[PATCH v2] earlycon: Allow specifying a uartclk in options

[Daniel Kurtz]

Currently when an earlycon is registered, the uartclk is assumed to be
BASE_BAUD * 16 = 1843200.  If a baud rate is specified in the earlycon
options, then 8250_early's init_port will program the UART clock divider
registers based on this assumed uartclk.

However, not all uarts have a UART clock of 1843200.  For example, the
8250_dw uart in AMD's CZ/ST uses a fixed 48 MHz clock (as specified in
cz_uart_desc in acpi_apd.c).  Thus, specifying a baud when using earlycon
on such a device will result in incorrect divider values and a wrong UART
clock.

Fix this by extending the earlycon options parameter to allow specification
of a uartclk, like so:

 earlycon=uart,mmio32,0xfedc6000,115200,48000000

If none is specified, fall-back to prior behavior - 1843200.

Signed-off-by: Daniel Kurtz <djku...@chromium.org>
---
 Documentation/admin-guide/kernel-parameters.txt | 3 +++
 drivers/tty/serial/earlycon.c                   | 8 ++++++--
 include/linux/serial_core.h                     | 2 +-
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 1d1d53f85ddd..20e72cada38e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -958,6 +958,9 @@
                        to be equivalent to 'mmio'. 'options' are specified
                        in the same format described for "console=ttyS<n>"; if
                        unspecified, the h/w is not initialized.
+                       A UART clock rate can also be appended after the baud,
+                       as in <options> = 115200n8,48000000; if unspecified,
+                       the default 1843200 will be used.
 
                pl011,<addr>
                pl011,mmio32,<addr>
diff --git a/drivers/tty/serial/earlycon.c b/drivers/tty/serial/earlycon.c
index 870e84fb6e39..e846f406a1c6 100644
--- a/drivers/tty/serial/earlycon.c
+++ b/drivers/tty/serial/earlycon.c
@@ -115,12 +115,17 @@ static int __init parse_options(struct earlycon_device 
*device, char *options)
        }
 
        if (options) {
-               device->baud = simple_strtoul(options, NULL, 0);
+               char *uartclk;
+               device->baud = simple_strtoul(options, &uartclk, 0);
+               if (*uartclk++ == ',')
+                       port->uartclk = simple_strtoul(uartclk, NULL, 0);
                length = min(strcspn(options, " ") + 1,
                             (size_t)(sizeof(device->options)));
                strlcpy(device->options, options, length);
        }
 
+       port->uartclk = (port->uartclk) ?: BASE_BAUD * 16;
+
        return 0;
 }
 
@@ -134,7 +139,6 @@ static int __init register_earlycon(char *buf, const struct 
earlycon_id *match)
                buf = NULL;
 
        spin_lock_init(&port->lock);
-       port->uartclk = BASE_BAUD * 16;
        if (port->mapbase)
                port->membase = earlycon_map(port->mapbase, 64);
 
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index b32df49a3bd5..b772f0d20b18 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -343,7 +343,7 @@ static inline int uart_poll_timeout(struct uart_port *port)
 struct earlycon_device {
        struct console *con;
        struct uart_port port;
-       char options[16];               /* e.g., 115200n8 */
+       char options[32];               /* e.g., 115200n8,48000000 */
        unsigned int baud;
 };
 
-- 
2.16.2.395.g2e18187dfd-goog