[PATCH v2 2/4] serial: core: allow drivers to reserve flushed transmit storage

[James Hilliard <james.hilliard1@gmail.com>]

uart_flush_buffer() resets the transmit FIFO before calling the driver's
flush callback. An asynchronous DMA transfer can still access that storage
after the callback returns, while a new write can immediately reuse it.

Add uart_xmit_block() and uart_xmit_unblock() so a driver can reserve the
flushed storage until its transfer and completion callbacks have retired.
Serialize the reservation with FIFO producers using the port lock.

While the storage is reserved, write_room() reports no space and write()
and put_char() accept no data. Releasing the reservation wakes writers
through the existing TTY wakeup path. This preserves immediate logical
flushing while applying write backpressure until the storage is reusable.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
---
 drivers/tty/serial/serial_core.c | 47 +++++++++++++++++++++++++++++++++++++---
 include/linux/serial_core.h      | 16 ++++++++++++--
 2 files changed, 58 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index c0deec0e6af4..f82c812f842f 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -123,6 +123,46 @@ void uart_write_wakeup(struct uart_port *port)
 }
 EXPORT_SYMBOL(uart_write_wakeup);
 
+/**
+ * uart_xmit_block - reserve flushed transmit storage
+ * @port: port whose transmit storage is still in use
+ *
+ * A driver may call this from ->flush_buffer() if DMA still owns the old
+ * transmit data. The logical FIFO has already been emptied, but writers
+ * must not overwrite its storage until uart_xmit_unblock() is called.
+ * write_room() reports zero and write()/put_char() accept no data meanwhile.
+ *
+ * The driver must unblock after retiring the transfer, including on shutdown.
+ * Repeated calls are idempotent, not nested reservations.
+ *
+ * Context: @port->lock held.
+ */
+void uart_xmit_block(struct uart_port *port)
+{
+	lockdep_assert_held_once(&port->lock);
+	port->state->xmit_blocked = true;
+}
+EXPORT_SYMBOL_GPL(uart_xmit_block);
+
+/**
+ * uart_xmit_unblock - release reserved transmit storage and wake writers
+ * @port: port whose storage is available again
+ *
+ * The old transfer must no longer access the FIFO storage, and any required
+ * DMA synchronization must have completed before this call.
+ *
+ * Context: @port->lock held.
+ */
+void uart_xmit_unblock(struct uart_port *port)
+{
+	lockdep_assert_held_once(&port->lock);
+	if (!port->state->xmit_blocked)
+		return;
+	port->state->xmit_blocked = false;
+	uart_write_wakeup(port);
+}
+EXPORT_SYMBOL_GPL(uart_xmit_unblock);
+
 static void uart_stop(struct tty_struct *tty)
 {
 	struct uart_state *state = tty->driver_data;
@@ -260,6 +300,7 @@ static int uart_alloc_xmit_buf(struct tty_port *port)
 	uport = uart_port_ref_lock(state, &flags);
 	if (!state->port.xmit_buf) {
 		state->port.xmit_buf = buf;
+		state->xmit_blocked = false;
 		kfifo_init(&state->port.xmit_fifo, state->port.xmit_buf,
 				PAGE_SIZE);
 		uart_port_unlock_deref(uport, flags);
@@ -593,7 +634,7 @@ static int uart_put_char(struct tty_struct *tty, u8 c)
 		return 0;
 	}
 
-	if (port)
+	if (port && !state->xmit_blocked)
 		ret = kfifo_put(&state->port.xmit_fifo, c);
 	uart_port_unlock_deref(port, flags);
 	return ret;
@@ -624,7 +665,7 @@ static ssize_t uart_write(struct tty_struct *tty, const u8 *buf, size_t count)
 		return 0;
 	}
 
-	if (port)
+	if (port && !state->xmit_blocked)
 		ret = kfifo_in(&state->port.xmit_fifo, buf, count);
 
 	__uart_start(state);
@@ -640,7 +681,7 @@ static unsigned int uart_write_room(struct tty_struct *tty)
 	unsigned int ret;
 
 	port = uart_port_ref_lock(state, &flags);
-	if (!state->port.xmit_buf)
+	if (!state->port.xmit_buf || state->xmit_blocked)
 		ret = 0;
 	else
 		ret = kfifo_avail(&state->port.xmit_fifo);
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 4eeb8253752a..92164bd598ba 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -208,8 +208,16 @@ struct gpio_desc;
  *	Flush any write buffers, reset any DMA state and stop any ongoing DMA
  *	transfers.
  *
- *	This will be called whenever the @port->state->xmit circular buffer is
- *	cleared.
+ *	This is called after the transmit FIFO has been cleared, with @port->lock
+ *	still held. A driver whose asynchronous transfer can still access the
+ *	flushed storage must call uart_xmit_block() before returning. This keeps
+ *	write_room() at zero and prevents write() and put_char() from reusing the
+ *	storage until the driver calls uart_xmit_unblock().
+ *
+ *	Release the reservation only after the transfer and its callbacks have
+ *	retired and any required DMA synchronization has completed. Shutdown must
+ *	also retire the transfer and release the reservation before returning,
+ *	since serial core then frees the transmit buffer.
  *
  *	Locking: @port->lock taken.
  *	Interrupts: locally disabled.
@@ -834,6 +842,8 @@ enum uart_pm_state {
  */
 struct uart_state {
 	struct tty_port		port;
+	/* Flushed TX storage is reserved; protected by the UART port lock. */
+	bool			xmit_blocked;
 
 	enum uart_pm_state	pm_state;
 
@@ -909,6 +919,8 @@ struct uart_driver {
 };
 
 void uart_write_wakeup(struct uart_port *port);
+void uart_xmit_block(struct uart_port *port);
+void uart_xmit_unblock(struct uart_port *port);
 
 /**
  * enum UART_TX_FLAGS -- flags for uart_port_tx_flags()

-- 
2.53.0