linux-kernel - [PATCH] tty: hvc: dcc: Check for TXfull condition while setting up early console

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Date:   Tue, 5 Dec 2023 12:35:34 +0000
From:   Ayan Kumar Halder <ayan.kumar.halder@....com>
To:     <linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>,
        <ayan.kumar.halder@....com>
CC:     <michal.simek@....com>, <gregkh@...uxfoundation.org>,
        <jirislaby@...nel.org>, <stefano.stabellini@....com>,
        <michal.orzel@....com>, <julien@....org>,
        <linuxppc-dev@...ts.ozlabs.org>
Subject: [PATCH] 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@....com>
---
 drivers/tty/hvc/hvc_dcc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c
index 1751108cf763..136381bec7cd 100644
--- 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;
-- 
2.25.1