[PATCH u-boot-marvell 4/5] serial: a37xx: Switch to XTAL clock when booting Linux kernel
Stefan Roese
sr at denx.de
Thu May 27 08:17:41 CEST 2021
On 25.05.21 19:42, Marek Behún wrote:
> From: Pali Rohár <pali at kernel.org>
>
> Unfortunately the UART driver in current Linux for Armada 3700 expects
> UART's parent clock to be XTAL and calculats baudrate divisor according
> to XTAL clock. Therefore we must switch back to XTAL clock before
> booting kernel.
Do you plan to enhance the Linux driver as well to support TBG as
clock in input at some time?
> Implement .remove method for this driver with DM_FLAG_OS_PREPARE flag
> set.
>
> If current baudrate is unsuitable for XTAL clock then we do not change
> anything. This can only happen if the user either configured unsupported
> settings or knows what they are doing and has kernel patches which allow
> usage of non-XTAL parent clock.
>
> Signed-off-by: Pali Rohár <pali at kernel.org>
> Reviewed-by: Marek Behún <marek.behun at nic.cz>
Reviewed-by: Stefan Roese <sr at denx.de>
Thanks,
Stefan
> ---
> drivers/serial/serial_mvebu_a3700.c | 67 +++++++++++++++++++++++++++++
> 1 file changed, 67 insertions(+)
>
> diff --git a/drivers/serial/serial_mvebu_a3700.c b/drivers/serial/serial_mvebu_a3700.c
> index ba2ac5917f..c7e66fef87 100644
> --- a/drivers/serial/serial_mvebu_a3700.c
> +++ b/drivers/serial/serial_mvebu_a3700.c
> @@ -204,6 +204,71 @@ static int mvebu_serial_probe(struct udevice *dev)
> return 0;
> }
>
> +static int mvebu_serial_remove(struct udevice *dev)
> +{
> + struct mvebu_plat *plat = dev_get_plat(dev);
> + void __iomem *base = plat->base;
> + ulong new_parent_rate, parent_rate;
> + u32 new_divider, divider;
> + u32 new_oversampling;
> + u32 oversampling;
> + u32 d1, d2;
> +
> + /*
> + * Switch UART base clock back to XTAL because older Linux kernel
> + * expects it. Otherwise it does not calculate UART divisor correctly
> + * and therefore UART does not work in kernel.
> + */
> + divider = readl(base + UART_BAUD_REG);
> + if (!(divider & BIT(19))) /* UART already uses XTAL */
> + return 0;
> +
> + /* Read current divisors settings */
> + d1 = (divider >> 15) & 7;
> + d2 = (divider >> 12) & 7;
> + parent_rate = plat->tbg_rate;
> + divider &= 1023;
> + oversampling = readl(base + UART_POSSR_REG) & 63;
> + if (!oversampling)
> + oversampling = 16;
> +
> + /* Calculate new divisor against XTAL clock without changing baudrate */
> + new_oversampling = 0;
> + new_parent_rate = get_ref_clk() * 1000000;
> + new_divider = DIV_ROUND_CLOSEST(new_parent_rate * divider * d1 * d2 *
> + oversampling, parent_rate * 16);
> +
> + /*
> + * UART does not work reliably when XTAL divisor is smaller than 4.
> + * In this case we do not switch UART parent to XTAL. User either
> + * configured unsupported settings or has newer kernel with patches
> + * which allow usage of non-XTAL clock as a parent clock.
> + */
> + if (new_divider < 4)
> + return 0;
> +
> + /*
> + * If new divisor is larger than maximal supported, try to switch
> + * from default x16 scheme to oversampling with maximal factor 63.
> + */
> + if (new_divider > 1023) {
> + new_oversampling = 63;
> + new_divider = DIV_ROUND_CLOSEST(new_parent_rate * divider * d1 *
> + d2 * oversampling,
> + parent_rate * new_oversampling);
> + if (new_divider < 4 || new_divider > 1023)
> + return 0;
> + }
> +
> + while (!(readl(base + UART_STATUS_REG) & UART_STATUS_TX_EMPTY))
> + ;
> +
> + writel(new_divider, base + UART_BAUD_REG);
> + writel(new_oversampling, base + UART_POSSR_REG);
> +
> + return 0;
> +}
> +
> static int mvebu_serial_of_to_plat(struct udevice *dev)
> {
> struct mvebu_plat *plat = dev_get_plat(dev);
> @@ -232,6 +297,8 @@ U_BOOT_DRIVER(serial_mvebu) = {
> .of_to_plat = mvebu_serial_of_to_plat,
> .plat_auto = sizeof(struct mvebu_plat),
> .probe = mvebu_serial_probe,
> + .remove = mvebu_serial_remove,
> + .flags = DM_FLAG_OS_PREPARE,
> .ops = &mvebu_serial_ops,
> };
>
>
Viele Grüße,
Stefan
--
DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-51 Fax: (+49)-8142-66989-80 Email: sr at denx.de
More information about the U-Boot
mailing list