tty: hvc: dcc: Check for TXfull condition while setting up early console

Refer ARM DDI 0487I.a ID081822, D17.3.8, DBGDTRTX_EL0,
"If TXfull is set to 1, set DTRRX and DTRTX to UNKNOWN"

Thus one should always check for TXfull condition before hvc can be
used as an early console. This is similar to what is being done
today in hvc_dcc_console_init() and hvc_dcc_init().

The count 0x4000000 has been obtained from uboot (v2024.01-rc3)
drivers/serial/arm_dcc.c "TIMEOUT_COUNT".
It reads the dcc status and waits for 0x4000000  times for the TX Fifo
to be available before returning an error. Thus, it will prevent DCC
to be used as early console.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Link: https://lore.kernel.org/r/20231205123534.3376883-1-ayan.kumar.halder@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c
index 1751108cf763ebdab3266f51006e9aa11e04b620..136381bec7cd74176b644683d24443f81f4b4341 100644 (file)
--- a/drivers/tty/hvc/hvc_dcc.c
+++ b/drivers/tty/hvc/hvc_dcc.c
@@ -47,6 +47,14 @@ static void dcc_early_write(struct console *con, const char *s, unsigned n)
 static int __init dcc_early_console_setup(struct earlycon_device *device,
                                          const char *opt)
 {
+       unsigned int count = 0x4000000;
+
+       while (--count && (__dcc_getstatus() & DCC_STATUS_TX))
+               cpu_relax();
+
+       if (__dcc_getstatus() & DCC_STATUS_TX)
+               return -ENODEV;
+
        device->con->write = dcc_early_write;
 
        return 0;