PCI: microchip: Fix potential race in interrupt handling

PCI: microchip: Fix potential race in interrupt handling

[ Upstream commit 7013654af694f6e1a2e699a6450ea50d309dd0e5 ]

Clear the MSI bit in ISTATUS_LOCAL register after reading it, but
before reading and handling individual MSI bits from the ISTATUS_MSI
register. This avoids a potential race where new MSI bits may be set
on the ISTATUS_MSI register after it was read and be missed when the
MSI bit in the ISTATUS_LOCAL register is cleared.

ISTATUS_LOCAL is a read/write/clear register; the register's bits
are set when the corresponding interrupt source is activated. Each
source is independent and thus multiple sources may be active
simultaneously. The processor can monitor and clear status
bits. If one or more ISTATUS_LOCAL interrupt sources are active,
the RootPort issues an interrupt towards the processor (on
the AXI domain). Bit 28 of this register reports an MSI has been
received by the RootPort.

ISTATUS_MSI is a read/write/clear register. Bits 31-0 are asserted
when an MSI with message number 31-0 is received by the RootPort.
The processor must monitor and clear these bits.

Effectively, Bit 28 of ISTATUS_LOCAL informs the processor that
an MSI has arrived at the RootPort and ISTATUS_MSI informs the
processor which MSI (in the range 0 - 31) needs handling.

Reported by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/linux-pci/20220127202000.GA126335@bhelgaas/
Link: https://lore.kernel.org/r/20220517141622.145581-1-daire.mcnamara@microchip.com
Fixes: 6f15a9c9f941 ("PCI: microchip: Add Microchip PolarFire PCIe controller driver")
Signed-off-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>