[PATCH] misc: rp1: Send IACK on IRQ activate to fix kdump/kexec

[Xiaolei Wang <xiaolei.wang@windriver.com>]

After a kexec/kdump reboot, the macb Ethernet controller fails to
receive any packets, causing DHCP to hang indefinitely and the network
interface to be unusable despite link being up.

The root cause is that RP1's level-triggered MSI-X interrupt sources
(such as macb on hwirq 6) may have their internal state machines stuck
in the "waiting for IACK" state. This happens because the previous
kernel crashed before sending the acknowledgment for a pending level
interrupt.

In this stuck state, RP1 will not generate new MSI-X writes even though
the interrupt source remains asserted. Since no new MSI-X is sent, the
GIC never sees a new edge, the chained IRQ handler is never invoked,
and the interrupt is permanently lost.

Fix this by sending MSIX_CFG_IACK in rp1_irq_activate(). This
unconditionally resets the MSI-X state machine back to idle when a
child device requests its interrupt. If the interrupt source is still
asserted, RP1 will immediately issue a new MSI-X with the freshly
configured msg_addr/msg_data, and normal interrupt delivery resumes.

Writing IACK when the state machine is already idle (i.e., on a normal
cold boot) is harmless — it has no effect.

Fixes: 49d63971f963 ("misc: rp1: RaspberryPi RP1 misc driver")
Cc: stable@vger.kernel.org
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
---
 drivers/misc/rp1/rp1_pci.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/misc/rp1/rp1_pci.c b/drivers/misc/rp1/rp1_pci.c
index d210da84c30a..81685e3f3296 100644
--- a/drivers/misc/rp1/rp1_pci.c
+++ b/drivers/misc/rp1/rp1_pci.c
@@ -143,6 +143,7 @@ static int rp1_irq_activate(struct irq_domain *d, struct irq_data *irqd,
 	struct rp1_dev *rp1 = d->host_data;
 
 	msix_cfg_set(rp1, (unsigned int)irqd->hwirq, MSIX_CFG_ENABLE);
+	msix_cfg_set(rp1, (unsigned int)irqd->hwirq, MSIX_CFG_IACK);
 
 	return 0;
 }
-- 
2.43.0