[PATCH v2] PCI: dwc: Use level-triggered handler for MSI IRQs

[Brian Norris]

From: Brian Norris <briannorris@xxxxxxxxxx>

Per Synopsis's documentation [1], the msi_ctrl_int signal is
level-triggered, not edge-triggered.

The use of handle_edge_trigger() was chosen in commit 7c5925afbc58
("PCI: dwc: Move MSI IRQs allocation to IRQ domains hierarchical API"),
which actually dropped preexisting use of handle_level_trigger().
Looking at the patch history, this change was only made by request:

  Subject: Re: [PATCH v6 1/9] PCI: dwc: Add IRQ chained API support
  https://lore.kernel.org/all/04d3d5b6-9199-218d-476f-c77d04b8d2e7@xxxxxxx/

  "Are you sure about this "handle_level_irq"? MSIs are definitely edge
   triggered, not level."

However, while the underlying MSI protocol is edge-triggered in a sense,
the DesignWare IP is actually level-triggered.

So, let's switch back to level-triggered.

In many cases, the distinction doesn't really matter here, because this
signal is hidden behind another (chained) top-level IRQ which can be
masked on its own. But it's still wise to manipulate this interrupt line
according to its actual specification -- specifically, to mask it while
we handle it.

[1] From:

  DesignWare Cores PCI Express RP Controller Reference Manual
  Version 6.00a, June 2022
  Section 2.89 MSI Interface Signals

msi_ctrl_int is described as:

  "Asserted when an MSI interrupt is pending. De-asserted when there is
  no MSI interrupt pending.
  ...
  Active State: High (level)"

It also points at the databook for more info. One relevant excerpt from
the databook:

  DesignWare Cores PCI Express Controller Databook
  Version 6.00a, June 2022
  Section 3.9.2.3 iMSI-RX: Integrated MSI Receiver [AXI Bridge]

  "When any status bit remains set, then msi_ctrl_int remains asserted.
  The interrupt status register provides a status bit for up to 32
  interrupt vectors per Endpoint. When the decoded interrupt vector is
  enabled but is masked, then the controller sets the corresponding bit
  in interrupt status register but the it [sic] does not assert the
  top-level controller output msi_ctrl_int.

Signed-off-by: Brian Norris <briannorris@xxxxxxxxxx>
Signed-off-by: Brian Norris <briannorris@xxxxxxxxxxxx>
---

Changes in v2:
 * add documentation references

 drivers/pci/controller/dwc/pcie-designware-host.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
index ffaded8f2df7..89a1207754d3 100644
--- a/drivers/pci/controller/dwc/pcie-designware-host.c
+++ b/drivers/pci/controller/dwc/pcie-designware-host.c
@@ -198,7 +198,7 @@ static int dw_pcie_irq_domain_alloc(struct irq_domain *domain,
 	for (i = 0; i < nr_irqs; i++)
 		irq_domain_set_info(domain, virq + i, bit + i,
 				    pp->msi_irq_chip,
-				    pp, handle_edge_irq,
+				    pp, handle_level_irq,
 				    NULL, NULL);
 
 	return 0;
-- 
2.48.1.362.g079036d154-goog