Hi Andy,
On 03/11/2015 04:19 PM, Andy Shevchenko wrote:
> The fractional divider is used to provide different frequencies from one
> source. The following formulas are used to show the relations between Fref,
> Fuart, prescaler, nominator, and denominator values:
>
> Fuart = Fref * m / n
> baud = Fuart / (prescaler * DLAB)
>
> where prescaler value is in range 1..16.
>
> The helper function returns prescaler, nominator, and denominator values. It
> also sets port UART clock to the corresponding value. Since serial core
> considers base Fuart = baud * 16, the given value of Fuart will be in
> proportion to that:
>
> Port UART clock = Fuart * 16 / prescaler
>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
> drivers/tty/serial/8250/8250.h | 4 +++
> drivers/tty/serial/8250/8250_core.c | 54 +++++++++++++++++++++++++++++++++++++
> 2 files changed, 58 insertions(+)
>
> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
> index 656ecc6..feed6a3 100644
> --- a/drivers/tty/serial/8250/8250.h
> +++ b/drivers/tty/serial/8250/8250.h
> @@ -198,3 +198,7 @@ static inline int serial8250_request_dma(struct uart_8250_port *p)
> }
> static inline void serial8250_release_dma(struct uart_8250_port *p) { }
> #endif
> +
> +unsigned short serial8250_get_ps(struct uart_port *port, unsigned int baud,
> + unsigned int fref, unsigned short width,
> + unsigned int *m, unsigned int *n);
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 7b502c0..09803f5 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -27,6 +27,7 @@
> #include <linux/console.h>
> #include <linux/sysrq.h>
> #include <linux/delay.h>
> +#include <linux/gcd.h>
> #include <linux/platform_device.h>
> #include <linux/tty.h>
> #include <linux/ratelimit.h>
> @@ -2412,6 +2413,59 @@ static void serial8250_shutdown(struct uart_port *port)
> serial8250_do_shutdown(port);
> }
>
> +/**
> + * serial8250_get_ps - calculates prescaler, nominator, and denominator values
> + * @port: pointer to struct uart_port object
> + * @baud: baud rate
> + * @fref: reference frequency in Hz
> + * @width: maximum width of @m and @n in bits
> + * @m: nominator of fractional divider
> + * @n: denominator of fractional divider
> + *
> + * The fractional divider is used to provide different frequencies from one
> + * source. The following formulas are used to show the relations between Fref,
> + * Fuart, prescaler, nominator, and denominator values:
> + *
> + * Fuart = Fref * m / n, where m <= n
> + *
> + * baud = Fuart / (prescaler * DLAB)
> + *
> + * Return:
> + * Prescaler value from 16 to 1, and sets port UART clock to the corresponding
> + * value. Since serial core considers base Fuart = baud * 16, the given value
> + * of Fuart will be in proportion to that:
> + *
> + * Port UART clock = Fuart * 16 / prescaler
> + */
> +unsigned short serial8250_get_ps(struct uart_port *port, unsigned int baud,
> + unsigned int fref, unsigned short width,
> + unsigned int *m, unsigned int *n)
I don't think this belongs in the 8250 core; this should be a static helper
function to those 2 Intel parts.
Unless you think this is applicable to any other designs?
Regards,
Peter Hurley
> +{
> + unsigned int fuart;
> + unsigned int ps = 16, divisor, denominator;
> +
> + /* Find prescaler value that satisfies Fuart < Fref */
> + do {
> + fuart = baud * ps;
> + } while ((fuart > fref) && (--ps > 1));
> +
> + /* Get Fuart closer to Fref */
> + fuart *= rounddown_pow_of_two(fref / fuart);
> +
> + /* Get a greatest common divisor */
> + divisor = gcd(fref, fuart);
> +
> + denominator = fref / divisor;
> + while (denominator > BIT(width)) {
> + divisor <<= 1;
> + denominator >>= 1;
> + }
> + *n = denominator;
> + *m = fuart / divisor;
> + port->uartclk = fuart * 16 / ps;
> + return ps;
> +}
> +
> /*
> * XR17V35x UARTs have an extra fractional divisor register (DLD)
> * Calculate divisor with extra 4-bit fractional portion
>
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