dt-bindings: serial: stm32: override FIFO threshold properties

Override rx-threshold and tx-threshold properties:
- extend description
- provide default and expected values

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>

Changes in v2:
Change added properties naming and factorize it in serial.yaml as proposed
by Rob Herring.

Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210413174015.23011-3-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/st,stm32-uart.yaml

@@ -65,6 +65,18 @@ properties:
   linux,rs485-enabled-at-boot-time: true
   rs485-rx-during-tx: true
 
+  rx-threshold:
+    description:
+      If value is set to 1, RX FIFO threshold is disabled.
+    enum: [1, 2, 4, 8, 12, 14, 16]
+    default: 8
+
+  tx-threshold:
+    description:
+      If value is set to 1, TX FIFO threshold is disabled.
+    enum: [1, 2, 4, 8, 12, 14, 16]
+    default: 8
+
 allOf:
   - $ref: rs485.yaml#
   - $ref: serial.yaml#
@@ -82,6 +94,17 @@ allOf:
     then:
       properties:
         rx-tx-swap: false
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - st,stm32-uart
+              - st,stm32f7-uart
+    then:
+      properties:
+        rx-threshold: false
+        tx-threshold: false
 
 required:
   - compatible
@@ -96,13 +119,15 @@ examples:
   - |
     #include <dt-bindings/clock/stm32mp1-clks.h>
     usart1: serial@40011000 {
-      compatible = "st,stm32-uart";
+      compatible = "st,stm32h7-uart";
       reg = <0x40011000 0x400>;
       interrupts = <37>;
       clocks = <&rcc 0 164>;
       dmas = <&dma2 2 4 0x414 0x0>,
              <&dma2 7 4 0x414 0x0>;
       dma-names = "rx", "tx";
+      rx-threshold = <4>;
+      tx-threshold = <4>;
       rs485-rts-active-low;
     };