linux-kernel - Fractional divider on the Atmel USART controller

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Date:   Mon, 6 Feb 2017 12:56:42 +0100
From:   Romain Izard <romain.izard.pro@...il.com>
To:     linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Ludovic Desroches <ludovic.desroches@...el.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>
Cc:     Richard Genoud <richard.genoud@...il.com>
Subject: Fractional divider on the Atmel USART controller

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.

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 ?

Best regards,
-- 
Romain Izard