8. Poll Mode Driver

8.4.1. Device Identification

Each NIC port is uniquely designated by its (bus/bridge, device, function) PCI identifiers assigned by the PCI probing/enumeration function executed at DPDK initialization. Based on their PCI identifier, NIC ports are assigned two other identifiers:

• A port index used to designate the NIC port in all functions exported by the PMD API.
• A port name used to designate the port in console messages, for administration or debugging purposes. For ease of use, the port name includes the port index.

8.4.2. Device Configuration

The configuration of each NIC port includes the following operations:

• Allocate PCI resources
• Reset the hardware (issue a Global Reset) to a well-known default state
• Set up the PHY and the link
• Initialize statistics counters

The PMD API must also export functions to start/stop the all-multicast feature of a port and functions to set/unset the port in promiscuous mode.

Some hardware offload features must be individually configured at port initialization through specific configuration parameters. This is the case for the Receive Side Scaling (RSS) and Data Center Bridging (DCB) features for example.

8.4.3. On-the-Fly Configuration

All device features that can be started or stopped “on the fly” (that is, without stopping the device) do not require the PMD API to export dedicated functions for this purpose.

All that is required is the mapping address of the device PCI registers to implement the configuration of these features in specific functions outside of the drivers.

For this purpose, the PMD API exports a function that provides all the information associated with a device that can be used to set up a given device feature outside of the driver. This includes the PCI vendor identifier, the PCI device identifier, the mapping address of the PCI device registers, and the name of the driver.

The main advantage of this approach is that it gives complete freedom on the choice of the API used to configure, to start, and to stop such features.

As an example, refer to the configuration of the IEEE1588 feature for the Intel® 82576 Gigabit Ethernet Controller and the Intel® 82599 10 Gigabit Ethernet Controller controllers in the testpmd application.

Other features such as the L3/L4 5-Tuple packet filtering feature of a port can be configured in the same way. Ethernet* flow control (pause frame) can be configured on the individual port. Refer to the testpmd source code for details. Also, L4 (UDP/TCP/ SCTP) checksum offload by the NIC can be enabled for an individual packet as long as the packet mbuf is set up correctly. See Hardware Offload for details.

8.4.4. Configuration of Transmit Queues

Each transmit queue is independently configured with the following information:

• The number of descriptors of the transmit ring
• The socket identifier used to identify the appropriate DMA memory zone from which to allocate the transmit ring in NUMA architectures
• The values of the Prefetch, Host and Write-Back threshold registers of the transmit queue
• The minimum transmit packets to free threshold (tx_free_thresh). When the number of descriptors used to transmit packets exceeds this threshold, the network adaptor should be checked to see if it has written back descriptors. A value of 0 can be passed during the TX queue configuration to indicate the default value should be used. The default value for tx_free_thresh is 32. This ensures that the PMD does not search for completed descriptors until at least 32 have been processed by the NIC for this queue.
• The minimum RS bit threshold. The minimum number of transmit descriptors to use before setting the Report Status (RS) bit in the transmit descriptor. Note that this parameter may only be valid for Intel 10 GbE network adapters. The RS bit is set on the last descriptor used to transmit a packet if the number of descriptors used since the last RS bit setting, up to the first descriptor used to transmit the packet, exceeds the transmit RS bit threshold (tx_rs_thresh). In short, this parameter controls which transmit descriptors are written back to host memory by the network adapter. A value of 0 can be passed during the TX queue configuration to indicate that the default value should be used. The default value for tx_rs_thresh is 32. This ensures that at least 32 descriptors are used before the network adapter writes back the most recently used descriptor. This saves upstream PCIe* bandwidth resulting from TX descriptor write-backs. It is important to note that the TX Write-back threshold (TX wthresh) should be set to 0 when tx_rs_thresh is greater than 1. Refer to the Intel® 82599 10 Gigabit Ethernet Controller Datasheet for more details.

The following constraints must be satisfied for tx_free_thresh and tx_rs_thresh:

• tx_rs_thresh must be greater than 0.
• tx_rs_thresh must be less than the size of the ring minus 2.
• tx_rs_thresh must be less than or equal to tx_free_thresh.
• tx_free_thresh must be greater than 0.
• tx_free_thresh must be less than the size of the ring minus 3.
• For optimal performance, TX wthresh should be set to 0 when tx_rs_thresh is greater than 1.

One descriptor in the TX ring is used as a sentinel to avoid a hardware race condition, hence the maximum threshold constraints.

8.4.5. Free Tx mbuf on Demand

Many of the drivers do not release the mbuf back to the mempool, or local cache, immediately after the packet has been transmitted. Instead, they leave the mbuf in their Tx ring and either perform a bulk release when the tx_rs_thresh has been crossed or free the mbuf when a slot in the Tx ring is needed.

An application can request the driver to release used mbufs with the rte_eth_tx_done_cleanup() API. This API requests the driver to release mbufs that are no longer in use, independent of whether or not the tx_rs_thresh has been crossed. There are two scenarios when an application may want the mbuf released immediately:

• When a given packet needs to be sent to multiple destination interfaces (either for Layer 2 flooding or Layer 3 multi-cast). One option is to make a copy of the packet or a copy of the header portion that needs to be manipulated. A second option is to transmit the packet and then poll the rte_eth_tx_done_cleanup() API until the reference count on the packet is decremented. Then the same packet can be transmitted to the next destination interface. The application is still responsible for managing any packet manipulations needed between the different destination interfaces, but a packet copy can be avoided. This API is independent of whether the packet was transmitted or dropped, only that the mbuf is no longer in use by the interface.
• Some applications are designed to make multiple runs, like a packet generator. For performance reasons and consistency between runs, the application may want to reset back to an initial state between each run, where all mbufs are returned to the mempool. In this case, it can call the rte_eth_tx_done_cleanup() API for each destination interface it has been using to request it to release of all its used mbufs.

To determine if a driver supports this API, check for the Free Tx mbuf on demand feature in the Network Interface Controller Drivers document.

8.4.6. Hardware Offload

Depending on driver capabilities advertised by rte_eth_dev_info_get(), the PMD may support hardware offloading feature like checksumming, TCP segmentation, VLAN insertion or lockfree multithreaded TX burst on the same TX queue.

The support of these offload features implies the addition of dedicated status bit(s) and value field(s) into the rte_mbuf data structure, along with their appropriate handling by the receive/transmit functions exported by each PMD. The list of flags and their precise meaning is described in the mbuf API documentation and in the in Mbuf Library, section “Meta Information”.

8.5.1. Generalities

By default, all functions exported by a PMD are lock-free functions that are assumed not to be invoked in parallel on different logical cores to work on the same target object. For instance, a PMD receive function cannot be invoked in parallel on two logical cores to poll the same RX queue of the same port. Of course, this function can be invoked in parallel by different logical cores on different RX queues. It is the responsibility of the upper-level application to enforce this rule.

If needed, parallel accesses by multiple logical cores to shared queues can be explicitly protected by dedicated inline lock-aware functions built on top of their corresponding lock-free functions of the PMD API.

8.5.2. Generic Packet Representation

A packet is represented by an rte_mbuf structure, which is a generic metadata structure containing all necessary housekeeping information. This includes fields and status bits corresponding to offload hardware features, such as checksum computation of IP headers or VLAN tags.

The rte_mbuf data structure includes specific fields to represent, in a generic way, the offload features provided by network controllers. For an input packet, most fields of the rte_mbuf structure are filled in by the PMD receive function with the information contained in the receive descriptor. Conversely, for output packets, most fields of rte_mbuf structures are used by the PMD transmit function to initialize transmit descriptors.

The mbuf structure is fully described in the Mbuf Library chapter.

8.5.3. Ethernet Device API

The Ethernet device API exported by the Ethernet PMDs is described in the DPDK API Reference.

8.5.4. Extended Statistics API

The extended statistics API allows a PMD to expose all statistics that are available to it, including statistics that are unique to the device. Each statistic has three properties name, id and value:

• name: A human readable string formatted by the scheme detailed below.
• id: An integer that represents only that statistic.
• value: A unsigned 64-bit integer that is the value of the statistic.

Note that extended statistic identifiers are driver-specific, and hence might not be the same for different ports. The API consists of various rte_eth_xstats_*() functions, and allows an application to be flexible in how it retrieves statistics.

struct rte_eth_xstat_name *xstats_names;
uint64_t *values;
int len, i;

/* Get number of stats */
len = rte_eth_xstats_get_names_by_id(port_id, NULL, NULL, 0);
if (len < 0) {
    printf("Cannot get xstats count\n");
    goto err;
}

xstats_names = malloc(sizeof(struct rte_eth_xstat_name) * len);
if (xstats_names == NULL) {
    printf("Cannot allocate memory for xstat names\n");
    goto err;
}

/* Retrieve xstats names, passing NULL for IDs to return all statistics */
if (len != rte_eth_xstats_get_names_by_id(port_id, xstats_names, NULL, len)) {
    printf("Cannot get xstat names\n");
    goto err;
}

values = malloc(sizeof(values) * len);
if (values == NULL) {
    printf("Cannot allocate memory for xstats\n");
    goto err;
}

/* Getting xstats values */
if (len != rte_eth_xstats_get_by_id(port_id, NULL, values, len)) {
    printf("Cannot get xstat values\n");
    goto err;
}

/* Print all xstats names and values */
for (i = 0; i < len; i++) {
    printf("%s: %"PRIu64"\n", xstats_names[i].name, values[i]);
}

uint64_t id;
uint64_t value;
const char *xstat_name = "rx_errors";

if(!rte_eth_xstats_get_id_by_name(port_id, xstat_name, &id)) {
    rte_eth_xstats_get_by_id(port_id, &id, &value, 1);
    printf("%s: %"PRIu64"\n", xstat_name, value);
}
else {
    printf("Cannot find xstats with a given name\n");
    goto err;
}

#define APP_NUM_STATS 4
/* application cached these ids previously; see above */
uint64_t ids_array[APP_NUM_STATS] = {3,4,7,21};
uint64_t value_array[APP_NUM_STATS];

/* Getting multiple xstats values from array of IDs */
rte_eth_xstats_get_by_id(port_id, ids_array, value_array, APP_NUM_STATS);

uint32_t i;
for(i = 0; i < APP_NUM_STATS; i++) {
    printf("%d: %"PRIu64"\n", ids_array[i], value_array[i]);
}