[PATCH 6.18 074/212] serial: 8250_dw: Rework IIR_NO_INT handling to stop interrupt storm

Greg Kroah-Hartman
2026-03-23T14:04:46Z

From: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

commit 73a4ed8f9efaaaf8207614ccc1c9d5ca1888f23a upstream.

INTC10EE UART can end up into an interrupt storm where it reports
IIR_NO_INT (0x1). If the storm happens during active UART operation, it
is promptly stopped by IIR value change due to Rx or Tx events.
However, when there is no activity, either due to idle serial line or
due to specific circumstances such as during shutdown that writes
IER=0, there is nothing to stop the storm.

During shutdown the storm is particularly problematic because
serial8250_do_shutdown() calls synchronize_irq() that will hang in
waiting for the storm to finish which never happens.

This problem can also result in triggering a warning:

irq 45: nobody cared (try booting with the "irqpoll" option)
[...snip...]
handlers:
serial8250_interrupt
Disabling IRQ #45

Normal means to reset interrupt status by reading LSR, MSR, USR, or RX
register do not result in the UART deasserting the IRQ.

Add a quirk to INTC10EE UARTs to enable Tx interrupts if UART's Tx is
currently empty and inactive. Rework IIR_NO_INT to keep track of the
number of consecutive IIR_NO_INT, and on fourth one perform the quirk.
Enabling Tx interrupts should change IIR value from IIR_NO_INT to
IIR_THRI which has been observed to stop the storm.

Fixes: e92fad024929 ("serial: 8250_dw: Add ACPI ID for Granite Rapids-D UART")
Cc: stable <stable@kernel.org>
Reported-by: Bandal, Shankar <shankar.bandal@intel.com>
Tested-by: Bandal, Shankar <shankar.bandal@intel.com>
Tested-by: Murthy, Shanth <shanth.murthy@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://patch.msgid.link/20260203171049.4353-6-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/tty/serial/8250/8250_dw.c | 67 +++++++++++++++++++++++++++++++++++---
1 file changed, 63 insertions(+), 4 deletions(-)

--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -61,6 +61,13 @@
#define DW_UART_QUIRK_IS_DMA_FC BIT(3)
#define DW_UART_QUIRK_APMC0D08 BIT(4)
#define DW_UART_QUIRK_CPR_VALUE BIT(5)
+#define DW_UART_QUIRK_IER_KICK BIT(6)
+
+/*
+ * Number of consecutive IIR_NO_INT interrupts required to trigger interrupt
+ * storm prevention code.
+ */
+#define DW_UART_QUIRK_IER_KICK_THRES 4

struct dw8250_platform_data {
u8 usr_reg;
@@ -82,6 +89,8 @@ struct dw8250_data {

unsigned int skip_autocfg:1;
unsigned int uart_16550_compatible:1;
+
+ u8 no_int_count;
};

static inline struct dw8250_data *to_dw8250_data(struct dw8250_port_data *data)
@@ -308,6 +317,29 @@ static u32 dw8250_serial_in32be(struct u
return dw8250_modify_msr(p, offset, value);
}

+/*
+ * INTC10EE UART can IRQ storm while reporting IIR_NO_INT. Inducing IIR value
+ * change has been observed to break the storm.
+ *
+ * If Tx is empty (THRE asserted), we use here IER_THRI to cause IIR_NO_INT ->
+ * IIR_THRI transition.
+ */
+static void dw8250_quirk_ier_kick(struct uart_port *p)
+{
+ struct uart_8250_port *up = up_to_u8250p(p);
+ u32 lsr;
+
+ if (up->ier & UART_IER_THRI)
+ return;
+
+ lsr = serial_lsr_in(up);
+ if (!(lsr & UART_LSR_THRE))
+ return;
+
+ serial_port_out(p, UART_IER, up->ier | UART_IER_THRI);
+ serial_port_in(p, UART_LCR); /* safe, no side-effects */
+ serial_port_out(p, UART_IER, up->ier);
+}

static int dw8250_handle_irq(struct uart_port *p)
{
@@ -318,18 +350,30 @@ static int dw8250_handle_irq(struct uart
unsigned int quirks = d->pdata->quirks;
unsigned int status;

+ guard(uart_port_lock_irqsave)(p);
+
switch (FIELD_GET(DW_UART_IIR_IID, iir)) {
case UART_IIR_NO_INT:
+ if (d->uart_16550_compatible || up->dma)
+ return 0;
+
+ if (quirks & DW_UART_QUIRK_IER_KICK &&
+ d->no_int_count == (DW_UART_QUIRK_IER_KICK_THRES - 1))
+ dw8250_quirk_ier_kick(p);
+ d->no_int_count = (d->no_int_count + 1) % DW_UART_QUIRK_IER_KICK_THRES;
+
return 0;

case UART_IIR_BUSY:
/* Clear the USR */
serial_port_in(p, d->pdata->usr_reg);

+ d->no_int_count = 0;
+
return 1;
}

- guard(uart_port_lock_irqsave)(p);
+ d->no_int_count = 0;

/*
* There are ways to get Designware-based UARTs into a state where
@@ -562,6 +606,14 @@ static void dw8250_reset_control_assert(
reset_control_assert(data);
}

+static void dw8250_shutdown(struct uart_port *port)
+{
+ struct dw8250_data *d = to_dw8250_data(port->private_data);
+
+ serial8250_do_shutdown(port);
+ d->no_int_count = 0;
+}
+
static int dw8250_probe(struct platform_device *pdev)
{
struct uart_8250_port uart = {}, *up = &uart;
@@ -685,10 +737,12 @@ static int dw8250_probe(struct platform_
dw8250_quirks(p, data);

/* If the Busy Functionality is not implemented, don't handle it */
- if (data->uart_16550_compatible)
+ if (data->uart_16550_compatible) {
p->handle_irq = NULL;
- else if (data->pdata)
+ } else if (data->pdata) {
p->handle_irq = dw8250_handle_irq;
+ p->shutdown = dw8250_shutdown;
+ }

dw8250_setup_dma_filter(p, data);

@@ -822,6 +876,11 @@ static const struct dw8250_platform_data
.quirks = DW_UART_QUIRK_SKIP_SET_RATE,
};

+static const struct dw8250_platform_data dw8250_intc10ee = {
+ .usr_reg = DW_UART_USR,
+ .quirks = DW_UART_QUIRK_IER_KICK,
+};
+
static const struct of_device_id dw8250_of_match[] = {
{ .compatible = "snps,dw-apb-uart", .data = &dw8250_dw_apb },
{ .compatible = "cavium,octeon-3860-uart", .data = &dw8250_octeon_3860_data },
@@ -851,7 +910,7 @@ static const struct acpi_device_id dw825
{ "INT33C5", (kernel_ulong_t)&dw8250_dw_apb },
{ "INT3434", (kernel_ulong_t)&dw8250_dw_apb },
{ "INT3435", (kernel_ulong_t)&dw8250_dw_apb },
- { "INTC10EE", (kernel_ulong_t)&dw8250_dw_apb },
+ { "INTC10EE", (kernel_ulong_t)&dw8250_intc10ee },
{ },
};
MODULE_DEVICE_TABLE(acpi, dw8250_acpi_match);