[PATCH] serial: tegra: Fixes lower tolerance baud rate limit for older tegra chips introduced by d781ec21bae6

[Patrik John]

The current implementation uses 0 as lower limit for the baud rate tolerance which contradicts the initial commit description (https://git.kernel.org/pub/scm/linux/kernel/git/tegra/linux.git/commit/drivers/tty/serial/serial-tegra.c?h=for-next&id=d781ec21bae6ff8f9e07682e8947a654484611f5) of +4/-4% tolerance for older tegra chips other than Tegra186 and Tegra194.
This causes issues on UART initilization as soon as the actual baud rate clock is slightly lower than required which we have seen on the Tegra124-based Toradex Apalis TK1 which also uses tegra30-hsuart as compatible in the DT serial node (for reference line 1540ff https://git.kernel.org/pub/scm/linux/kernel/git/tegra/linux.git/tree/arch/arm/boot/dts/tegra124-apalis-v1.2.dtsi?h=for-next)

The standard baud rate tolerance limits are also stated in the tegra20-hsuart driver description (https://www.kernel.org/doc/Documentation/devicetree/bindings/serial/nvidia%2Ctegra20-hsuart.txt).

The previously introduced check_rate_in_range() always fails due to the lower limit set to 0 even if the actual baud rate is within the required -4% tolerance.

static int tegra_check_rate_in_range(struct tegra_uart_port *tup)
{
    long diff;
    diff = ((long)(tup->configured_rate - tup->required_rate) * 10000)
        / tup->required_rate;
    if (diff < (tup->cdata->error_tolerance_low_range * 100) ||
        diff > (tup->cdata->error_tolerance_high_range * 100)) {
        dev_err(tup->uport.dev,
            "configured baud rate is out of range by %ld", diff);
        return -EIO;
    }
    return 0;
}

Changing the lower tolerance limit to the actual -4% resolved the issues for the Tegra124 and should resolve potential issues for other Tegra20/Tegra30 based platforms as well.

Signed-off-by: Patrik John <patrik.john@xxxxxxxxxx>
---
 drivers/tty/serial/serial-tegra.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial-tegra.c b/drivers/tty/serial/serial-tegra.c
index 45e2e4109acd..b6223fab0687 100644
--- a/drivers/tty/serial/serial-tegra.c
+++ b/drivers/tty/serial/serial-tegra.c
@@ -1506,7 +1506,7 @@ static struct tegra_uart_chip_data tegra20_uart_chip_data = {
 	.fifo_mode_enable_status	= false,
 	.uart_max_port			= 5,
 	.max_dma_burst_bytes		= 4,
-	.error_tolerance_low_range	= 0,
+	.error_tolerance_low_range	= -4,
 	.error_tolerance_high_range	= 4,
 };
 
@@ -1517,7 +1517,7 @@ static struct tegra_uart_chip_data tegra30_uart_chip_data = {
 	.fifo_mode_enable_status	= false,
 	.uart_max_port			= 5,
 	.max_dma_burst_bytes		= 4,
-	.error_tolerance_low_range	= 0,
+	.error_tolerance_low_range	= -4,
 	.error_tolerance_high_range	= 4,
 };
 
-- 
2.25.1