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SubjectRe: [PATCH 11/14] serial: tegra: DT for Adjusted baud rates
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@nvidia.com>
> ---
> .../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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