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 DMA transfer queues and descriptions.

A quick sales pitch on why RTIO works well in many scenarios:

API is DMA and interrupt friendly
No buffer copying
No callbacks
Blocking or non-blocking operation

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 and Chaining 

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 and IODev 

Turning submission queue entries (sqe) into completion queue events (cqe) is the job of objects implementing the executor and iodev APIs. These APIs enable coordination between themselves to enable things like DMA transfers.

The end result of these APIs should be a method to resolve the request by deciding some of the following questions with heuristic/constraint based decision making.

Polling, Interrupt, or DMA transfer?
If DMA, are the requirements met (peripheral supported by DMAC, etc).

The executor is meant to provide policy for when to use each transfer type, and provide the common code for walking through submission queue chains by providing calls the iodev may use to signal completion, error, or a need to suspend and wait.

Memory pools 

In some cases, the consumer may not know how much data will be produced. Alternatively, a consumer might be handling data from multiple producers where the frequency of the data is unpredictable. In these cases, read operations may not want to bind memory at the time of allocation, but leave it to the IODev. In such cases, there exists a macro RTIO_DEFINE_WITH_MEMPOOL. It allows creating the 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_EXECUTOR_SIMPLE_DEFINE(simple_exec);
RTIO_DEFINE_WITH_MEMPOOL(rtio_context, (struct rtio_executor *)&simple_exec,
    SQ_SIZE, CQ_SIZE, MEM_BLK_COUNT, MEM_BLK_SIZE, MEM_BLK_ALIGN);

When a read is needed, the consumer 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 c:func: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);

Outstanding Questions 

RTIO is not a complete API and solution, and is currently evolving to best fit the nature of an RTOS. The general ideas behind a pair of queues to describe requests and completions seems sound and has been proven out in other contexts. Questions remain though.

Timeouts and Deadlines 

Timeouts and deadlines are key to being Real-Time. Real-Time in Zephyr means being able to do things when an application wants them done. That could mean different things from a deadline with best effort attempts or a timeout and failure.

These features would surely be useful in many cases, but would likely add some significant complexities. It’s something to decide upon, and even if enabled would likely be a compile time optional feature leading to complex testing.

Cancellation 

Canceling an already queued operation could be possible with a small API addition to perhaps take both the RTIO context and a pointer to the submission queue entry. However, cancellation as an API induces many potential complexities that might not be appropriate. It’s something to be decided upon.

Userspace Support 

RTIO with userspace is certainly plausible but would require the equivalent of a memory map call to map the shared ringbuffers and also potentially dma buffers.

Additionally a DMA buffer interface would likely need to be provided for coherence and MMU usage.

IODev and Executor API 

Lastly the API between an executor and iodev is incomplete.

There are certain interactions that should be supported. Perhaps things like expanding a submission queue entry into multiple submission queue entries in order to split up work that can be done by a device and work that can be done by a DMA controller.

In some SoCs only specific DMA channels may be used with specific devices. In others there are requirements around needing a DMA handshake or specific triggering setups to tell the DMA when to start its operation.

None of that, from the outward facing API, is an issue.

It is however an unresolved task and issue from an internal API between the executor and iodev. This requires some SoC specifics and enabling those generically isn’t likely possible. That’s ok, an iodev and dma executor should be vendor specific, but an API needs to be there between them that is not!

Special Hardware: Intel HDA 

In some cases there’s a need to always do things in a specific order with a specific buffer allocation strategy. Consider a DMA that requires the usage of a circular buffer segmented into blocks that may only be transferred one after another. This is the case of the Intel HDA stream for audio.

In this scenario the above API can still work, but would require an additional buffer allocator to work with fixed sized segments.

When to Use 

It’s important to understand when DMA like transfers are useful and when they are not. It’s a poor idea to assume that something made for high throughput will work for you. There is a computational, memory, and latency cost to setup the description of transfers.

Polling at 1Hz an air sensor will almost certainly result in a net negative result compared to ad-hoc sensor (i2c/spi) requests to get the sample.

Continuous transfers, driven by timer or interrupt, of data from a peripheral’s on board FIFO over I2C, I3C, SPI, MIPI, I2S, etc… maybe, but not always!

Examples 

Examples speak loudly about the intended uses and goals of an API. So several key examples are presented below. Some are entirely plausible today without a big leap. Others (the sensor example) would require additional work in other APIs outside of RTIO as a sub system and are theoretical.

Chained Blocking Requests 

A common scenario is needing to write the register address to then read from. This can be accomplished by chaining a write into a read operation.

The transaction on i2c is implicit for each operation chain.

RTIO_I2C_IODEV(i2c_dev, I2C_DT_SPEC_INST(n));
RTIO_DEFINE(ez_io, 4, 4);
static uint16_t reg_addr;
static uint8_t buf[32];

int do_some_io(void)
{
        struct rtio_sqe *write_sqe = rtio_spsc_acquire(ez_io.sq);
        struct rtio_sqe *read_sqe = rtio_spsc_acquire(ez_io.sq);

        rtio_sqe_prep_write(write_sqe, i2c_dev, RTIO_PRIO_LOW, &reg_addr, 2);
        write_sqe->flags = RTIO_SQE_CHAINED; /* the next item in the queue will wait on this one */

        rtio_sqe_prep_read(read_sqe, i2c_dev, RTIO_PRIO_LOW, buf, 32);

        rtio_submit(rtio_inplace_executor, &ez_io, 2);

        struct rtio_cqe *read_cqe = rtio_spsc_consume(ez_io.cq);
        struct rtio_cqe *write_cqe = rtio_spsc_consume(ez_io.cq);

        if(read_cqe->result < 0) {
                LOG_ERR("read failed!");
        }

        if(write_cqe->result < 0) {
                LOG_ERR("write failed!");
        }

        rtio_spsc_release(ez_io.cq);
        rtio_spsc_release(ez_io.cq);
}

Non blocking device to device 

Imagine wishing to read from one device on an I2C bus and then write the same buffer to a device on a SPI bus without blocking the thread or setting up callbacks or other IPC notification mechanisms.

Perhaps an I2C temperature sensor and a SPI lowrawan module. The following is a simplified version of that potential operation chain.

RTIO_I2C_IODEV(i2c_dev, I2C_DT_SPEC_INST(n));
RTIO_SPI_IODEV(spi_dev, SPI_DT_SPEC_INST(m));

RTIO_DEFINE(ez_io, 4, 4);
static uint8_t buf[32];

int do_some_io(void)
{
        uint32_t read, write;
        struct rtio_sqe *read_sqe = rtio_spsc_acquire(ez_io.sq);
        rtio_sqe_prep_read(read_sqe, i2c_dev, RTIO_PRIO_LOW, buf, 32);
        read_sqe->flags = RTIO_SQE_CHAINED; /* the next item in the queue will wait on this one */

        /* Safe to do as the chained operation *ensures* that if one fails all subsequent ops fail */
        struct rtio_sqe *write_sqe = rtio_spsc_acquire(ez_io.sq);
        rtio_sqe_prep_write(write_sqe, spi_dev, RTIO_PRIO_LOW, buf, 32);

        /* call will return immediately without blocking if possible */
        rtio_submit(rtio_inplace_executor, &ez_io, 0);

        /* These calls might return NULL if the operations have not yet completed! */
        for (int i = 0; i < 2; i++) {
                struct rtio_cqe *cqe = rtio_spsc_consume(ez_io.cq);
                while(cqe == NULL) {
                        cqe = rtio_spsc_consume(ez_io.cq);
                        k_yield();
                }
                if(cqe->userdata == &read && cqe->result < 0) {
                        LOG_ERR("read from i2c failed!");
                }
                if(cqe->userdata == &write && cqe->result < 0) {
                        LOG_ERR("write to spi failed!");
                }
                /* Must release the completion queue event after consume */
                rtio_spsc_release(ez_io.cq);
        }
}

Nested iodevs for Devices on Buses (Sensors), Theoretical 

Consider a device like a sensor or audio codec sitting on a bus.

Its useful to consider that the sensor driver can use RTIO to do I/O on the SPI bus, while also being an RTIO device itself. The sensor iodev can set aside a small portion of the buffer in front or in back to store some metadata describing the format of the data. This metadata could then be used in creating a sensor readings iterator which lazily lets you map over each reading, doing calculations such as FIR/IIR filtering, or perhaps translating the readings into other numerical formats with useful measurement units such as SI. RTIO is a common movement API and allows for such uses while not deciding the mechanism.

This same sort of setup could be done for other data streams such as audio or video.

/* Note that the sensor device itself can use RTIO to get data over I2C/SPI
 * potentially with DMA, but we don't need to worry about that here
 * All we need to know is the device tree node_id and that it can be an iodev
 */
RTIO_SENSOR_IODEV(sensor_dev, DEVICE_DT_GET(DT_NODE(super6axis));

RTIO_DEFINE(ez_io, 4, 4);


/* The sensor driver decides the minimum buffer size for us, we decide how
 * many bufs. This could be a typical multiple of a fifo packet the sensor
 * produces, ICM42688 for example produces a FIFO packet of 20 bytes in
 * 20bit mode at 32KHz so perhaps we'd like to get 4 buffers of 4ms of data
 * each in this setup to process on. and its already been defined here for us.
 */
#include <sensors/icm42688_p.h>
static uint8_t bufs[4][ICM42688_RTIO_BUF_SIZE];

int do_some_sensors(void) {
        /* Obtain a dmac executor from the DMA device */
        struct device *dma = DEVICE_DT_GET(DT_NODE(dma0));
        const struct rtio_executor *rtio_dma_exec =
                        dma_rtio_executor(dma);

        /*
         * Set the executor for our queue context
         */
         rtio_set_executor(ez_io, rtio_dma_exec);

        /* Mostly we want to feed the sensor driver enough buffers to fill while
         * we wait and process! Small enough to process quickly with low latency,
         * big enough to not spend all the time setting transfers up.
         *
         * It's assumed here that the sensor has been configured already
         * and each FIFO watermark interrupt that occurs it attempts
         * to pull from the queue, fill the buffer with a small metadata
         * offset using its own rtio request to the SPI bus using DMA.
         */
        for(int i = 0; i < 4; i++) {
                struct rtio_sqe *read_sqe = rtio_spsc_acquire(ez_io.sq);

                rtio_sqe_prep_read(read_sqe, sensor_dev, RTIO_PRIO_HIGH, bufs[i], ICM42688_RTIO_BUF_SIZE);
        }
        struct device *sensor = DEVICE_DT_GET(DT_NODE(super6axis));
        struct sensor_reader reader;
        struct sensor_channels channels[4] = {
                SENSOR_TIMESTAMP_CHANNEL,
                SENSOR_CHANNEL(int32_t, SENSOR_ACC_X, 0, SENSOR_RAW),
                SENSOR_CHANNEL(int32_t SENSOR_ACC_Y, 0, SENSOR_RAW),
                SENSOR_CHANNEL(int32_t, SENSOR_ACC_Z, 0, SENSOR_RAW),
        };
        while (true) {
                /* call will wait for one completion event */
                rtio_submit(ez_io, 1);
                struct rtio_cqe *cqe = rtio_spsc_consume(ez_io.cq);
                if(cqe->result < 0) {
                        LOG_ERR("read failed!");
                        goto next;
                }

                /* Bytes read into the buffer */
                int32_t bytes_read = cqe->result;

                /* Retrieve soon to be reusable buffer pointer from completion */
                uint8_t *buf = cqe->userdata;


                /* Get an iterator (reader) that obtains sensor readings in integer
                 * form, 16 bit signed values in the native sensor reading format
                 */
                res = sensor_reader(sensor, buf, cqe->result, &reader, channels,
                                                    sizeof(channels));
                __ASSERT(res == 0);
                while(sensor_reader_next(&reader)) {
                        printf("time(raw): %d, acc (x,y,z): (%d, %d, %d)\n",
                        channels[0].value.u32, channels[1].value.i32,
                        channels[2].value.i32, channels[3].value.i32);
                }

next:
                /* Release completion queue event */
                rtio_spsc_release(ez_io.cq);

                /* resubmit a read request with the newly freed buffer to the sensor */
                struct rtio_sqe *read_sqe = rtio_spsc_acquire(ez_io.sq);
                rtio_sqe_prep_read(read_sqe, sensor_dev, RTIO_PRIO_HIGH, buf, ICM20649_RTIO_BUF_SIZE);
        }
}

API Reference 

RTIO API 

group rtio_api

RTIO API.

Defines

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_SQ_DEFINE(name, len)

Statically define and initialize a fixed length submission queue.

Parameters:

name – Name of the submission queue.
len – Queue length, power of 2 required (2, 4, 8).

RTIO_CQ_DEFINE(name, len)

Statically define and initialize a fixed length completion queue.

Parameters:

name – Name of the completion queue.
len – Queue length, power of 2 required (2, 4, 8).

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_DEFINE(name, exec, sq_sz, cq_sz)

Statically define and initialize an RTIO context.

Parameters:

name – Name of the RTIO
exec – Symbol for rtio_executor (pointer)
sq_sz – Size of the submission queue, must be power of 2
cq_sz – Size of the completion queue, must be power of 2

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_WITH_MEMPOOL(name, exec, sq_sz, cq_sz, num_blks, blk_size, balign)

Statically define and initialize an RTIO context.

Parameters:

name – Name of the RTIO
exec – Symbol for rtio_executor (pointer)
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

Enums

enum rtio_mempool_entry_state

Internal state of the mempool sqe map entry.

Values:

enumerator RTIO_MEMPOOL_ENTRY_STATE_FREE = 0: The SQE has no mempool buffer allocated

enumerator RTIO_MEMPOOL_ENTRY_STATE_ALLOCATED: The SQE has an active mempool buffer allocated

enumerator RTIO_MEMPOOL_ENTRY_STATE_ZOMBIE: The SQE has a mempool buffer allocated that is currently owned by a CQE

enumerator RTIO_MEMPOOL_ENTRY_STATE_COUNT

Functions

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.

RTIO SPSC API 

group rtio_spsc

RTIO Single Producer Single Consumer (SPSC) Queue API.

Defines

RTIO_SPSC_INITIALIZER(sz, buf)

Statically initialize an rtio_spsc.

Parameters:

sz – Size of the spsc, must be power of 2 (ex: 2, 4, 8)
buf – Buffer pointer

RTIO_SPSC_DECLARE(name, type)

Declare an anonymous struct type for an rtio_spsc.

Parameters:

name – Name of the spsc symbol to be provided
type – Type stored in the spsc

RTIO_SPSC_DEFINE(name, type, sz)

Define an rtio_spsc with a fixed size.

Parameters:

name – Name of the spsc symbol to be provided
type – Type stored in the spsc
sz – Size of the spsc, must be power of 2 (ex: 2, 4, 8)

rtio_spsc_size(spsc)

Size of the SPSC queue.

Parameters:

spsc – SPSC reference

rtio_spsc_reset(spsc)

Initialize/reset a spsc such that its empty.

Note that this is not safe to do while being used in a producer/consumer situation with multiple calling contexts (isrs/threads).

Parameters:

spsc – SPSC to initialize/reset

rtio_spsc_acquire(spsc)

Acquire an element to produce from the SPSC.

Parameters:

spsc – SPSC to acquire an element from for producing

Returns:

A pointer to the acquired element or null if the spsc is full

rtio_spsc_produce(spsc)

Produce one previously acquired element to the SPSC.

This makes one element available to the consumer immediately

Parameters:

spsc – SPSC to produce the previously acquired element or do nothing

rtio_spsc_produce_all(spsc)

Produce all previously acquired elements to the SPSC.

This makes all previous acquired elements available to the consumer immediately

Parameters:

spsc – SPSC to produce all previously acquired elements or do nothing

rtio_spsc_drop_all(spsc)

Drop all previously acquired elements.

This makes all previous acquired elements available to be acquired again

Parameters:

spsc – SPSC to drop all previously acquired elements or do nothing

rtio_spsc_consume(spsc)

Consume an element from the spsc.

Parameters:

spsc – Spsc to consume from

Returns:

Pointer to element or null if no consumable elements left

rtio_spsc_release(spsc)

Release a consumed element.

Parameters:

spsc – SPSC to release consumed element or do nothing

rtio_spsc_release_all(spsc)

Release all consumed elements.

Parameters:

spsc – SPSC to release consumed elements or do nothing

rtio_spsc_acquirable(spsc)

Count of acquirable in spsc.

Parameters:

spsc – SPSC to get item count for

rtio_spsc_consumable(spsc)

Count of consumables in spsc.

Parameters:

spsc – SPSC to get item count for

rtio_spsc_peek(spsc)

Peek at the first available item in queue.

Parameters:

spsc – Spsc to peek into

Returns:

Pointer to element or null if no consumable elements left

rtio_spsc_next(spsc, item)

Peek at the next item in the queue from a given one.

Parameters:

spsc – SPSC to peek at
item – Pointer to an item in the queue

Returns:

Pointer to element or null if none left

rtio_spsc_prev(spsc, item)

Get the previous item in the queue from a given one.

Parameters:

spsc – SPSC to peek at
item – Pointer to an item in the queue

Returns:

Pointer to element or null if none left

struct rtio_spsc: Common SPSC attributes.

Warning

Not to be manipulated without the macros!