serial: tegra: DT for Adjusted baud rates

Tegra186 chip has a hardware issue resulting in frame errors when
tolerance level for baud rate is negative. Provided entries to adjust
baud rate to be within acceptable range and work with devices that
can send negative baud rate. Also report error when baud rate set is
out of tolerance range of controller updated in device tree.

Signed-off-by: Krishna Yarlagadda <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Documentation/devicetree/bindings/serial/nvidia,tegra20-hsuart.txt

@@ -24,6 +24,37 @@ Required properties:
 Optional properties:
 - nvidia,enable-modem-interrupt: Enable modem interrupts. Should be enable
 		only if all 8 lines of UART controller are pinmuxed.
+- nvidia,adjust-baud-rates: List of entries providing percentage of baud rate
+  adjustment within a range.
+  Each entry contains sets of 3 values. Range low/high and adjusted rate.
+  <range_low range_high adjusted_rate>
+  When baud rate set on controller falls within the range mentioned in this
+  field, baud rate will be adjusted by percentage mentioned here.
+  Ex: <9600 115200 200>
+  Increase baud rate by 2% when set baud rate falls within range 9600 to 115200
+
+Baud Rate tolerance:
+  Standard UART devices are expected to have tolerance for baud rate error by
+  -4 to +4 %. All Tegra devices till Tegra210 had this support. However,
+  Tegra186 chip has a known hardware issue. UART Rx baud rate tolerance level
+  is 0% to +4% in 1-stop config. Otherwise, the received data will have
+  corruption/invalid framing errors. Parker errata suggests adjusting baud
+  rate to be higher than the deviations observed in Tx.
+
+  Tx deviation of connected device can be captured over scope (or noted from
+  its spec) for valid range and Tegra baud rate has to be set above actual
+  Tx baud rate observed. To do this we use nvidia,adjust-baud-rates
+
+  As an example, consider there is deviation observed in Tx for baud rates as
+  listed below.
+  0 to 9600 has 1% deviation
+  9600 to 115200 2% deviation
+  This slight deviation is expcted and Tegra UART is expected to handle it. Due
+  to the issue stated above, baud rate on Tegra UART should be set equal to or
+  above deviation observed for avoiding frame errors.
+  Property should be set like this
+  nvidia,adjust-baud-rates = <0 9600 100>,
+  			     <9600 115200 200>;
 
 Example:
 
@@ -38,4 +69,5 @@ serial@70006000 {
 	reset-names = "serial";
 	dmas = <&apbdma 8>, <&apbdma 8>;
 	dma-names = "rx", "tx";
+	nvidia,adjust-baud-rates = <1000000 4000000 136>; /* 1.36% shift */
 };