RTL8169: Difference between revisions
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Rx Descriptors are used to tell the NIC where to put packets once received. Because this is an asynchronous operation, we must setup the descriptors before we enable the reception of packets so that the NIC knows where to put things. A major function of descriptors is to tell the NIC which descriptors are owned by the OS (host) and which ones are owned by the NIC. If the descriptor is owned by the NIC, the NIC can then read it and use it for the next packet it receives, if not, it either skips over it or sends a RDU (Rx Descriptor Unavailable) interrupt and halts all further Rx operations. Because there can be multiple descriptors used, there is also a EOR (End of Rx descriptor Ring) bit used when the NIC has reached the end of the descriptors and needs to loop back to the beginning of the descriptors. There is one more big 'gotcha' with all of this, the beginning of the descriptor arrays must be aligned on a 256-byte boundary.
====Descriptor format====
{| {{wikitable}}
! Offset
! Name
! Description
|-
| dword 0 || Flags || 31 bit=OWN (if set, card own this descriptor) 30 bit=EOR (last descriptor in list)
|-
| dword 1 || VLAN || Unused
|-
| dword 2 || Pointer to buffer low 32 bits ||
|-
| dword 3 || Pointer to buffer high 32 bits ||
|-
|}
====Code example====
<source lang="c">
struct Descriptor {
| |||