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    SubjectRe: [PATCH 5/5] tty: serial: atmel: Make the driver aware of the existence of GCLK
    Date
    On 30.08.2022 20:29, Richard Genoud wrote:
    >
    > Le 17/08/2022 à 09:55, Sergiu Moga a écrit :
    >> Previously, the atmel serial driver did not take into account the
    >> possibility of using the more customizable generic clock as its
    >> baudrate generator. Unless there is a Fractional Part available to
    >> increase accuracy, there is a high chance that we may be able to
    >> generate a baudrate closer to the desired one by using the GCLK as the
    >> clock source. Now, depending on the error rate between
    >> the desired baudrate and the actual baudrate, the serial driver will
    >> fallback on the generic clock. The generic clock must be provided
    >> in the DT node of the serial that may need a more flexible clock source.
    >>
    >> Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
    >> ---
    >>   drivers/tty/serial/atmel_serial.c | 52 ++++++++++++++++++++++++++++++-
    >>   drivers/tty/serial/atmel_serial.h |  1 +
    >>   2 files changed, 52 insertions(+), 1 deletion(-)
    >>
    >> diff --git a/drivers/tty/serial/atmel_serial.c
    >> b/drivers/tty/serial/atmel_serial.c
    >> index 30ba9eef7b39..0a0b46ee0955 100644
    >> --- a/drivers/tty/serial/atmel_serial.c
    >> +++ b/drivers/tty/serial/atmel_serial.c
    >> @@ -15,6 +15,7 @@
    >>   #include <linux/init.h>
    >>   #include <linux/serial.h>
    >>   #include <linux/clk.h>
    >> +#include <linux/clk-provider.h>
    >>   #include <linux/console.h>
    >>   #include <linux/sysrq.h>
    >>   #include <linux/tty_flip.h>
    >> @@ -77,6 +78,8 @@ static void atmel_stop_rx(struct uart_port *port);
    >>   #endif
    >>
    >>   #define ATMEL_ISR_PASS_LIMIT        256
    >> +#define ERROR_RATE(desired_value, actual_value) \
    >> +     ((int)(100 - ((desired_value) * 100) / (actual_value)))
    >>
    >>   struct atmel_dma_buffer {
    >>       unsigned char   *buf;
    >> @@ -110,6 +113,7 @@ struct atmel_uart_char {
    >>   struct atmel_uart_port {
    >>       struct uart_port        uart;           /* uart */
    >>       struct clk              *clk;           /* uart clock */
    >> +     struct clk              *gclk;          /* uart generic clock */
    >>       int                     may_wakeup;     /* cached value of
    >> device_may_wakeup for times we need to disable it */
    >>       u32                     backup_imr;     /* IMR saved during
    >> suspend */
    >>       int                     break_active;   /* break being received */
    >> @@ -2115,6 +2119,8 @@ static void atmel_serial_pm(struct uart_port
    >> *port, unsigned int state,
    >>                * This is called on uart_close() or a suspend event.
    >>                */
    >>               clk_disable_unprepare(atmel_port->clk);
    >> +             if (atmel_port->gclk && __clk_is_enabled(atmel_port->gclk))
    >> +                     clk_disable_unprepare(atmel_port->gclk);
    >>               break;
    >>       default:
    >>               dev_err(port->dev, "atmel_serial: unknown pm %d\n", state);
    >> @@ -2129,7 +2135,8 @@ static void atmel_set_termios(struct uart_port
    >> *port, struct ktermios *termios,
    >>   {
    >>       struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
    >>       unsigned long flags;
    >> -     unsigned int old_mode, mode, imr, quot, baud, div, cd, fp = 0;
    >> +     unsigned int old_mode, mode, imr, quot, div, cd, fp = 0;
    >> +     unsigned int baud, actual_baud, gclk_rate;
    >>
    >>       /* save the current mode register */
    >>       mode = old_mode = atmel_uart_readl(port, ATMEL_US_MR);
    >> @@ -2288,6 +2295,37 @@ static void atmel_set_termios(struct uart_port
    >> *port, struct ktermios *termios,
    >>               cd /= 8;
    >>               mode |= ATMEL_US_USCLKS_MCK_DIV8;
    >>       }
    >> +
    >> +     /*
    >> +      * If there is no Fractional Part, there is a high chance that
    >> +      * we may be able to generate a baudrate closer to the desired one
    >> +      * if we use the GCLK as the clock source driving the baudrate
    >> +      * generator.
    >> +      */
    >> +     if (!fp && atmel_port->gclk) {
    >> +             if (__clk_is_enabled(atmel_port->gclk))
    >> +                     clk_disable_unprepare(atmel_port->gclk);
    >> +             clk_set_rate(atmel_port->gclk, 16 * baud);
    >> +             gclk_rate = clk_get_rate(atmel_port->gclk);
    >> +             actual_baud = clk_get_rate(atmel_port->clk) / (16 * cd);
    >> +             if (abs(ERROR_RATE(baud, actual_baud)) >
    >> +                 abs(ERROR_RATE(baud, gclk_rate / 16))) {
    >> +                     mode |= ATMEL_US_GCLK;
    >> +
    >> +                     /*
    >> +                      * Set the Clock Divisor for GCLK to 1.
    >> +                      * Since we were able to generate the smallest
    >> +                      * multiple of the desired baudrate times 16,
    >> +                      * then we surely can generate a bigger multiple
    >> +                      * with the exact error rate for an equally
    >> increased
    >> +                      * CD. Thus no need to take into account
    >> +                      * a higher value for CD.
    >> +                      */
    >> +                     cd = 1;
    >> +                     clk_prepare_enable(atmel_port->gclk);
    >> +             }
    >> +     }
    >> +
    >>       quot = cd | fp << ATMEL_US_FP_OFFSET;
    >>
    >>       if (!(port->iso7816.flags & SER_ISO7816_ENABLED))
    >> @@ -2883,6 +2921,16 @@ static int atmel_serial_probe(struct
    >> platform_device *pdev)
    >>       if (ret)
    >>               goto err;
    >>
    >> +     atmel_port->gclk = devm_clk_get_optional(&pdev->dev, "gclk");
    >> +     if (atmel_port->gclk) {
    >> +             ret = clk_prepare_enable(atmel_port->gclk);
    >> +             if (ret) {
    >> +                     atmel_port->gclk = NULL;
    >> +                     return ret;
    >> +             }
    >> +             clk_disable_unprepare(atmel_port->gclk);
    >> +     }
    >> +
    >>       ret = atmel_init_port(atmel_port, pdev);
    >>       if (ret)
    >>               goto err_clk_disable_unprepare;
    >> @@ -2929,6 +2977,8 @@ static int atmel_serial_probe(struct
    >> platform_device *pdev)
    >>        * is used
    >>        */
    >>       clk_disable_unprepare(atmel_port->clk);
    >> +     if (atmel_port->gclk && __clk_is_enabled(atmel_port->gclk))
    >> +             clk_disable_unprepare(atmel_port->gclk);
    >>
    >>       return 0;
    >>
    >> diff --git a/drivers/tty/serial/atmel_serial.h
    >> b/drivers/tty/serial/atmel_serial.h
    >> index 0d8a0f9cc5c3..fb718972f81a 100644
    >> --- a/drivers/tty/serial/atmel_serial.h
    >> +++ b/drivers/tty/serial/atmel_serial.h
    >> @@ -63,6 +63,7 @@
    >>   #define             ATMEL_US_PAR_MARK               (3 <<  9)
    >>   #define             ATMEL_US_PAR_NONE               (4 <<  9)
    >>   #define             ATMEL_US_PAR_MULTI_DROP         (6 <<  9)
    >> +#define ATMEL_US_GCLK                          BIT(12)
    >>   #define     ATMEL_US_NBSTOP         GENMASK(13, 12) /* Number of
    >> Stop Bits */
    >>   #define             ATMEL_US_NBSTOP_1               (0 << 12)
    >>   #define             ATMEL_US_NBSTOP_1_5             (1 << 12)
    >
    > Correct me if I'm wrong, but GCLK is selected by the bit 12 only in
    > UART_MR, not in USART_MR.
    > In USART_MR, it seems to be controlled by bits 4-5 (and bit 12 is for
    > stop bits, as we can see above, and in the datasheet).
    > cf
    > https://ww1.microchip.com/downloads/aemDocuments/documents/MPU32/ProductDocuments/DataSheets/SAMA5D2-Series-Datasheet-DS60001476H.pdf
    >
    > page 1637
    >
    > Regards,
    > Richard.

    Yes, you are correct, this should have been called ATMEL_UA_GCLK
    instead. I think I will add both ATMEL_UA_GCLK and ATMEL_US_GCLK bits
    and an additional field in struct atmel_uart_port to hold ATMEL_UA_GCLK
    for UART or ATMEL_US_GCLK for USART. I guess this field should be set in
    atmel_get_ip_name(), the same place where the decision between
    ATMEL_UA_RTOR and ATMEL_US_RTOR is taken.

    Thanks,
    Sergiu
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