[Qemu-devel] [RFC PATCH] PCI: Introduce INTx check & mask API

[Alexey Kardashevskiy]

[Found while debugging VFIO on POWER but it is platform independent]

There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and
PCI_STATUS registers.

And there is some API to support that (commit 
a2e27787f893621c5a6b865acf6b7766f8671328).

I have a network adapter:
0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE Single 
Port Adapter
        Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ 
Stepping- SERR+ FastB2B- DisINTx-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- 
<MAbort- >SERR- <PERR- INTx-

pci_intx_mask_supported() reports that the feature is supported for this adapter
BUT the adapter does not set PCI_STATUS_INTERRUPT so 
pci_check_and_set_intx_mask()
never changes PCI_COMMAND and INTx does not work on it when we use it as 
VFIO-PCI device.

If I remove the check of this bit, it works fine as it is called from an 
interrupt handler and
Status bit check is redundant.

Opened a spec:
PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
===
3       This read-only bit reflects the state of the interrupt in the
device/function. Only when the Interrupt Disable bit in the command
register is a 0 and this Interrupt Status bit is a 1, will the
device’s/function’s INTx# signal be asserted. Setting the Interrupt
   Disable bit to a 1 has no effect on the state of this bit.
===
With this adapter, INTx# is asserted but Status bit is still 0.

Is it mandatory for a device to set Status bit if it supports INTx masking?

2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in 
VFIO-PCI
somehow.


Signed-off-by: Alexey Kardashevskiy <address@hidden>
---
 drivers/pci/pci.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index ab6c2a6..ee4c804 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2978,60 +2978,60 @@ EXPORT_SYMBOL_GPL(pci_intx_mask_supported);
 
 static bool pci_check_and_set_intx_mask(struct pci_dev *dev, bool mask)
 {
        struct pci_bus *bus = dev->bus;
        bool mask_updated = true;
        u32 cmd_status_dword;
        u16 origcmd, newcmd;
        unsigned long flags;
        bool irq_pending;
 
        /*
         * We do a single dword read to retrieve both command and status.
         * Document assumptions that make this possible.
         */
        BUILD_BUG_ON(PCI_COMMAND % 4);
        BUILD_BUG_ON(PCI_COMMAND + 2 != PCI_STATUS);
 
        raw_spin_lock_irqsave(&pci_lock, flags);
 
        bus->ops->read(bus, dev->devfn, PCI_COMMAND, 4, &cmd_status_dword);
 
        irq_pending = (cmd_status_dword >> 16) & PCI_STATUS_INTERRUPT;
 
        /*
         * Check interrupt status register to see whether our device
         * triggered the interrupt (when masking) or the next IRQ is
         * already pending (when unmasking).
         */
-       if (mask != irq_pending) {
+/*     if (mask != irq_pending) {
                mask_updated = false;
                goto done;
-       }
+       }*/
 
        origcmd = cmd_status_dword;
        newcmd = origcmd & ~PCI_COMMAND_INTX_DISABLE;
        if (mask)
                newcmd |= PCI_COMMAND_INTX_DISABLE;
        if (newcmd != origcmd)
                bus->ops->write(bus, dev->devfn, PCI_COMMAND, 2, newcmd);
 
 done:
        raw_spin_unlock_irqrestore(&pci_lock, flags);
 
        return mask_updated;
 }
 
 /**
  * pci_check_and_mask_intx - mask INTx on pending interrupt
  * @dev: the PCI device to operate on
  *
  * Check if the device dev has its INTx line asserted, mask it and
  * return true in that case. False is returned if not interrupt was
  * pending.
  */
 bool pci_check_and_mask_intx(struct pci_dev *dev)
 {
        return pci_check_and_set_intx_mask(dev, true);
 }
 EXPORT_SYMBOL_GPL(pci_check_and_mask_intx);
 
-- 
1.7.7.3