Re: [PATCH 11/14] serial: tegra: DT for Adjusted baud rates

[Thierry Reding]

On Mon, Aug 12, 2019 at 04:58:20PM +0530, Krishna Yarlagadda wrote:
> 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 <kyarlagadda@xxxxxxxxxx>
> ---
>  .../bindings/serial/nvidia,tegra20-hsuart.txt      | 32 ++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/serial/nvidia,tegra20-hsuart.txt b/Documentation/devicetree/bindings/serial/nvidia,tegra20-hsuart.txt
> index 187ec78..1ce3fd4 100644
> --- a/Documentation/devicetree/bindings/serial/nvidia,tegra20-hsuart.txt
> +++ b/Documentation/devicetree/bindings/serial/nvidia,tegra20-hsuart.txt
> @@ -20,6 +20,37 @@ 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.

The above sounds like the tolerance deviation is a characteristic of the
Tegra186 chip. If the board design does not influence the deviation, why
can't we encode this in the driver? Why do we need a description of this
in device tree?

Thierry

> +
> +  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:
>  
> @@ -34,4 +65,5 @@ serial@70006000 {
>  	reset-names = "serial";
>  	dmas = <&apbdma 8>, <&apbdma 8>;
>  	dma-names = "rx", "tx";
> +	nvidia,adjust-baud-rates = <1000000 4000000 136>; /* 1.36% shift */
>  };
> -- 
> 2.7.4
>

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