Real Time I/O (RTIO)

RTIO provides a framework for doing asynchronous operation chains with event driven I/O. This section covers the RTIO API, queues, executor, iodev, and common usage patterns with peripheral devices.

RTIO takes a lot of inspiration from Linux’s io_uring in its operations and API as that API matches up well with hardware transfer queues and descriptions such as DMA transfer lists.

Problem 

An application wishing to do complex DMA or interrupt driven operations today in Zephyr requires direct knowledge of the hardware and how it works. There is no understanding in the DMA API of other Zephyr devices and how they relate.

This means doing complex audio, video, or sensor streaming requires direct hardware knowledge or leaky abstractions over DMA controllers. Neither is ideal.

To enable asynchronous operations, especially with DMA, a description of what to do rather than direct operations through C and callbacks is needed. Enabling DMA features such as channels with priority, and sequences of transfers requires more than a simple list of descriptions.

Using DMA and/or interrupt driven I/O shouldn’t dictate whether or not the call is blocking or not.

Inspiration, introducing io_uring 

It’s better not to reinvent the wheel (or ring in this case) and io_uring as an API from the Linux kernel provides a winning model. In io_uring there are two lock-free ring buffers acting as queues shared between the kernel and a userland application. One queue for submission entries which may be chained and flushed to create concurrent sequential requests. A second queue for completion queue events. Only a single syscall is actually required to execute many operations, the io_uring_submit call. This call may block the caller when a number of operations to wait on is given.

This model maps well to DMA and interrupt driven transfers. A request to do a sequence of operations in an asynchronous way directly relates to the way hardware typically works with interrupt driven state machines potentially involving multiple peripheral IPs like bus and DMA controllers.

Submission Queue 

The submission queue (sq), is the description of the operations to perform in concurrent chains.

For example imagine a typical SPI transfer where you wish to write a register address to then read from. So the sequence of operations might be…

Chip Select

Clock Enable

Write register address into SPI transmit register

Read from the SPI receive register into a buffer

Disable clock

Disable Chip Select

If anything in this chain of operations fails give up. Some of those operations can be embodied in a device abstraction that understands a read or write implicitly means setup the clock and chip select. The transactional nature of the request also needs to be embodied in some manner. Of the operations above perhaps the read could be done using DMA as its large enough make sense. That requires an understanding of how to setup the device’s particular DMA to do so.

The above sequence of operations is embodied in RTIO as chain of submission queue entries (sqe). Chaining is done by setting a bitflag in an sqe to signify the next sqe must wait on the current one.

Because the chip select and clocking is common to a particular SPI controller and device on the bus it is embodied in what RTIO calls an iodev.

Multiple operations against the same iodev are done in the order provided as soon as possible. If two operation chains have varying points using the same device its possible one chain will have to wait for another to complete.

Completion Queue 

In order to know when a sqe has completed there is a completion queue (cq) with completion queue events (cqe). A sqe once completed results in a cqe being pushed into the cq. The ordering of cqe may not be the same order of sqe. A chain of sqe will however ensure ordering and failure cascading.

Other potential schemes are possible but a completion queue is a well trod idea with io_uring and other similar operating system APIs.

Executor 

The RTIO executor is a low overhead concurrent I/O task scheduler. It ensures certain request flags provide the expected behavior. It takes a list of submissions working through them in order. Various flags allow for changing the behavior of how submissions are worked through. Flags to form in order chains of submissions, transactional sets of submissions, or create multi-shot (continuously producing) requests are all possible!

IO Device 

Turning submission queue entries (sqe) into completion queue events (cqe) is the job of objects implementing the iodev (IO device) API. This API accepts requests in the form of the iodev submit API call. It is the io devices job to work through its internal queue of submissions and convert them into completions. In effect every io device can be viewed as an independent, event driven actor like object, that accepts a never ending queue of I/O like requests. How the iodev does this work is up to the author of the iodev, perhaps the entire queue of operations can be converted to a set of DMA transfer descriptors, meaning the hardware does almost all of the real work.

Cancellation 

Canceling an already queued operation is possible but not guaranteed. If the SQE has not yet started, it’s likely that a call to rtio_sqe_cancel() will remove the SQE and never run it. If, however, the SQE already started running, the cancel request will be ignored.

Memory pools 

In some cases requests to read may not know how much data will be produced. Alternatively, a reader might be handling data from multiple io devices where the frequency of the data is unpredictable. In these cases it may be wasteful to bind memory to in flight read requests. Instead with memory pools the memory to read into is left to the iodev to allocate from a memory pool associated with the RTIO context that the read was associated with. To create such an RTIO context the RTIO_DEFINE_WITH_MEMPOOL can be used. It allows creating an RTIO context with a dedicated pool of “memory blocks” which can be consumed by the iodev. Below is a snippet setting up the RTIO context with a memory pool. The memory pool has 128 blocks, each block has the size of 16 bytes, and the data is 4 byte aligned.

#include <zephyr/rtio/rtio.h>

#define SQ_SIZE       4
#define CQ_SIZE       4
#define MEM_BLK_COUNT 128
#define MEM_BLK_SIZE  16
#define MEM_BLK_ALIGN 4

RTIO_DEFINE_WITH_MEMPOOL(rtio_context,
    SQ_SIZE, CQ_SIZE, MEM_BLK_COUNT, MEM_BLK_SIZE, MEM_BLK_ALIGN);

When a read is needed, the caller simply needs to replace the call rtio_sqe_prep_read() (which takes a pointer to a buffer and a length) with a call to rtio_sqe_prep_read_with_pool(). The iodev requires only a small change which works with both pre-allocated data buffers as well as the mempool. When the read is ready, instead of getting the buffers directly from the rtio_iodev_sqe, the iodev should get the buffer and count by calling rtio_sqe_rx_buf() like so:

uint8_t *buf;
uint32_t buf_len;
int rc = rtio_sqe_rx_buff(iodev_sqe, MIN_BUF_LEN, DESIRED_BUF_LEN, &buf, &buf_len);

if (rc != 0) {
  LOG_ERR("Failed to get buffer of at least %u bytes", MIN_BUF_LEN);
  return;
}

Finally, the consumer will be able to access the allocated buffer via rtio_cqe_get_mempool_buffer().

uint8_t *buf;
uint32_t buf_len;
int rc = rtio_cqe_get_mempool_buffer(&rtio_context, &cqe, &buf, &buf_len);

if (rc != 0) {
  LOG_ERR("Failed to get mempool buffer");
  return rc;
}

/* Release the cqe events (note that the buffer is not released yet */
rtio_cqe_release_all(&rtio_context);

/* Do something with the memory */

/* Release the mempool buffer */
rtio_release_buffer(&rtio_context, buf);

When to Use 

RTIO is useful in cases where concurrent or batch like I/O flows are useful.

From the driver/hardware perspective the API enables batching of I/O requests, potentially in an optimal way. Many requests to the same SPI peripheral for example might be translated to hardware command queues or DMA transfer descriptors entirely. Meaning the hardware can potentially do more than ever.

There is a small cost to each RTIO context and iodev. This cost could be weighed against using a thread for each concurrent I/O operation or custom queues and threads per peripheral. RTIO is much lower cost than that.

API Reference 

group rtio

RTIO.

Since: 3.2
Version: 0.1.0

Defines

RTIO_IODEV_I2C_STOP: Equivalent to the I2C_MSG_STOP flag.

RTIO_IODEV_I2C_RESTART: Equivalent to the I2C_MSG_RESTART flag.

RTIO_IODEV_I2C_10_BITS: Equivalent to the I2C_MSG_ADDR_10_BITS.

RTIO_OP_NOP: An operation that does nothing and will complete immediately.

RTIO_OP_RX: An operation that receives (reads)

RTIO_OP_TX: An operation that transmits (writes)

RTIO_OP_TINY_TX: An operation that transmits tiny writes by copying the data to write.

RTIO_OP_CALLBACK: An operation that calls a given function (callback)

RTIO_OP_TXRX: An operation that transceives (reads and writes simultaneously)

RTIO_OP_I2C_RECOVER: An operation to recover I2C buses.

RTIO_OP_I2C_CONFIGURE: An operation to configure I2C buses.

RTIO_IODEV_DEFINE(name, iodev_api, iodev_data)

Statically define and initialize an RTIO IODev.

Parameters:

name – Name of the iodev
iodev_api – Pointer to struct rtio_iodev_api
iodev_data – Data pointer

RTIO_BMEM

Allocate to bss if available.

If CONFIG_USERSPACE is selected, allocate to the rtio_partition bss. Maps to: K_APP_BMEM(rtio_partition) static

If CONFIG_USERSPACE is disabled, allocate as plain static: static

RTIO_DMEM

Allocate as initialized memory if available.

If CONFIG_USERSPACE is selected, allocate to the rtio_partition init. Maps to: K_APP_DMEM(rtio_partition) static

If CONFIG_USERSPACE is disabled, allocate as plain static: static

RTIO_DEFINE(name, sq_sz, cq_sz)

Statically define and initialize an RTIO context.

Parameters:

name – Name of the RTIO
sq_sz – Size of the submission queue entry pool
cq_sz – Size of the completion queue entry pool

RTIO_DEFINE_WITH_MEMPOOL(name, sq_sz, cq_sz, num_blks, blk_size, balign)

Statically define and initialize an RTIO context.

Parameters:

name – Name of the RTIO
sq_sz – Size of the submission queue, must be power of 2
cq_sz – Size of the completion queue, must be power of 2
num_blks – Number of blocks in the memory pool
blk_size – The number of bytes in each block
balign – The block alignment

Typedefs

typedef void (*rtio_callback_t)(struct rtio *r, const struct rtio_sqe *sqe, void *arg0)

Callback signature for RTIO_OP_CALLBACK.

Param r:: RTIO context being used with the callback
Param sqe:: Submission for the callback op
Param arg0:: Argument option as part of the sqe

Functions

static inline size_t rtio_mempool_block_size(const struct rtio *r)

Get the mempool block size of the RTIO context.

Parameters:

r – [in] The RTIO context

Returns:

The size of each block in the context’s mempool

Returns:

0 if the context doesn’t have a mempool

static inline void rtio_sqe_prep_nop(struct rtio_sqe *sqe, const struct rtio_iodev *iodev, void *userdata): Prepare a nop (no op) submission.

static inline void rtio_sqe_prep_read(struct rtio_sqe *sqe, const struct rtio_iodev *iodev, int8_t prio, uint8_t *buf, uint32_t len, void *userdata): Prepare a read op submission.

static inline void rtio_sqe_prep_read_with_pool(struct rtio_sqe *sqe, const struct rtio_iodev *iodev, int8_t prio, void *userdata)

Prepare a read op submission with context’s mempool.