The patch works around two UART interrupt bugs when the serial console is
flooded with inputs:
1. syslog shows "serial8250: too much works for irq"
2. serial console stops responding to key stroke
serial8250_handle_irq() checks UART_IIR_RDI before reading receive fifo
and clears bogus interrupt UART_IIR_RDI without accompanying UART_LSR_DR,
otherwise RDI interrupt could freeze or too many unhandled RDI interrupts.
Added module parameter skip_rdi_check to opt out this workaround.
Tested on Radisys ATCA 46XX which uses FPGA 16550-compatible and
other generic 16550 UART. It takes from an hour to days to reproduce by
pumping inputs to serial console continously using TeraTerm script:
---
drivers/tty/serial/8250/8250.c | 55 ++++++++++++++++++++++++++++++++++++++++
1 files changed, 55 insertions(+), 0 deletions(-)
diff --git a/drivers/tty/serial/8250/8250.c b/drivers/tty/serial/8250/8250.c
index 3ba4234..838dd22 100644
--- a/drivers/tty/serial/8250/8250.c
+++ b/drivers/tty/serial/8250/8250.c
@@ -64,6 +64,7 @@ static int serial_index(struct uart_port *port)
}
static unsigned int skip_txen_test; /* force skip of txen test at init time */
+static unsigned int skip_rdi_check; /* skip of IIR RDI check in interrupt */
/*
* Debugging.
@@ -1479,6 +1480,51 @@ unsigned int serial8250_modem_status(struct uart_8250_port *up)
EXPORT_SYMBOL_GPL(serial8250_modem_status);
/*
+ * Check if status UART_LSR_RD accompanies with interrupt UART_IIR_RDI.
+ * If they are mismatch, massage the status or interupt cause accordingly:
+ *
+ * Return a cleared UART_LSR_RD status if there is no accompanying
+ * UART_IIR_RDI. Hopefully the new status is used by interrupt handler
+ * to skip reading receive FIFO. Otherwise some UART controller stops
+ * generating RDI interrupt after this unnotified FIFO read, until other
+ * interrupts maybe transmit interrupt reads UART_LSR again.
+ *
+ * Or clear interrupt cause UART_IIR_RDI without UART_LSR_RD. The UART sets
+ * UART_IIR_RDI *even* if the received data has been read out from the FIFO
+ * before the timeout occurs. To clear UART_IIR_RDI, read receive buffer
+ * register. Reading it also clears timeout interrupt for 16550+. Otherwise
+ * the uncleared UART_IIR_RDI will keep triggering IRQ but interrupt
+ * handler finds nothing to do.
+ *
+ * Skip this workaround if interrupt is not expected, such as backup timer,
+ * so that handler can still solely rely on original status register.
+ */
+static unsigned char serial8250_iir_rdi_check(struct uart_8250_port *up,
+ unsigned char status,
+ unsigned int iir)
+{
+ unsigned int ier, rdi_stat, rdi_intr;
+
+ /* skip for handler without interrupt */
+ if (!up->port.irq)
+ return status;
+
+ /* skip for polling handler such as backup timer */
+ ier = serial_in(up, UART_IER);
+ if (!(ier & UART_IER_RDI))
+ return status;
+
+ rdi_stat = status & UART_LSR_DR;
+ rdi_intr = iir & UART_IIR_RDI;
+
+ if (rdi_stat && !rdi_intr)
+ status &= ~UART_LSR_DR;
+ else if (!rdi_stat && rdi_intr)
+ serial_in(up, UART_RX);
+ return status;
+}
+
+/*
* This handles the interrupt from one port.
*/
int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
@@ -1497,6 +1543,12 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
DEBUG_INTR("status = %x...", status);
+ /* Some UART controller has mismatched UART_IIR_RDI and UART_LSR_DR,
+ which causes either too many interrupts or interrupt freeze
+ */
+ if (!skip_rdi_check)
+ status = serial8250_iir_rdi_check(up, status, iir);
+
if (status & (UART_LSR_DR | UART_LSR_BI))
status = serial8250_rx_chars(up, status);
serial8250_modem_status(up);
@@ -3338,6 +3390,9 @@ MODULE_PARM_DESC(nr_uarts, "Maximum number of UARTs supported. (1-" __MODULE_STR
module_param(skip_txen_test, uint, 0644);
MODULE_PARM_DESC(skip_txen_test, "Skip checking for the TXEN bug at init time");
+module_param(skip_rdi_check, uint, 0644);
+MODULE_PARM_DESC(skip_rdi_check, "Skip checking IIR RDI bug in interrupt");
+
#ifdef CONFIG_SERIAL_8250_RSA
module_param_array(probe_rsa, ulong, &probe_rsa_count, 0444);
MODULE_PARM_DESC(probe_rsa, "Probe I/O ports for RSA");
--
1.7.7.4
--
To unsubscribe from this list: send the line "unsubscribe linux-serial" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
