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drivers: serial: ns16550: Fix TX IRQ not triggered when FIFO is empty

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When uart_ns16550_irq_tx_enable() is called and the TX FIFO is already
empty, no new interrupt is generated, causing data transmission to stall
in some cases. This patch introduces a workaround to simulate an ISR
callback if the FIFO is empty when enabling the TX IRQ.

Signed-off-by: Jacky Lee <jacky.lee@egistec.com>
This commit is contained in:
Jacky Lee 2025-05-23 11:40:31 +08:00 • committed by Benjamin Cabé
commit 47e43d552e
2 changed files with 60 additions and 0 deletions
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11
drivers/serial/Kconfig.ns16550
View file

@ -103,6 +103,17 @@ config UART_NS16550_WA_ISR_REENABLE_INTERRUPT
the IER is being toggled to re-assert interrupts at the end of ISR
to nudge the host interrupt controller to fire the ISR again.
config UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
bool "Callback directly when the TX FIFO is already empty"
default y if SHELL_BACKEND_SERIAL
depends on UART_INTERRUPT_DRIVEN
help
When calling uart_ns16550_irq_tx_enable() to wait for the TX FIFO
ready interrupt, but this interrupt will only be triggered if the
current state is not empty. Therefore, if the current state is
empty, you need to solve this problem by calling the callback
function directly.
endmenu
endif # UART_NS16550

49
drivers/serial/uart_ns16550.c
View file

@ -369,6 +369,10 @@ struct uart_ns16550_dev_data {
void *cb_data; /**< Callback function arg */
#endif
#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
uint8_t sw_tx_irq; /**< software tx ready flag */
#endif
#if UART_NS16550_DLF_ENABLED
uint8_t dlf; /**< DLF value */
#endif
@ -933,6 +937,10 @@ static void uart_ns16550_poll_out(const struct device *dev,
ns16550_outbyte(dev_cfg, THR(dev), c);
#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
data->sw_tx_irq = 0; /**< clean up */
#endif
k_spin_unlock(&data->lock, key);
}
@ -980,6 +988,12 @@ static int uart_ns16550_fifo_fill(const struct device *dev,
ns16550_outbyte(dev_cfg, THR(dev), tx_data[i]);
}
#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
if (i != 0) {
data->sw_tx_irq = 0; /**< clean up */
}
#endif
k_spin_unlock(&data->lock, key);
return i;
@ -1042,6 +1056,26 @@ static void uart_ns16550_irq_tx_enable(const struct device *dev)
#endif
ns16550_outbyte(dev_cfg, IER(dev), ns16550_inbyte(dev_cfg, IER(dev)) | IER_TBE);
#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
if (ns16550_inbyte(dev_cfg, LSR(dev)) & LSR_THRE) {
k_spin_unlock(&data->lock, key);
/*
* The TX FIFO ready interrupt will be triggered if only if
* when the pre-state is not empty. Thus, if the pre-state is
* already empty, try to call the callback routine directly
* to resolve it.
*/
int irq_lock_key = arch_irq_lock();
if (data->cb && (ns16550_inbyte(dev_cfg, LSR(dev)) & LSR_THRE)) {
data->sw_tx_irq = 1; /**< set tx ready */
data->cb(dev, data->cb_data);
}
arch_irq_unlock(irq_lock_key);
return;
}
#endif
k_spin_unlock(&data->lock, key);
}
@ -1056,6 +1090,10 @@ static void uart_ns16550_irq_tx_disable(const struct device *dev)
const struct uart_ns16550_dev_config * const dev_cfg = dev->config;
k_spinlock_key_t key = k_spin_lock(&data->lock);
#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
data->sw_tx_irq = 0; /**< clean up */
#endif
ns16550_outbyte(dev_cfg, IER(dev),
ns16550_inbyte(dev_cfg, IER(dev)) & (~IER_TBE));
@ -1096,6 +1134,17 @@ static int uart_ns16550_irq_tx_ready(const struct device *dev)
int ret = ((IIRC(dev) & IIR_ID) == IIR_THRE) ? 1 : 0;
#ifdef CONFIG_UART_NS16550_WA_TX_FIFO_EMPTY_INTERRUPT
if (ret == 0 && data->sw_tx_irq) {
/**< replace resoult when there is a software solution */
const struct uart_ns16550_dev_config * const dev_cfg = dev->config;
if (ns16550_inbyte(dev_cfg, IER(dev)) & IER_TBE) {
ret = 1;
}
}
#endif
k_spin_unlock(&data->lock, key);
return ret;