linux-kernel - [PATCH 2/2] can: m_can: fix missed interrupts with m_can

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Message-ID: <f6155510fbea33b0e18030a147b87c04395f7394.1726669005.git.matthias.schiffer@ew.tq-group.com>
Date: Wed, 18 Sep 2024 16:21:54 +0200
From: Matthias Schiffer <matthias.schiffer@...tq-group.com>
To: Chandrasekar Ramakrishnan <rcsekar@...sung.com>,
	Marc Kleine-Budde <mkl@...gutronix.de>,
	Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Martin Hundebøll <martin@...nix.com>,
	Markus Schneider-Pargmann <msp@...libre.com>,
	"Felipe Balbi (Intel)" <balbi@...nel.org>,
	Raymond Tan <raymond.tan@...el.com>,
	Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
	linux-can@...r.kernel.org,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux@...tq-group.com,
	Matthias Schiffer <matthias.schiffer@...tq-group.com>
Subject: [PATCH 2/2] can: m_can: fix missed interrupts with m_can_pci

The interrupt line of PCI devices is interpreted as edge-triggered,
however the interrupt signal of the m_can controller integrated in Intel
Elkhart Lake CPUs appears to be generated level-triggered.

Consider the following sequence of events:

- IR register is read, interrupt X is set
- A new interrupt Y is triggered in the m_can controller
- IR register is written to acknowledge interrupt X. Y remains set in IR

As at no point in this sequence no interrupt flag is set in IR, the
m_can interrupt line will never become deasserted, and no edge will ever
be observed to trigger another run of the ISR. This was observed to
result in the TX queue of the EHL m_can to get stuck under high load,
because frames were queued to the hardware in m_can_start_xmit(), but
m_can_finish_tx() was never run to account for their successful
transmission.

To fix the issue, repeatedly read and acknowledge interrupts at the
start of the ISR until no interrupt flags are set, so the next incoming
interrupt will also result in an edge on the interrupt line.

Fixes: cab7ffc0324f ("can: m_can: add PCI glue driver for Intel Elkhart Lake")
Signed-off-by: Matthias Schiffer <matthias.schiffer@...tq-group.com>
---
 drivers/net/can/m_can/m_can.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
index 47481afb9add3..363732517c3c5 100644
--- a/drivers/net/can/m_can/m_can.c
+++ b/drivers/net/can/m_can/m_can.c
@@ -1207,20 +1207,28 @@ static void m_can_coalescing_update(struct m_can_classdev *cdev, u32 ir)
 static int m_can_interrupt_handler(struct m_can_classdev *cdev)
 {
 	struct net_device *dev = cdev->net;
-	u32 ir;
+	u32 ir = 0, ir_read;
 	int ret;
 
 	if (pm_runtime_suspended(cdev->dev))
 		return IRQ_NONE;
 
-	ir = m_can_read(cdev, M_CAN_IR);
+	/* For m_can_pci, the interrupt line is interpreted as edge-triggered,
+	 * but the m_can controller generates them as level-triggered. We must
+	 * observe that IR is 0 at least once to be sure that the next
+	 * interrupt will generate an edge.
+	 */
+	while ((ir_read = m_can_read(cdev, M_CAN_IR)) != 0) {
+		ir |= ir_read;
+
+		/* ACK all irqs */
+		m_can_write(cdev, M_CAN_IR, ir);
+	}
+
 	m_can_coalescing_update(cdev, ir);
 	if (!ir)
 		return IRQ_NONE;
 
-	/* ACK all irqs */
-	m_can_write(cdev, M_CAN_IR, ir);
-
 	if (cdev->ops->clear_interrupts)
 		cdev->ops->clear_interrupts(cdev);
 
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