Re: [PATCH 1/4] serial: meson: add Amlogic S4 UART support

[Lucas Tanure <tanure@linux.com>]

On 10/08/2026 15:01, Ferass El Hafidi wrote:
> On Thu, 06 Aug 2026 10:18, "Lucas Tanure via groups.io" 
> <tanure=linux.com@groups.io> wrote:
>> The S4-generation UART (S4, T7, ...) derives its baud rate from the
>> 24 MHz crystal divided by 2, selected via the XTAL_DIV2 bit in the
>> baud rate register, while older SoCs divide the crystal by 3. Add the
>> "amlogic,meson-s4-uart" compatible with driver data selecting the
>> div-by-2 scheme, matching the Linux driver (meson_s4_uart_data) and
>> the configuration the vendor BL2 programs on T7 hardware.
>>
>> Older SoCs keep the existing div-by-3 behaviour: Linux also uses
>> div-by-2 on G12A, but both modes work there and switching would risk
>> regressing boards that have shipped with div-by-3 for years.
>>
>> Also drain the transmitter before changing the baud rate so in-flight
>> characters are not garbled, as the Linux driver does.
>>
>> Assisted-by: Claude:claude-fable-5
> 
> As I understand it, LLM-assisted patches are not welcome in U-Boot right
> now: https://lore.kernel.org/u-boot/20260515220758.GM1858239@bill-the-cat/
> 
> AFAIK there isn't an official AI policy yet, but it seems the concensus
> on that matter is currently "please don't". Sorry.

Ok, I will drop this line.

> 
>> Signed-off-by: Lucas Tanure <tanure@linux.com>
>> ---
>> drivers/serial/serial_meson.c | 51 ++++++++++++++++++++++++++---------
>> 1 file changed, 38 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/serial/serial_meson.c b/drivers/serial/ 
>> serial_meson.c
>> index cc71381f87e..d909fedcfc4 100644
>> --- a/drivers/serial/serial_meson.c
>> +++ b/drivers/serial/serial_meson.c
>> @@ -62,26 +62,49 @@ struct meson_serial_plat {
>> #define AML_UART_REG5_USE_NEW_BAUD    BIT(23) /* default 1 (use new 
>> baud rate register) */
>> #define AML_UART_REG5_BAUD_MASK        0x7fffff
>>
>> +/* Driver data flags */
>> +#define MESON_UART_XTAL_DIV2    BIT(0)
>> +
>> #if CONFIG_IS_ENABLED(DM_SERIAL)
>> -static u32 meson_calc_baud_divisor(ulong src_rate, u32 baud)
>> +static u32 meson_uart_xtal_div(struct udevice *dev)
>> +{
>> +    /*
>> +     * S4-generation UARTs (S4, T7, ...) derive the baud rate from the
>> +     * crystal divided by 2, older ones divide by 3.
>> +     */
>> +    return (dev_get_driver_data(dev) & MESON_UART_XTAL_DIV2) ? 2 : 3;
>> +}
>> +
>> +static u32 meson_calc_baud_divisor(struct udevice *dev, ulong 
>> src_rate, u32 baud)
>> {
>>     /*
>>      * Usually src_rate is 24 MHz (from crystal) as clock source for 
>> serial
>> -     * device. Since 8 Mb/s is the maximum supported baud rate, use 
>> div by 3
>> -     * to derive baud rate. This choice is used also in 
>> meson_serial_setbrg.
>> +     * device. Since 8 Mb/s is the maximum supported baud rate, use a
>> +     * divided crystal to derive the baud rate. This choice is used 
>> also in
>> +     * meson_serial_setbrg.
>>      */
>> -    return DIV_ROUND_CLOSEST(src_rate / 3, baud) - 1;
>> +    return DIV_ROUND_CLOSEST(src_rate / meson_uart_xtal_div(dev), 
>> baud) - 1;
>> }
>>
>> -static void meson_serial_set_baud(struct meson_uart *uart, ulong 
>> src_rate, u32 baud)
>> +static void meson_serial_set_baud(struct udevice *dev, struct 
>> meson_uart *uart,
>> +                  ulong src_rate, u32 baud)
>> {
>>     /*
>> -     * Set crystal divided by 3 (regardless of device tree clock 
>> property)
>> +     * Set the divided crystal (regardless of device tree clock 
>> property)
>>      * as clock source and the corresponding divisor to approximate baud
>>      */
>> -    u32 divisor = meson_calc_baud_divisor(src_rate, baud);
>> +    u32 divisor = meson_calc_baud_divisor(dev, src_rate, baud);
>>     u32 val = AML_UART_REG5_USE_XTAL_CLK | AML_UART_REG5_USE_NEW_BAUD |
>>         (divisor & AML_UART_REG5_BAUD_MASK);
>> +
>> +    if (meson_uart_xtal_div(dev) == 2)
>> +        val |= AML_UART_REG5_XTAL_DIV2;
>> +
>> +    /* Drain the transmitter before changing the baud rate */
>> +    while ((readl(&uart->status) & (AML_UART_TX_EMPTY | 
>> AML_UART_XMIT_BUSY))
>> +           != AML_UART_TX_EMPTY)
>> +        ;
>> +
>>     writel(val, &uart->reg5);
>> }
>>
>> @@ -109,7 +132,7 @@ static int meson_serial_probe(struct udevice *dev)
>>         return ret;
>>     ulong rate = clk_get_rate(&per_clk);
>>
>> -    meson_serial_set_baud(uart, rate, CONFIG_BAUDRATE);
>> +    meson_serial_set_baud(dev, uart, rate, CONFIG_BAUDRATE);
>>     meson_serial_init(uart);
>>
>>     return 0;
>> @@ -165,8 +188,9 @@ static int meson_serial_setbrg(struct udevice 
>> *dev, const int baud)
>> {
>>     /*
>>      * Change device baud rate if baud is reasonable (considering a 23 
>> bit
>> -     * counter with an 8 MHz clock input) and the actual baud
>> -     * rate is within 2% of the requested value (2% is arbitrary).
>> +     * counter with an 8 MHz, or 12 MHz for XTAL_DIV2 devices, clock 
>> input)
>> +     * and the actual baud rate is within 2% of the requested value 
>> (2% is
>> +     * arbitrary).
>>      */
>>     if (baud < 1 || baud > 8000000)
>>         return -EINVAL;
>> @@ -179,14 +203,14 @@ static int meson_serial_setbrg(struct udevice 
>> *dev, const int baud)
>>     if (ret)
>>         return ret;
>>     ulong rate = clk_get_rate(&per_clk);
>> -    u32 divisor = meson_calc_baud_divisor(rate, baud);
>> -    u32 calc_baud = (rate / 3) / (divisor + 1);
>> +    u32 divisor = meson_calc_baud_divisor(dev, rate, baud);
>> +    u32 calc_baud = (rate / meson_uart_xtal_div(dev)) / (divisor + 1);
>>     u32 calc_err = baud > calc_baud ? baud - calc_baud : calc_baud - 
>> baud;
>>
>>     if (((calc_err * 100) / baud) > 2)
>>         return -EINVAL;
>>
>> -    meson_serial_set_baud(uart, rate, baud);
>> +    meson_serial_set_baud(dev, uart, rate, baud);
>>
>>     return 0;
>> }
>> @@ -244,6 +268,7 @@ static const struct udevice_id meson_serial_ids[] = {
>>     { .compatible = "amlogic,meson-uart" },
>>     { .compatible = "amlogic,meson-gx-uart" },
>>     { .compatible = "amlogic,meson-a1-uart" },
>> +    { .compatible = "amlogic,meson-s4-uart", .data = 
>> MESON_UART_XTAL_DIV2 },
>>     { }
>> };
>>
>> -- 
>> 2.55.0
> 
> Best regards,
> Ferass