sdio_f0_writeb(9)

✨ sdio_f0_writeb(9) ✨

okay bestie, bestie, BESTIE — gather round, because we're in the Kernel API Manual today and this page is a lil tiny one but it's literally so important, ngl 💅

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🎀 NAME 🎀

sdio_f0_writeb — write a single byte to SDIO function 0

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🦋 SYNOPSIS 🦋

void sdio_f0_writeb (struct sdio_func *func , unsigned char b , unsigned
int addr , int *err_ret );

like, that's the whole squad right there 🫶

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🌸 ARGUMENTS 🌸

• func — an SDIO function of the card. yes, the actual function struct, she's the main character here 👑
• b — the byte to write. just one. that's her whole job 🍓
• addr — the address you're writing to 📍
• err_ret — optional status value from the transfer. optional, but like... your bestie over here? she wants to know how it went 😭

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💖 DESCRIPTION 💖

So the vibe is: this writes a single byte into the address space of SDIO function 0. one byte, no more, no cap. and err_ret will contain the status of the actual transfer — so if you passed a pointer in, you get the tea on whether it worked. ⭐

IMPORTANT VIBE CHECK 🚨 only writes to the vendor specific CCCR registers (0xF0 - 0xFF) are permitted. if you try to write anywhere outside that range, err_ret gets set to -EINVAL and the kernel is like "girl. no." 💀

so yeah — stay in your lane (0xF0 to 0xFF), and everything's slay. wander out of it and you get -EINVAL. that's the whole boundary situation, periodt. 🌈

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🔥 SEE ALSO 🔥

Kernel file ./drivers/mmc/core/sdio_io.c, plus the whole friend group:

• sdio_claim_host(9)
• sdio_release_host(9)
• sdio_enable_func(9)
• sdio_disable_func(9)
• sdio_set_block_size(9)
• sdio_align_size(9)
• sdio_readb(9)
• sdio_writeb(9)
• sdio_writeb_readb(9)
• sdio_memcpy_fromio(9)
• sdio_memcpy_toio(9)
• sdio_readsb(9)
• sdio_writesb(9)
• sdio_readw(9)
• sdio_writew(9)
• sdio_readl(9)
• sdio_writel(9)
• sdio_f0_readb(9)
• sdio_get_host_pm_caps(9)
• sdio_set_host_pm_flags(9)
• sdio_retune_crc_disable(9)
• sdio_retune_crc_enable(9)
• sdio_retune_hold_now(9)
• sdio_retune_release(9)

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and that's the page!! 🧚 this lil icon of a manpage lives under ./drivers/mmc/core, it's from October 2026, and you can find it filed as sdio_f0_writeb(9) — go off, write that one byte, stay inside 0xF0 - 0xFF, and stan a kernel dev today 💜✨🎀