[PATCH 1/3] Revert "PCI: dwc: Clear MSI interrupt status after it is handled, not before"

[Vignesh R]

Since commit 06e15e6883bed ("PCI: dwc: Clear MSI interrupt status after
it is handled, not before"), MSI IRQ status in PCIE_MSI_INTR0_STATUS
register is cleared after calling EP's IRQ handler.
But, MSI IRQs in case of PCIe are bit like edge interrupts. If a
another MSI IRQ is raised and the end of current EP's IRQ handler call
but before clearing the MSI IRQ status in PCIE_MSI_INTR0_STATUS register
then the new MSI IRQ is lost.

This issue has been observed in case of USB-Ethernet adapter connected
to PCIe USB card running iperf3 test. iperf3 client stalls after
sometime as reported here[1]. This is because XHCI raises MSI IRQ at the
end of its IRQ handler but before PCIe driver has cleared its status
in PCIE_MSI_INTR0_STATUS register. Hence, the new IRQ is never
registered by PCIe designware core. Since, XHCI does not raise any more
IRQs until the previous one is handled, it leads to a stall in
communication.
Therefore driver should always clear the MSI IRQ status in
PCIE_MSI_INTR0_STATUS before calling EP's IRQ handler. This make sure
that DW PCIe core catches new IRQ raised during the call to EP's IRQ
handler

[1] https://lkml.kernel.org/r/BN6PR18MB124994D85EAC4B5B1AD5EC56866E0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Signed-off-by: Vignesh R <vigneshr@xxxxxx>
---
 drivers/pci/dwc/pcie-designware-host.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/pci/dwc/pcie-designware-host.c b/drivers/pci/dwc/pcie-designware-host.c
index 8de2d5c69b1d..c29cbcd430f4 100644
--- a/drivers/pci/dwc/pcie-designware-host.c
+++ b/drivers/pci/dwc/pcie-designware-host.c
@@ -68,9 +68,9 @@ irqreturn_t dw_handle_msi_irq(struct pcie_port *pp)
 		while ((pos = find_next_bit((unsigned long *) &val, 32,
 					    pos)) != 32) {
 			irq = irq_find_mapping(pp->irq_domain, i * 32 + pos);
-			generic_handle_irq(irq);
 			dw_pcie_wr_own_conf(pp, PCIE_MSI_INTR0_STATUS + i * 12,
 					    4, 1 << pos);
+			generic_handle_irq(irq);
 			pos++;
 		}
 	}
-- 
2.16.1