Currently the Synopsys DesignWare 8250 driver assumes its UART clock
runs at a fixed rate. If a "real" clock was set up using the common
clock framework, and that clock's rate is adjustable, it may be
possible to support a wider range of baud rates by changing the
UART clock rate.
This is done by setting up a uart_port->set_termios method that
requests a clock rate change if a different rate might make it
more likely to achieve a specified baud.
Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
---
This patch is also available here:
http://git.linaro.org/landing-teams/working/broadcom/kernel.git
Branch review/dw8250_clock
drivers/tty/serial/8250/8250_dw.c | 38 ++++++++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 51b307a..013aa9b 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -242,6 +242,43 @@ dw8250_do_pm(struct uart_port *port, unsigned int state, unsigned int old)
pm_runtime_put_sync_suspend(port->dev);
}
+/*
+ * If our clock was configured through the common clock framework we
+ * might be able to support a wider range of baud rates by changing
+ * its frequency. For the "traditional" baud rates (115200 or
+ * less), request the conventional 1.8432 MHz clock. The clock
+ * frequency needs to be at least 16 times the baud rate, so
+ * use 16 times the requested rate for those higher than 115200.
+ */
+static void
+dw8250_set_termios(struct uart_port *p, struct ktermios *new,
+ struct ktermios *old)
+{
+ struct dw8250_data *data = p->private_data;
+
+ if (!IS_ERR(data->clk)) {
+ speed_t baud;
+ unsigned int rate;
+
+ baud = tty_termios_baud_rate(new);
+ BUG_ON(baud > (speed_t)UINT_MAX);
+ rate = 16 * max(115200U, (unsigned int)baud);
+
+ /*
+ * Request a different clock rate if necessary, and
+ * record it if successful.
+ */
+ if (rate != p->uartclk) {
+ BUG_ON(!data->clk);
+ if (!clk_set_rate(data->clk, (unsigned long)rate))
+ p->uartclk = rate;
+ }
+ }
+
+ /* The standard code does most of the work. */
+ serial8250_do_set_termios(p, new, old);
+}
+
static bool dw8250_dma_filter(struct dma_chan *chan, void *param)
{
struct dw8250_data *data = param;
@@ -417,6 +454,7 @@ static int dw8250_probe(struct platform_device *pdev)
uart.port.iotype = UPIO_MEM;
uart.port.serial_in = dw8250_serial_in;
uart.port.serial_out = dw8250_serial_out;
+ uart.port.set_termios = dw8250_set_termios;
uart.port.private_data = data;
if (pdev->dev.of_node) {
--
1.9.1
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