[PATCH RFC 2/3] Reapply "serial: 8250: Switch to nbcon console"

Michael Cobb
2025-05-14T17:35:44Z

This reverts commit f79b163c42314a1f46f4bcc40a19c8a75cf1e7a3.
---
drivers/tty/serial/8250/8250_core.c | 35 +++++-
drivers/tty/serial/8250/8250_port.c | 180 ++++++++++++++++++++++------
include/linux/serial_8250.h | 13 +-
3 files changed, 187 insertions(+), 41 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 6f676bb37ac3..3ab372b28cf3 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -388,12 +388,34 @@ void __init serial8250_register_ports(struct uart_driver *drv, struct device *de

#ifdef CONFIG_SERIAL_8250_CONSOLE

-static void univ8250_console_write(struct console *co, const char *s,
- unsigned int count)
+static void univ8250_console_write_atomic(struct console *co,
+ struct nbcon_write_context *wctxt)
{
struct uart_8250_port *up = &serial8250_ports[co->index];

- serial8250_console_write(up, s, count);
+ serial8250_console_write(up, wctxt, true);
+}
+
+static void univ8250_console_write_thread(struct console *co,
+ struct nbcon_write_context *wctxt)
+{
+ struct uart_8250_port *up = &serial8250_ports[co->index];
+
+ serial8250_console_write(up, wctxt, false);
+}
+
+static void univ8250_console_device_lock(struct console *co, unsigned long *flags)
+{
+ struct uart_port *up = &serial8250_ports[co->index].port;
+
+ __uart_port_lock_irqsave(up, flags);
+}
+
+static void univ8250_console_device_unlock(struct console *co, unsigned long flags)
+{
+ struct uart_port *up = &serial8250_ports[co->index].port;
+
+ __uart_port_unlock_irqrestore(up, flags);
}

static int univ8250_console_setup(struct console *co, char *options)
@@ -494,12 +516,15 @@ static int univ8250_console_match(struct console *co, char *name, int idx,

static struct console univ8250_console = {
.name = "ttyS",
- .write = univ8250_console_write,
+ .write_atomic = univ8250_console_write_atomic,
+ .write_thread = univ8250_console_write_thread,
+ .device_lock = univ8250_console_device_lock,
+ .device_unlock = univ8250_console_device_unlock,
.device = uart_console_device,
.setup = univ8250_console_setup,
.exit = univ8250_console_exit,
.match = univ8250_console_match,
- .flags = CON_PRINTBUFFER | CON_ANYTIME,
+ .flags = CON_PRINTBUFFER | CON_ANYTIME | CON_NBCON,
.index = -1,
.data = &serial8250_reg,
};
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index c57f44882abb..ef7dfab908d8 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -709,7 +709,12 @@ static void serial8250_set_sleep(struct uart_8250_port *p, int sleep)
serial8250_rpm_put(p);
}

-static void serial8250_clear_IER(struct uart_8250_port *up)
+/*
+ * Only to be used directly by the callback helper serial8250_console_write(),
+ * which may not require the port lock. Use serial8250_clear_IER() instead for
+ * all other cases.
+ */
+static void __serial8250_clear_IER(struct uart_8250_port *up)
{
if (up->capabilities & UART_CAP_UUE)
serial_out(up, UART_IER, UART_IER_UUE);
@@ -717,6 +722,11 @@ static void serial8250_clear_IER(struct uart_8250_port *up)
serial_out(up, UART_IER, 0);
}

+static inline void serial8250_clear_IER(struct uart_8250_port *up)
+{
+ __serial8250_clear_IER(up);
+}
+
#ifdef CONFIG_SERIAL_8250_RSA
/*
* Attempts to turn on the RSA FIFO. Returns zero on failure.
@@ -1404,9 +1414,6 @@ void serial8250_em485_stop_tx(struct uart_8250_port *p, bool toggle_ier)
{
unsigned char mcr = serial8250_in_MCR(p);

- /* Port locked to synchronize UART_IER access against the console. */
- lockdep_assert_held_once(&p->port.lock);
-
if (p->port.rs485.flags & SER_RS485_RTS_AFTER_SEND)
mcr |= UART_MCR_RTS;
else
@@ -1422,6 +1429,16 @@ void serial8250_em485_stop_tx(struct uart_8250_port *p, bool toggle_ier)
serial8250_clear_and_reinit_fifos(p);

if (toggle_ier) {
+ /*
+ * Port locked to synchronize UART_IER access against
+ * the console. The lockdep_assert must be restricted
+ * to this condition because only here is it
+ * guaranteed that the port lock is held. The other
+ * hardware access in this function is synchronized
+ * by console ownership.
+ */
+ lockdep_assert_held_once(&p->port.lock);
+
p->ier |= UART_IER_RLSI | UART_IER_RDI;
serial_port_out(&p->port, UART_IER, p->ier);
}
@@ -3310,7 +3327,11 @@ EXPORT_SYMBOL_GPL(serial8250_set_defaults);

static void serial8250_console_putchar(struct uart_port *port, unsigned char ch)
{
+ struct uart_8250_port *up = up_to_u8250p(port);
+
serial_port_out(port, UART_TX, ch);
+
+ up->console_line_ended = (ch == '\n');
}

static void serial8250_console_wait_putchar(struct uart_port *port, unsigned char ch)
@@ -3347,11 +3368,22 @@ static void serial8250_console_restore(struct uart_8250_port *up)
serial8250_out_MCR(up, up->mcr | UART_MCR_DTR | UART_MCR_RTS);
}

-static void fifo_wait_for_lsr(struct uart_8250_port *up, unsigned int count)
+static void fifo_wait_for_lsr(struct uart_8250_port *up,
+ struct nbcon_write_context *wctxt,
+ unsigned int count)
{
unsigned int i;

for (i = 0; i < count; i++) {
+ /*
+ * Pass the ownership as quickly as possible to a higher
+ * priority context. Otherwise, its attempt to take over
+ * the ownership might timeout. The new owner will wait
+ * for UART_LSR_THRE before reusing the fifo.
+ */
+ if (!nbcon_can_proceed(wctxt))
+ return;
+
if (wait_for_lsr(up, UART_LSR_THRE))
return;
}
@@ -3364,20 +3396,29 @@ static void fifo_wait_for_lsr(struct uart_8250_port *up, unsigned int count)
* to get empty.
*/
static void serial8250_console_fifo_write(struct uart_8250_port *up,
- const char *s, unsigned int count)
+ struct nbcon_write_context *wctxt)
{
- const char *end = s + count;
unsigned int fifosize = up->tx_loadsz;
struct uart_port *port = &up->port;
+ const char *s = wctxt->outbuf;
+ const char *end = s + wctxt->len;
unsigned int tx_count = 0;
bool cr_sent = false;
unsigned int i;

while (s != end) {
/* Allow timeout for each byte of a possibly full FIFO */
- fifo_wait_for_lsr(up, fifosize);
+ fifo_wait_for_lsr(up, wctxt, fifosize);

+ /*
+ * Fill the FIFO. If a handover or takeover occurs, writing
+ * must be aborted since wctxt->outbuf and wctxt->len are no
+ * longer valid.
+ */
for (i = 0; i < fifosize && s != end; ++i) {
+ if (!nbcon_enter_unsafe(wctxt))
+ return;
+
if (*s == '\n' && !cr_sent) {
serial8250_console_putchar(port, '\r');
cr_sent = true;
@@ -3385,6 +3426,8 @@ static void serial8250_console_fifo_write(struct uart_8250_port *up,
serial8250_console_putchar(port, *s++);
cr_sent = false;
}
+
+ nbcon_exit_unsafe(wctxt);
}
tx_count = i;
}
@@ -3393,39 +3436,57 @@ static void serial8250_console_fifo_write(struct uart_8250_port *up,
* Allow timeout for each byte written since the caller will only wait
* for UART_LSR_BOTH_EMPTY using the timeout of a single character
*/
- fifo_wait_for_lsr(up, tx_count);
+ fifo_wait_for_lsr(up, wctxt, tx_count);
+}
+
+static void serial8250_console_byte_write(struct uart_8250_port *up,
+ struct nbcon_write_context *wctxt)
+{
+ struct uart_port *port = &up->port;
+ const char *s = wctxt->outbuf;
+ const char *end = s + wctxt->len;
+
+ /*
+ * Write out the message. If a handover or takeover occurs, writing
+ * must be aborted since wctxt->outbuf and wctxt->len are no longer
+ * valid.
+ */
+ while (s != end) {
+ if (!nbcon_enter_unsafe(wctxt))
+ return;
+
+ uart_console_write(port, s++, 1, serial8250_console_wait_putchar);
+
+ nbcon_exit_unsafe(wctxt);
+ }
}

/*
- * Print a string to the serial port trying not to disturb
- * any possible real use of the port...
- *
- * The console_lock must be held when we get here.
+ * Print a string to the serial port trying not to disturb
+ * any possible real use of the port...
*
- * Doing runtime PM is really a bad idea for the kernel console.
- * Thus, we assume the function is called when device is powered up.
+ * Doing runtime PM is really a bad idea for the kernel console.
+ * Thus, assume it is called when device is powered up.
*/
-void serial8250_console_write(struct uart_8250_port *up, const char *s,
- unsigned int count)
+void serial8250_console_write(struct uart_8250_port *up,
+ struct nbcon_write_context *wctxt,
+ bool is_atomic)
{
struct uart_8250_em485 *em485 = up->em485;
struct uart_port *port = &up->port;
- unsigned long flags;
- unsigned int ier, use_fifo;
- int locked = 1;
-
- touch_nmi_watchdog();
+ unsigned int ier;
+ bool use_fifo;

- if (oops_in_progress)
- locked = uart_port_trylock_irqsave(port, &flags);
- else
- uart_port_lock_irqsave(port, &flags);
+ if (!nbcon_enter_unsafe(wctxt))
+ return;

/*
- * First save the IER then disable the interrupts
+ * First, save the IER, then disable the interrupts. The special
+ * variant to clear the IER is used because console printing may
+ * occur without holding the port lock.
*/
ier = serial_port_in(port, UART_IER);
- serial8250_clear_IER(up);
+ __serial8250_clear_IER(up);

/* check scratch reg to see if port powered off during system sleep */
if (up->canary && (up->canary != serial_port_in(port, UART_SCR))) {
@@ -3439,6 +3500,18 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
mdelay(port->rs485.delay_rts_before_send);
}

+ /* If ownership was lost, no writing is allowed */
+ if (!nbcon_can_proceed(wctxt))
+ goto skip_write;
+
+ /*
+ * If console printer did not fully output the previous line, it must
+ * have been handed or taken over. Insert a newline in order to
+ * maintain clean output.
+ */
+ if (!up->console_line_ended)
+ uart_console_write(port, "\n", 1, serial8250_console_wait_putchar);
+
use_fifo = (up->capabilities & UART_CAP_FIFO) &&
/*
* BCM283x requires to check the fifo
@@ -3459,10 +3532,23 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
*/
!(up->port.flags & UPF_CONS_FLOW);

+ nbcon_exit_unsafe(wctxt);
+
if (likely(use_fifo))
- serial8250_console_fifo_write(up, s, count);
+ serial8250_console_fifo_write(up, wctxt);
else
- uart_console_write(port, s, count, serial8250_console_wait_putchar);
+ serial8250_console_byte_write(up, wctxt);
+skip_write:
+ /*
+ * If ownership was lost, this context must reacquire ownership and
+ * re-enter the unsafe section in order to perform final actions
+ * (such as re-enabling interrupts).
+ */
+ if (!nbcon_can_proceed(wctxt)) {
+ do {
+ nbcon_reacquire_nobuf(wctxt);
+ } while (!nbcon_enter_unsafe(wctxt));
+ }

/*
* Finally, wait for transmitter to become empty
@@ -3485,11 +3571,18 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
* call it if we have saved something in the saved flags
* while processing with interrupts off.
*/
- if (up->msr_saved_flags)
- serial8250_modem_status(up);
+ if (up->msr_saved_flags) {
+ /*
+ * For atomic, it must be deferred to irq_work because this
+ * may be a context that does not permit waking up tasks.
+ */
+ if (is_atomic)
+ irq_work_queue(&up->modem_status_work);
+ else
+ serial8250_modem_status(up);
+ }

- if (locked)
- uart_port_unlock_irqrestore(port, flags);
+ nbcon_exit_unsafe(wctxt);
}

static unsigned int probe_baud(struct uart_port *port)
@@ -3507,8 +3600,24 @@ static unsigned int probe_baud(struct uart_port *port)
return (port->uartclk / 16) / quot;
}

+/*
+ * irq_work handler to perform modem control. Only triggered via
+ * ->write_atomic() callback because it may be in a scheduler or
+ * NMI context, unable to wake tasks.
+ */
+static void modem_status_handler(struct irq_work *iwp)
+{
+ struct uart_8250_port *up = container_of(iwp, struct uart_8250_port, modem_status_work);
+ struct uart_port *port = &up->port;
+
+ uart_port_lock(port);
+ serial8250_modem_status(up);
+ uart_port_unlock(port);
+}
+
int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
{
+ struct uart_8250_port *up = up_to_u8250p(port);
int baud = 9600;
int bits = 8;
int parity = 'n';
@@ -3518,6 +3627,9 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
if (!port->iobase && !port->membase)
return -ENODEV;

+ up->console_line_ended = true;
+ init_irq_work(&up->modem_status_work, modem_status_handler);
+
if (options)
uart_parse_options(options, &baud, &parity, &bits, &flow);
else if (probe)
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 144de7a7948d..57875c37023a 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -150,8 +150,17 @@ struct uart_8250_port {
#define LSR_SAVE_FLAGS UART_LSR_BRK_ERROR_BITS
u16 lsr_saved_flags;
u16 lsr_save_mask;
+
+ /*
+ * Track when a console line has been fully written to the
+ * hardware, i.e. true when the most recent byte written to
+ * UART_TX by the console was '\n'.
+ */
+ bool console_line_ended;
+
#define MSR_SAVE_FLAGS UART_MSR_ANY_DELTA
unsigned char msr_saved_flags;
+ struct irq_work modem_status_work;

struct uart_8250_dma *dma;
const struct uart_8250_ops *ops;
@@ -202,8 +211,8 @@ void serial8250_tx_chars(struct uart_8250_port *up);
unsigned int serial8250_modem_status(struct uart_8250_port *up);
void serial8250_init_port(struct uart_8250_port *up);
void serial8250_set_defaults(struct uart_8250_port *up);
-void serial8250_console_write(struct uart_8250_port *up, const char *s,
- unsigned int count);
+void serial8250_console_write(struct uart_8250_port *up,
+ struct nbcon_write_context *wctxt, bool in_atomic);
int serial8250_console_setup(struct uart_port *port, char *options, bool probe);
int serial8250_console_exit(struct uart_port *port);

--
2.45.2