s390_pci_mmio_write(2)

💖 s390_pci_mmio_write(2) — the manual page, but make it make sense ✨

okay bestie, gather round, because today we're talking about s390_pci_mmio_write and s390_pci_mmio_read — two syscalls from the System Calls Manual that live on Linux on s390 (that's IBM System z, the Big Iron™ 💅). They move data between your user-space buffer and a PCI MMIO memory page. Yes, it's giving main-character energy for mainframes. Let's go. 🎀

---

✨ NAME ✨

• s390_pci_mmio_write, s390_pci_mmio_read — transfer data to/from PCI MMIO memory page 🌸

---

📚 LIBRARY 📚

Standard C library — that's libc, -lc. Nothing exotic, she's just built different. 💖

---

🫶 SYNOPSIS 🫶

#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>

int syscall(size_t length;
            SYS_s390_pci_mmio_write, unsigned long mmio_addr,
            const void user_buffer[length], size_t length);
int syscall(size_t length;
            SYS_s390_pci_mmio_read, unsigned long mmio_addr,
            void user_buffer[length], size_t length);

⚠️ Note: glibc provides no wrappers for these system calls, necessitating the use of syscall(2). So yeah, you're raw-dogging the syscall interface. No cute libc helper is coming to save you. 😭

---

💜 DESCRIPTION 💜

The s390_pci_mmio_write() system call writes length bytes of data from the user-space buffer user_buffer to the PCI MMIO memory location specified by mmio_addr. 🦋

The s390_pci_mmio_read() system call reads length bytes of data from the PCI MMIO memory location specified by mmio_addr to the user-space buffer user_buffer. 💫

These system calls must be used instead of the simple assignment or data-transfer operations that are used to access the PCI MMIO memory areas mapped to user space on the Linux System z platform. Like, no cap, don't try to be cute with a plain pointer deref — that's not the vibe here. 🔥

The address specified by mmio_addr must belong to a PCI MMIO memory page mapping in the caller's address space, and the data being written or read must not cross a page boundary. And the length value cannot be greater than the system page size. Periodt. 👑

---

🍓 RETURN VALUE 🍓

On success, s390_pci_mmio_write() and s390_pci_mmio_read() return 0. Slay. ✨

On failure, -1 is returned and errno is set to indicate the error. Boo. 💀

---

🙈 ERRORS 🙈

• EFAULT — The address in mmio_addr is invalid.
• EFAULT — user_buffer does not point to a valid location in the caller's address space.
• EINVAL — Invalid length argument.
• ENODEV — PCI support is not enabled.
• ENOMEM — Insufficient memory.

(Yes, EFAULT shows up twice with two different explanations. She contains multitudes. 💅)

---

👑 STANDARDS 👑

Linux on s390. 🌈

---

🕰️ HISTORY 🕰️

Linux 3.19. System z EC12. Vintage but still iconic. ⭐

---

🌟 SEE ALSO 🌟

• syscall(2)

---

💌 tiny sign-off: this lil masterpiece comes to you from Linux man-pages 6.20, dated 2026-02-08, and she is the s390_pci_mmio_write(2) page. That's the whole vibe check. iykyk 🧚 💜