Fractional divider on the Atmel USART controller

[Ludovic Desroches]

Hello Romain,

On Mon, Feb 06, 2017 at 12:56:42PM +0100, Romain Izard wrote:
> Hello,
> 
> On Atmel SAMA5D2, when trying to configure a serial port for 3 Mbauds
> operation, I do not always get the requested baud rate. If the hardware
> flow control is disabled by software, the line works correctly. But if I
> set the crtscts option, the line does not work, and after checking the
> line I can observe that the signal is sent at 2.6 Mbauds.
> 
> This is due to the code used to manage fractional baud rate divisor: the
> existing code prevents the fractional bits from being used if the line
> is not configured in normal mode. This case occurs when the hardware
> flow control or the RS485 mode is set.
> 
> If I apply the following patch to drivers/tty/serial/atmel_serial.c,
> I get the required baudrate.
> 
> 8<----------------------------------------------------------------
> 
> @@ -2204,14 +2204,13
>       * baudrate = selected clock / (8 * (2 - OVER) * (CD + FP / 8))
>       * Currently, OVER is always set to 0 so we get
>       * baudrate = selected clock / (16 * (CD + FP / 8))
>       * then
>       * 8 CD + FP = selected clock / (2 * baudrate)
>       */
> -    if (atmel_port->has_frac_baudrate &&
> -        (mode & ATMEL_US_USMODE) == ATMEL_US_USMODE_NORMAL) {
> +    if (atmel_port->has_frac_baudrate) {
>          div = DIV_ROUND_CLOSEST(port->uartclk, baud * 2);
>          cd = div >> 3;
>          fp = div & ATMEL_US_FP_MASK;
>      } else {
>          cd = uart_get_divisor(port, baud);
>      }
> 
> 8<----------------------------------------------------------------
> 
> Unfortunately, I know that this will work on SAMA5D2, but this driver is
> used for many other Atmel chips. I do not know if the existing code is
> meant to respect a known limitation on other devices that use the same
> controller, or if it is just a bug.

It depends on the device used. In the SAMA5D4 datasheet, it is
mentionned: "This feature is only available when using USART normal mode".

In the SAMA5D2 datasheet, this constraint is no longer mentionned you
confirm that is works, that is great.

> 
> Ludovic, Nicolas,  what is your opinion on that matter? Should I just
> propose this as a patch, or is it necessary to add a limitation for
> supported devices only ?

Yes, only SAMA5D2 supports fractionnal baudrate in other modes than the
normal one. A new compatible string may be added, I am not sure you have
enough information in atmel_get_ip_name...

Regards

Ludovic

> 
> Best regards,
> -- 
> Romain Izard