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General description of rings and descriptors: TX ring is used for sending TX packet buffers to the NIC. It has the following descriptors: - `gve_tx_pkt_desc_dqo` - Data buffer descriptor - `gve_tx_tso_context_desc_dqo` - TSO context descriptor - `gve_tx_general_context_desc_dqo` - Generic metadata descriptor Metadata is a collection of 12 bytes. We define `gve_tx_metadata_dqo` which represents the logical interpetation of the metadata bytes. It's helpful to define this structure because the metadata bytes exist in multiple descriptor types (including `gve_tx_tso_context_desc_dqo`), and the device requires same field has the same value in all descriptors. The TX completion ring is used to receive completions from the NIC. Having a separate ring allows for completions to be out of order. The completion descriptor `gve_tx_compl_desc` has several different types, most important are packet and descriptor completions. Descriptor completions are used to notify the driver when descriptors sent on the TX ring are done being consumed. The descriptor completion is only used to signal that space is cleared in the TX ring. A packet completion will be received when a packet transmitted on the TX queue is done being transmitted. In addition there are "miss" and "reinjection" completions. The device implements a "flow-miss model". Most packets will simply receive a packet completion. The flow-miss system may choose to process a packet based on its contents. A TX packet which experiences a flow miss would receive a miss completion followed by a later reinjection completion. The miss-completion is received when the packet starts to be processed by the flow-miss system and the reinjection completion is received when the flow-miss system completes processing the packet and sends it on the wire. The RX buffer ring is used to send buffers to HW via the `gve_rx_desc_dqo` descriptor. Received packets are put into the RX queue by the device, which populates the `gve_rx_compl_desc_dqo` descriptor. The RX descriptors refer to buffers posted by the buffer queue. Received buffers may be returned out of order, such as when HW LRO is enabled. Important concepts: - "TX" and "RX buffer" queues, which send descriptors to the device, use MMIO doorbells to notify the device of new descriptors. - "RX" and "TX completion" queues, which receive descriptors from the device, use a "generation bit" to know when a descriptor was populated by the device. The driver initializes all bits with the "current generation". The device will populate received descriptors with the "next generation" which is inverted from the current generation. When the ring wraps, the current/next generation are swapped. - It's the driver's responsibility to ensure that the RX and TX completion queues are not overrun. This can be accomplished by limiting the number of descriptors posted to HW. - TX packets have a 16 bit completion_tag and RX buffers have a 16 bit buffer_id. These will be returned on the TX completion and RX queues respectively to let the driver know which packet/buffer was completed. Bitfields are used to describe descriptor fields. This notation is more concise and readable than shift-and-mask. It is possible because the driver is restricted to little endian platforms. Signed-off-by: Bailey Forrest <bcf@google.com> Reviewed-by: Willem de Bruijn <willemb@google.com> Reviewed-by: Catherine Sullivan <csully@google.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.