file changed, 45 insertions(+), 2 deletions(-)

diff --git a/Documentation/devicetree/bindings/mmc/cavium-mmc.txt b/Documentation/devicetree/bindings/mmc/cavium-mmc.txt
index 1433e6201dff..d0b750e23332 100644
--- a/Documentation/devicetree/bindings/mmc/cavium-mmc.txt
+++ b/Documentation/devicetree/bindings/mmc/cavium-mmc.txt
@@ -28,6 +28,46 @@ Deprecated properties:
 - power-gpios : use vmmc-supply instead
 - cavium,octeon-6130-mmc-slot : use mmc-slot instead
 
+GPIO control via vmmc-supply & vqmmc-supply:
+  Two types of regulator object can be specified as mmc properties,
+  typically regulator-fixed controlled by GPIO pins.
+
+  Octeon/OcteonTX chips commonly use GPIO8 as an MMC-reset pin.
+  In systems which may boot from MMC, it starts as input, and is gently
+  pulled up/down by board logic to indicate the active sense of the
+  signal. Chip reset then drives the signal in the opposite direction
+  to effect a reset of target devices.
+  Device tree should model this with a vmmc-supply regulator, gated by
+  GPIO8, so GPIO8 is driven in the non-reset direction when MMC devices
+  are probed, and held there until rmmod/shutdown/suspend.
+  This allows a warm reboot to reset the MMC devices.
+
+  Octeon/OcteonTX MMC supports up to 3 mmc slots, but any
+  level-shifting to accommodate different signal voltages is
+  done by external hardware, under control of an optional
+  vqmmc regulator object, typically controlled by GPIO.
+
+  If any mmc-slots have a vqmmc-supply property, it is taken as a warning
+  that we must switch carefully between slots (unless they have the same
+  vqmmc object), tri-stating MMC signals to avoid any transient states
+  as level-shifters are enabled/disabled.
+
+  Even when so-called bi-directional level shifters are used,
+  this technique should be employed when using different bus-widths
+  on different slots, disabling level shifters to avoid presenting
+  non-uniform impedance across DATA0-7 & CMD when non-selected
+  4-wide slots are left enabled, while accessing 8-wide targets.
+
+  Note that it's not possible to specify multiple regulators
+  controlled by same GPIO pin, but with different active state.
+  If one GPIO line is require to switch voltage/routing between
+  different mmc-slots, specify a vqmmc-supply on one slot, but
+  not the other. The regulator_disable call on leaving that slot
+  will implicitly switch the state to support the unmarked slot.
+
+  There's no need to list vqmmc-supply if all the mmc-slots on
+  a board run at same voltage, and have same width.
+
 Examples:
 	mmc_1_4: mmc@1,4 {
 		compatible = "cavium,thunder-8390-mmc";
@@ -40,7 +80,8 @@ Examples:
 			compatible = "mmc-slot";
 			reg = <0>;
 			vmmc-supply = <&mmc_supply_3v3>;
-			max-frequency = <42000000>;
+			vqmmc-supply = <&vqmmc_3v3>;
+			max-frequency = <52000000>;
 			bus-width = <4>;
 			cap-sd-highspeed;
 		};
@@ -49,9 +90,11 @@ Examples:
 			compatible = "mmc-slot";
 			reg = <1>;
 			vmmc-supply = <&mmc_supply_3v3>;
-			max-frequency = <42000000>;
+			vqmmc-supply = <&vqmmc_1v8>;
+			max-frequency = <100000000>;
 			bus-width = <8>;
 			cap-mmc-highspeed;
 			non-removable;
 		};
 	};
+
-- 
2.17.1