This is a note to let you know that I've just added the patch titled serial: mvebu-uart: clarify the baud rate derivation to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: serial-mvebu-uart-clarify-the-baud-rate-derivation.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 0e4cf69ede8751d25f733cd7a6f954c5b505fa03 Mon Sep 17 00:00:00 2001 From: Miquel Raynal <miquel.raynal@xxxxxxxxxxx> Date: Fri, 23 Nov 2018 16:45:29 +0100 Subject: serial: mvebu-uart: clarify the baud rate derivation From: Miquel Raynal <miquel.raynal@xxxxxxxxxxx> commit 0e4cf69ede8751d25f733cd7a6f954c5b505fa03 upstream. The current comment in ->set_baud_rate() is rather incomplete as it fails to describe what are the actual stages for the baudrate derivation. Replace this comment with something more explicit and close to the functional specification. Also adapt the variable names to it. Signed-off-by: Miquel Raynal <miquel.raynal@xxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/tty/serial/mvebu-uart.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-) --- a/drivers/tty/serial/mvebu-uart.c +++ b/drivers/tty/serial/mvebu-uart.c @@ -72,6 +72,7 @@ #define BRDV_BAUD_MASK 0x3FF #define UART_OSAMP 0x14 +#define OSAMP_DEFAULT_DIVISOR 16 #define MVEBU_NR_UARTS 2 @@ -444,23 +445,28 @@ static void mvebu_uart_shutdown(struct u static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud) { struct mvebu_uart *mvuart = to_mvuart(port); - unsigned int baud_rate_div; + unsigned int d_divisor, m_divisor; u32 brdv; if (IS_ERR(mvuart->clk)) return -PTR_ERR(mvuart->clk); /* - * The UART clock is divided by the value of the divisor to generate - * UCLK_OUT clock, which is 16 times faster than the baudrate. - * This prescaler can achieve all standard baudrates until 230400. - * Higher baudrates could be achieved for the extended UART by using the - * programmable oversampling stack (also called fractional divisor). + * The baudrate is derived from the UART clock thanks to two divisors: + * > D ("baud generator"): can divide the clock from 2 to 2^10 - 1. + * > M ("fractional divisor"): allows a better accuracy for + * baudrates higher than 230400. + * + * As the derivation of M is rather complicated, the code sticks to its + * default value (x16) when all the prescalers are zeroed, and only + * makes use of D to configure the desired baudrate. */ - baud_rate_div = DIV_ROUND_UP(port->uartclk, baud * 16); + m_divisor = OSAMP_DEFAULT_DIVISOR; + d_divisor = DIV_ROUND_UP(port->uartclk, baud * m_divisor); + brdv = readl(port->membase + UART_BRDV); brdv &= ~BRDV_BAUD_MASK; - brdv |= baud_rate_div; + brdv |= d_divisor; writel(brdv, port->membase + UART_BRDV); return 0; Patches currently in stable-queue which might be from miquel.raynal@xxxxxxxxxxx are queue-4.19/mtd-rawnand-marvell-add-missing-clk_disable_unprepar.patch queue-4.19/serial-mvebu-uart-clarify-the-baud-rate-derivation.patch