Question about em_irq_fast

[Sreekanth Rupavatharam]

We have this code snippet in em_irq_fast

        ifp = adapter->ifp;



        reg_icr = E1000_READ_REG(&adapter->hw, E1000_ICR);



        /* Hot eject?  */

        if (reg_icr == 0xffffffff)

                return FILTER_STRAY;



        /* Definitely not our interrupt.  */

        if (reg_icr == 0x0)

return FILTER_STRAY;

I don’t understand why the function returns if the read value is 0. From the programmer’s manual of Intel NIC, I see the following definition.










TXDW

0

0b

Transmit Descriptor Written Back
Set when hardware processes a transmit descriptor with the RS bit set (and possibly IDE set). If using delayed interrupts (IDE set), the interrupt occurs after the timer expires.

TXQE

1

0b

Transmit Queue Empty
Set when the last descriptor block for a transmit queue has been used.

LSC

2

0b

Link Status Change
This bit is set each time the link status changes (either from up to
down, or from down to up). This bit is affected by the internal link indication when configured for internal PHY mode.

RXSEQ

3

0b

Receive Sequence Error
In TBI mode/internal SerDes1, incoming packets with a bad delimiter sequence set this bit. In other 802.3 implementations, this would be classified as a framing error. A valid sequence consists of:
idle  SOF  data  pad (opt) EOF  fill (opt)  idle.
This is a reserved bit for the 82541xx, 82547GI/EI, and 82540EP/ EM. Set to 0b.

RXDMT0

4

0b

Receive Descriptor Minimum Threshold Reached
Indicates that the minimum number of receive descriptors are available and software should load more receive descriptors.

Reserved

5

0b

Reserved Reads as 0b.

RXO

6

0b

Receiver Overrun
Set on receive data FIFO overrun. Could be caused either
because there are no available receive buffers or because PCI receive bandwidth is inadequate.




If an interrupt happens due to a normal receive, shouldn’t the value of this register be 0 and still be valid? I am seeing this issue on a VM guest(QEMU hypervisor) where during a flood test, the driver starts rejecting packets because the register value is 0.  Can anyone tell me if it’s ok or not to remove the check for 0 value ?



Thanks,



-Sreekanth