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serial: 8250: Switch to nbcon console, take 2

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Implement the necessary callbacks to switch the 8250 console driver
to perform as an nbcon console.

Add implementations for the nbcon console callbacks:

  ->write_atomic()
  ->write_thread()
  ->device_lock()
  ->device_unlock()

and add CON_NBCON to the initial @flags.

All hardware access in the callbacks is within unsafe sections.
The ->write_atomic() and ->write_thread() callbacks allow safe
handover/takeover per byte and add a preceding newline if they
take over from another context mid-line.

For the ->write_atomic() callback, a new irq_work is used to defer
modem control since it may be called from a context that does not
allow waking up tasks. During suspend/resume the irq_work is not
used as this has been shown to cause suspend problems for some
hardware. Upon resume, any pending modem control is performed.

Note: A new __serial8250_clear_IER() is introduced for direct
clearing of UART_IER during console writing (which will not be
holding the port lock for atomic printing or KDB/KGDB). This
allows restoring a lockdep check to serial8250_clear_IER() in
a follow-up commit.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20260729120439.281252-2-john.ogness@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
John Ogness
2026-07-29 14:10:33 +02:06
committed by Greg Kroah-Hartman
parent 050f2ba1cb
commit d353934702
5 changed files with 274 additions and 55 deletions
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4
drivers/tty/serial/8250/8250.h
View File

@@ -177,7 +177,9 @@ static unsigned int __maybe_unused serial_icr_read(struct uart_8250_port *up,
void serial8250_clear_fifos(struct uart_8250_port *p);
void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p);
void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count);
void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up,
struct nbcon_write_context *wctxt,
unsigned int count);
void serial8250_rpm_get(struct uart_8250_port *p);
void serial8250_rpm_put(struct uart_8250_port *p);

65
drivers/tty/serial/8250/8250_core.c
View File

@@ -390,12 +390,34 @@ 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)
@@ -496,12 +518,15 @@ 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,
};
@@ -584,13 +609,19 @@ void serial8250_suspend_port(int line)
struct uart_8250_port *up = &serial8250_ports[line];
struct uart_port *port = &up->port;
if (!console_suspend_enabled && uart_console(port) &&
port->type != PORT_8250) {
unsigned char canary = 0xa5;
if (uart_console(port)) {
/* No irq_work may be queued when suspending */
scoped_guard(uart_port_lock_irq, port)
up->console_msr_work_allow = false;
irq_work_sync(&up->console_msr_work);
serial_out(up, UART_SCR, canary);
if (serial_in(up, UART_SCR) == canary)
up->canary = canary;
if (!console_suspend_enabled && port->type != PORT_8250) {
unsigned char canary = 0xa5;
serial_out(up, UART_SCR, canary);
if (serial_in(up, UART_SCR) == canary)
up->canary = canary;
}
}
uart_suspend_port(&serial8250_reg, port);
@@ -620,6 +651,18 @@ void serial8250_resume_port(int line)
port->uartclk = 921600*16;
}
uart_resume_port(&serial8250_reg, port);
if (uart_console(port)) {
guard(uart_port_lock_irq)(port);
/* irq_work allowed again */
up->console_msr_work_allow = true;
/* Handle any pending MSR changes */
if (up->msr_saved_flags)
serial8250_modem_status(up);
}
}
EXPORT_SYMBOL(serial8250_resume_port);

2
drivers/tty/serial/8250/8250_dw.c
View File

@@ -156,7 +156,7 @@ static int dw8250_idle_enter(struct uart_port *p)
*
* FIXME: frame_time delay is too long with very low baudrates.
*/
serial8250_fifo_wait_for_lsr_thre(up, p->fifosize);
serial8250_fifo_wait_for_lsr_thre(up, NULL, p->fifosize);
ndelay(p->frame_time);
serial_port_out(p, UART_MCR, up->mcr | UART_MCR_LOOP);

242
drivers/tty/serial/8250/8250_port.c
View File

@@ -704,7 +704,12 @@ static void serial8250_clear_interrupts(struct uart_port *port)
serial_port_in(port, UART_MSR);
}
static void serial8250_clear_IER(struct uart_8250_port *up)
/*
* Only to be directly used by serial8250_console_write() and
* serial8250_put_poll_char(), which do 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);
@@ -712,6 +717,11 @@ 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);
}
/*
* This is a quickie test to see how big the FIFO is.
* It doesn't work at all the time, more's the pity.
@@ -1284,9 +1294,6 @@ 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
@@ -1302,6 +1309,16 @@ 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);
}
@@ -2053,10 +2070,13 @@ static void serial8250_put_poll_char(struct uart_port *port,
guard(serial8250_rpm)(up);
/*
* 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 KDB/KGDB printing
* is synchronized via CPU quiescence without holding the port
* lock.
*/
ier = serial_port_in(port, UART_IER);
serial8250_clear_IER(up);
__serial8250_clear_IER(up);
wait_for_xmitr(up, UART_LSR_BOTH_EMPTY);
/*
@@ -3198,13 +3218,32 @@ void serial8250_set_defaults(struct uart_8250_port *up)
}
EXPORT_SYMBOL_GPL(serial8250_set_defaults);
void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count)
void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up,
struct nbcon_write_context *wctxt,
unsigned int count)
{
unsigned int i;
/*
* For console writing, enter/exit an unsafe section for each byte
* in order to pass the ownership as quickly as possible if a higher
* priority context wants ownership. Otherwise, an attempt to take
* over the ownership might timeout. The new owner will wait for
* UART_LSR_THRE before reusing the fifo.
*/
for (i = 0; i < count; i++) {
if (wait_for_lsr(up, UART_LSR_THRE))
bool tx_ready;
if (wctxt && !nbcon_enter_unsafe(wctxt))
return;
tx_ready = wait_for_lsr(up, UART_LSR_THRE);
if (wctxt)
nbcon_exit_unsafe(wctxt);
if (tx_ready)
break;
}
}
EXPORT_SYMBOL_NS_GPL(serial8250_fifo_wait_for_lsr_thre, "SERIAL_8250");
@@ -3213,7 +3252,11 @@ EXPORT_SYMBOL_NS_GPL(serial8250_fifo_wait_for_lsr_thre, "SERIAL_8250");
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)
@@ -3256,8 +3299,9 @@ static void serial8250_console_restore(struct uart_8250_port *up)
* It sends fifosize bytes and then waits for the fifo
* to get empty.
*/
static void serial8250_console_fifo_write(struct uart_8250_port *up,
const char *s, unsigned int count)
static void __serial8250_console_fifo_write(struct uart_8250_port *up,
struct nbcon_write_context *wctxt,
const char *s, unsigned int count)
{
const char *end = s + count;
unsigned int fifosize = up->tx_loadsz;
@@ -3268,9 +3312,16 @@ static void serial8250_console_fifo_write(struct uart_8250_port *up,
while (s != end) {
/* Allow timeout for each byte of a possibly full FIFO */
serial8250_fifo_wait_for_lsr_thre(up, fifosize);
serial8250_fifo_wait_for_lsr_thre(up, wctxt, fifosize);
/*
* Fill the FIFO. If a handover or takeover occurs, writing
* must be aborted since the string data is 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;
@@ -3278,6 +3329,8 @@ 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;
}
@@ -3286,39 +3339,92 @@ 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
*/
serial8250_fifo_wait_for_lsr_thre(up, tx_count);
serial8250_fifo_wait_for_lsr_thre(up, wctxt, tx_count);
}
static void serial8250_console_fifo_write(struct uart_8250_port *up,
struct nbcon_write_context *wctxt)
{
__serial8250_console_fifo_write(up, wctxt, wctxt->outbuf, wctxt->len);
}
static void __serial8250_console_byte_write(struct uart_8250_port *up,
struct nbcon_write_context *wctxt,
const char *s, unsigned int count)
{
struct uart_port *port = &up->port;
const char *end = s + count;
/*
* Write out the message. If a handover or takeover occurs, writing
* must be aborted since the string data is 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);
}
}
static void serial8250_console_byte_write(struct uart_8250_port *up,
struct nbcon_write_context *wctxt)
{
__serial8250_console_byte_write(up, wctxt, wctxt->outbuf, wctxt->len);
}
/*
* 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.
*
* Doing runtime PM is really a bad idea for the kernel console.
* Thus, we assume the function is called when device is powered up.
* Print the console line using the appropriate variant. If ownership is lost
* at any time during printing, the printing is aborted.
*/
void serial8250_console_write(struct uart_8250_port *up, const char *s,
unsigned int count)
static void __serial8250_console_write(struct uart_8250_port *up,
struct nbcon_write_context *wctxt,
bool use_fifo)
{
/*
* If the 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) {
if (use_fifo)
__serial8250_console_fifo_write(up, wctxt, "\n", 1);
else
__serial8250_console_byte_write(up, wctxt, "\n", 1);
}
if (use_fifo)
serial8250_console_fifo_write(up, wctxt);
else
serial8250_console_byte_write(up, wctxt);
}
/*
* Print a string to the serial port trying not to disturb
* any possible real use of the port...
*/
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;
unsigned int ier;
bool use_fifo;
touch_nmi_watchdog();
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 emergency and panic
* printing is synchronized only by nbcon ownership 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))) {
@@ -3352,10 +3458,17 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
*/
!uart_console_hwflow_active(&up->port);
if (likely(use_fifo))
serial8250_console_fifo_write(up, s, count);
else
uart_console_write(port, s, count, serial8250_console_wait_putchar);
nbcon_exit_unsafe(wctxt);
__serial8250_console_write(up, wctxt, use_fifo);
/*
* Re-enter an unsafe section in order to perform final actions
* (such as re-enabling interrupts). If ownership was lost, this
* context must reacquire ownership.
*/
while (!nbcon_enter_unsafe(wctxt))
nbcon_reacquire_nobuf(wctxt);
/*
* Finally, wait for transmitter to become empty
@@ -3365,10 +3478,21 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
if (em485) {
mdelay(port->rs485.delay_rts_after_send);
/* Toggle unsafe after possibly long delay */
nbcon_exit_unsafe(wctxt);
while (!nbcon_enter_unsafe(wctxt))
nbcon_reacquire_nobuf(wctxt);
if (em485->tx_stopped)
up->rs485_stop_tx(up, false);
}
/* Toggle unsafe after possibly long delay */
nbcon_exit_unsafe(wctxt);
while (!nbcon_enter_unsafe(wctxt))
nbcon_reacquire_nobuf(wctxt);
serial_port_out(port, UART_IER, ier);
/*
@@ -3378,11 +3502,25 @@ 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) {
if (is_atomic) {
/*
* For atomic, MSR handling must be deferred to
* irq_work because this may be a context that does
* not permit waking up tasks.
*
* But no irq_work may be queued when suspending.
* In that case, the MSR handling will occur during
* resume in serial8250_resume_port().
*/
if (up->console_msr_work_allow)
irq_work_queue(&up->console_msr_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)
@@ -3400,8 +3538,24 @@ static unsigned int probe_baud(struct uart_port *port)
return (port->uartclk / 16) / quot;
}
/*
* irq_work handler to perform modem control during console output.
* Only triggered via ->write_atomic() callback because it may be
* in a scheduler or NMI context, unable to wake tasks.
*/
static void console_msr_handler(struct irq_work *iwp)
{
struct uart_8250_port *up = container_of(iwp, struct uart_8250_port, console_msr_work);
struct uart_port *port = &up->port;
guard(uart_port_lock)(port);
serial8250_modem_status(up);
}
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';
@@ -3411,6 +3565,10 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
if (!port->iobase && !port->membase)
return -ENODEV;
up->console_line_ended = true;
up->console_msr_work_allow = true;
init_irq_work(&up->console_msr_work, console_msr_handler);
if (options)
uart_parse_options(options, &baud, &parity, &bits, &flow);
else if (probe)
@@ -3431,6 +3589,10 @@ int serial8250_console_setup(struct uart_port *port, char *options, bool probe)
int serial8250_console_exit(struct uart_port *port)
{
struct uart_8250_port *up = up_to_u8250p(port);
irq_work_sync(&up->console_msr_work);
if (port->dev)
pm_runtime_put_sync(port->dev);

16
include/linux/serial_8250.h
View File

@@ -150,8 +150,20 @@ 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;
/* Allow queuing irq_work for MSR handling */
bool console_msr_work_allow;
#define MSR_SAVE_FLAGS UART_MSR_ANY_DELTA
unsigned char msr_saved_flags;
struct irq_work console_msr_work;
struct uart_8250_dma *dma;
const struct uart_8250_ops *ops;
@@ -203,8 +215,8 @@ 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);