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Subject[PATCH 2/2] dt-bindings: mmc: Add vmmc/vqmmc for Cavium driver
Date
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@marvell.com>
---
.../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
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