linux-kernel - [PATCH v2] serial: max310x: improve interrupt handling

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Message-Id: <20250905-master-max310x-improve-interrupt-handling-v2-1-7387651a5ed2@vaisala.com>
Date: Fri, 05 Sep 2025 08:54:19 +0000
From: Tapio Reijonen <tapio.reijonen@...sala.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 Jiri Slaby <jirislaby@...nel.org>, Alexander Shiyan <shc_work@...l.ru>, 
 Hugo Villeneuve <hvilleneuve@...onoff.com>
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org, 
 Tapio Reijonen <tapio.reijonen@...sala.com>
Subject: [PATCH v2] serial: max310x: improve interrupt handling

When there is a heavy load of receiving characters to all
four UART's, the warning 'Hardware RX FIFO overrun' is
sometimes detected.
The current implementation always service first the highest UART
until no more interrupt and then service another UART
(ex: UART3 will be serviced for as long as there are interrupts
for it, then UART2, etc).

This commit handle all individual interrupt sources before
reading the global IRQ register again.

This commit has also a nice side-effect of improving the efficiency
of the driver by reducing the number of reads of the global
IRQ register.

Signed-off-by: Tapio Reijonen <tapio.reijonen@...sala.com>
---
Changes in v2:
- Improve content of the commit message
- Fix a line indention in prevoius patch
- According review comments, changed to use for_each_clear_bit
  to simplify serve all IRQs in a loop.
  NOTE: When a bit in IRQ[n] is set 0 the associated UART’s
  internal IRQ is generated.
- Link to v1: https://lore.kernel.org/r/20250903-master-max310x-improve-interrupt-handling-v1-1-bfb44829e760@vaisala.com
---
 drivers/tty/serial/max310x.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/max310x.c b/drivers/tty/serial/max310x.c
index ce260e9949c3c268e706b2615d6fc01adc21e49b..464f2828bd304198d79046e79b2a8a0893e5cd77 100644
--- a/drivers/tty/serial/max310x.c
+++ b/drivers/tty/serial/max310x.c
@@ -825,14 +825,24 @@ static irqreturn_t max310x_ist(int irq, void *dev_id)
 	if (s->devtype->nr > 1) {
 		do {
 			unsigned int val = ~0;
+			unsigned long irq;
+			unsigned long port;
+			bool done = true;
 
 			WARN_ON_ONCE(regmap_read(s->regmap,
 						 MAX310X_GLOBALIRQ_REG, &val));
-			val = ((1 << s->devtype->nr) - 1) & ~val;
-			if (!val)
+
+			irq = val;
+
+			for_each_clear_bit(port, &irq, s->devtype->nr) {
+				done = false;
+
+				if (max310x_port_irq(s, port) == IRQ_HANDLED)
+					handled = true;
+			}
+
+			if (done)
 				break;
-			if (max310x_port_irq(s, fls(val) - 1) == IRQ_HANDLED)
-				handled = true;
 		} while (1);
 	} else {
 		if (max310x_port_irq(s, 0) == IRQ_HANDLED)

---
base-commit: c8bc81a52d5a2ac2e4b257ae123677cf94112755
change-id: 20250903-master-max310x-improve-interrupt-handling-aa22b7ba1c1d

Best regards,
-- 
Tapio Reijonen <tapio.reijonen@...sala.com>