[PATCH v4] dt-binding: mtd: nand: Document the cs-gpios property

[Miquel Raynal]

To reach higher capacities, arrays of chips are now pretty common.
Unfortunately, most of the controllers have been designed a decade ago
and did not all anticipate the need for several chip-selects. The new
cs-gpios property allows to workaround this limitation by adding as many
GPIO chip-select as needed.

Signed-off-by: Miquel Raynal <miquel.raynal at bootlin.com>
---

Resending only the binding patch of the series, with the following
changes requested by Rob:
* Fixed the coherency between cs-gpios and gpios-cs
* Added maxItems: 8 (this is a good enough value for most of the cases I
  guess, this can be increased later if needed).
* Adding maxItems: 8 lead to an error when checking the example,
  minItems: 8 had to be added as well to the schema to fix it, not sure
  this was expected or not.

 .../bindings/mtd/nand-controller.yaml          | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/mtd/nand-controller.yaml b/Documentation/devicetree/bindings/mtd/nand-controller.yaml
index 678b39952502..bd217e6f5018 100644
--- a/Documentation/devicetree/bindings/mtd/nand-controller.yaml
+++ b/Documentation/devicetree/bindings/mtd/nand-controller.yaml
@@ -38,6 +38,17 @@ properties:
 
   ranges: true
 
+  cs-gpios:
+    minItems: 1
+    maxItems: 8
+    description:
+      Array of chip-select available to the controller. The first
+      entries are a 1:1 mapping of the available chip-select on the
+      NAND controller (even if they are not used). As many additional
+      chip-select as needed may follow and should be phandles of GPIO
+      lines. 'reg' entries of the NAND chip subnodes become indexes of
+      this array when this property is present.
+
 patternProperties:
   "^nand@[a-f0-9]$":
     type: object
@@ -164,14 +175,19 @@ examples:
     nand-controller {
       #address-cells = <1>;
       #size-cells = <0>;
+      cs-gpios = <0>, <&gpioA 1>; /* A single native CS is available */
 
       /* controller specific properties */
 
       nand at 0 {
-        reg = <0>;
+        reg = <0>; /* Native CS */
         nand-use-soft-ecc-engine;
         nand-ecc-algo = "bch";
 
         /* controller specific properties */
       };
+
+      nand at 1 {
+        reg = <1>; /* GPIO CS */
+      };
     };
-- 
2.27.0