SPI(9)

💅 SPI(9) — the Serial Peripheral Interface kernel interface, bestie edition 🎀

okay so this is the SPI(9) page from the Kernel Developer's Manual, and it's literally your whole guide to talking to SPI devices from inside the kernel. no cap, it's a good one. let's get into it ✨

---

🌸 NAME 🌸

spi_configure, spi_transfer, spi_transfer_init, spi_chunk_init, spi_transfer_add, spi_wait, spi_done, spi_send, spi_recv, spi_send_recv — Serial Peripheral Interface (SPI) kernel interface 🫶

---

✨ SYNOPSIS ✨

like, here's the whole cast, in their natural habitat:

#include <dev/spi/spivar.h>

int
spi_configure(struct spi_handle *sh, int mode, int speed);

int
spi_transfer(struct spi_handle *sh, struct spi_transfer *st);

void
spi_transfer_init(struct spi_transfer *st);

void
spi_chunk_init(struct spi_chunk *chunk, int cnt, const uint8_t *wptr,
    uint8_t *rptr);

void
spi_transfer_add(struct spi_transfer *st, struct spi_chunk *chunk);

void
spi_wait(struct spi_transfer *st);

void
spi_done(struct spi_transfer *st, int err);

int
spi_recv(struct spi_handle *sh, int cnt, uint8_t *data);

int
spi_send(struct spi_handle *sh, int cnt, const uint8_t *data);

int
spi_send_recv(struct spi_handle *sh, int scnt, const uint8_t *snd,
    int rcnt, const uint8_t *rcv);

---

🦋 DESCRIPTION 🦋

so, SPI? it's a 4-wire synchronous full-duplex serial bus 💫 and it's the moment for hooking up stuff like EEPROMs, displays, and honestly just, like, a whole aesthetic of integrated circuits. the spi interface is literally the way you communicate with SPI-connected devices. it's giving matchmaker, honestly. 🔥

---

💜 FUNCTIONS 💜

these are the functions that make up the API you get, as a driver author, for SPI-connected devices. main character energy, all of them. 👑

the struct spi_handle that corresponds to your device gets passed in through the driver attachment. (iykyk 🫶)

• spi_configure(sh, mode, speed) — sets mode and speed for all the communication that comes after this, with a SPI slave. set the vibe, then go. ✨
• spi_transfer(sh, st) — queues a transfer to the SPI controller. and if it couldn't be queued? it hands you back an errno value, so you're not left guessing. 😭
• spi_transfer_init(st) — initializes a transfer structure. fresh start, blank slate, new era. 🎀
• spi_chunk_init(chunk, cnt, wptr, rptr") — initializes a chunk structure. every chunk is a possibly bi-directional transfer, where the same amount of bytes gets shifted in and out. balanced. symmetric. iconic. ⚖️
• spi_transfer_add(st, chunk) — appends a chunk onto a transfer structure. little chunk, big transfer, we love a team effort. 🫶
• spi_wait(st) — waits for a transfer to complete. and if the transfer failed for some reason? the field st->st_errno gets set to a non-zero value, so you will know. 💀
• spi_done(st, err) — this is called back by the machine-dependent backend to signal that a transfer completed. the backend is basically tapping you on the shoulder like "hey bestie, it's done". 🔔

---

for simplicity there are also convenience functions that bundle up common operations for you. these return an errno value if the transfer failed. slay, honestly. 🧚

• spi_recv(sh, cnt, data) — prepares a chunk for receiving data, queues a transfer, then waits for it to complete. the whole routine, no thinking required. 💖
• spi_send(sh, cnt, data) — prepares a chunk for sending data, queues a transfer, then waits for it to complete. same energy, opposite direction. 🌈
• spi_send_recv(sh, scnt, snd, rcnt, rcv) — prepares two chunks: one for sending data first, then one for receiving an answer. queues a transfer and waits for it to complete. ⭐

heads up bestie: this is not a full-duplex operation. it's send, then receive. do not confuse the two, i'm begging. 🙈

---

👀 SEE ALSO 👀

• spi(4)

---

🕰️ HISTORY 🕰️

the spi API first appeared in NetBSD 4.0. a legacy. a pioneer. we stan. 👑

---

and that's the page, bestie!! 💅 mwah 💋 — this little gem came to you courtesy of Debian, dated February 23, 2019, and yes i'm absolutely signing off with that because she deserves her credit. 🍓🌸