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.
Furthermore, define the bit that represents the choice of having GCLK
as a baudrate source clock inside the USCLKS bitmask of the Mode Register
of USART IP's.
Signed-off-by: Sergiu Moga <sergiu.moga@xxxxxxxxxxxxx>
---
v1 -> v2:
- take into account the different placement of the baudrate clock source
into the IP's Mode Register (USART vs UART)
- don't check for atmel_port->gclk != NULL
- use clk_round_rate instead of clk_set_rate + clk_get_rate
- remove clk_disable_unprepare from the end of the probe method
v2 -> v3:
- add the error rate calculation function as an inline function instead of
a macro definition
- add `gclk_fail` goto
- replace `goto err` with `goto err_clk_disable_unprepare;`
v3 -> v4:
- Nothing, this was previously [PATCH 14]
v4 -> v5:
- Squashed the previous
`[PATCH v4 6/9] tty: serial: atmel: Define GCLK as USART baudrate source clock`
into this current commit
- No more BRSRCCK bitmask as it is only 1 bit
drivers/tty/serial/atmel_serial.c | 58 ++++++++++++++++++++++++++++++-
drivers/tty/serial/atmel_serial.h | 1 +
2 files changed, 58 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index acbf6b82d687..bd07f79a2df9 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>
@@ -110,6 +111,7 @@ 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 */
@@ -229,6 +231,11 @@ static inline int atmel_uart_is_half_duplex(struct uart_port *port)
(port->iso7816.flags & SER_ISO7816_ENABLED);
}
+static inline int atmel_error_rate(int desired_value, int actual_value)
+{
+ return 100 - (desired_value * 100) / actual_value;
+}
+
#ifdef CONFIG_SERIAL_ATMEL_PDC
static bool atmel_use_pdc_rx(struct uart_port *port)
{
@@ -2117,6 +2124,8 @@ 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 (__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);
@@ -2132,7 +2141,9 @@ static void atmel_set_termios(struct uart_port *port,
{
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;
+ int ret;
/* save the current mode register */
mode = old_mode = atmel_uart_readl(port, ATMEL_US_MR);
@@ -2305,6 +2316,45 @@ static void atmel_set_termios(struct uart_port *port,
cd = min_t(unsigned int, cd, ATMEL_US_CD);
}
+ /*
+ * 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 (!atmel_port->has_frac_baudrate) {
+ if (__clk_is_enabled(atmel_port->gclk))
+ clk_disable_unprepare(atmel_port->gclk);
+ gclk_rate = clk_round_rate(atmel_port->gclk, 16 * baud);
+ actual_baud = clk_get_rate(atmel_port->clk) / (16 * cd);
+ if (gclk_rate && abs(atmel_error_rate(baud, actual_baud)) >
+ abs(atmel_error_rate(baud, gclk_rate / 16))) {
+ clk_set_rate(atmel_port->gclk, 16 * baud);
+ ret = clk_prepare_enable(atmel_port->gclk);
+ if (ret)
+ goto gclk_fail;
+
+ if (atmel_port->is_usart) {
+ mode &= ~ATMEL_US_USCLKS;
+ mode |= ATMEL_US_USCLKS_GCLK;
+ } else {
+ mode |= ATMEL_UA_BRSRCCK;
+ }
+
+ /*
+ * 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;
+ }
+ }
+
+gclk_fail:
quot = cd | fp << ATMEL_US_FP_OFFSET;
if (!(port->iso7816.flags & SER_ISO7816_ENABLED))
@@ -2900,6 +2950,12 @@ static int atmel_serial_probe(struct platform_device *pdev)
if (ret)
goto err;
+ atmel_port->gclk = devm_clk_get_optional(&pdev->dev, "gclk");
+ if (IS_ERR(atmel_port->gclk)) {
+ ret = PTR_ERR(atmel_port->gclk);
+ goto err_clk_disable_unprepare;
+ }
+
ret = atmel_init_port(atmel_port, pdev);
if (ret)
goto err_clk_disable_unprepare;
diff --git a/drivers/tty/serial/atmel_serial.h b/drivers/tty/serial/atmel_serial.h
index 2a525b58e11a..0fcadbeabc6c 100644
--- a/drivers/tty/serial/atmel_serial.h
+++ b/drivers/tty/serial/atmel_serial.h
@@ -49,6 +49,7 @@
#define ATMEL_US_USCLKS GENMASK(5, 4) /* Clock Selection */
#define ATMEL_US_USCLKS_MCK (0 << 4)
#define ATMEL_US_USCLKS_MCK_DIV8 (1 << 4)
+#define ATMEL_US_USCLKS_GCLK (2 << 4)
#define ATMEL_US_USCLKS_SCK (3 << 4)
#define ATMEL_UA_FILTER BIT(4)
#define ATMEL_US_CHRL GENMASK(7, 6) /* Character Length */
--
2.34.1
