RTSX(4)

💳 RTSX(4) — Realtek SD card reader, but make it understood

okay bestie, so this is a kernel interfaces manual page for rtsx(4), and it's genuinely not that deep once I explain it. you got this 💖

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✨ NAME ✨

rtsx — the Realtek SD card reader driver. that's it. that's the whole vibe.

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🌸 SYNOPSIS 🌸

so like, you want this bad boy inside your kernel? then drop these lines into the kernel configuration file, no cap:

device mmc
device mmcsd
device rtsx

or, alternatively, if you'd rather load it as a module at boot time — which, honestly, is giving ✨main character energy✨ — you put these into loader.conf(5):

mmc_load="YES"
mmcsd_load="YES"
rtsx_load="YES"

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🎀 DESCRIPTION 🎀

the rtsx driver provides support for the Realtek SD card reader. the way it works is genuinely so clean: the driver attaches the mmc bus on card insertion and detaches it on card removal. in and out, periodt. 🦋

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👑 HARDWARE 👑

the rtsx driver supports the following Realtek SD card readers, and I'm listing every single one because we don't skip:

• RTS5209
• RTS5227
• RTS5229
• RTS522A
• RTS525A
• RTS5260
• RTL8411B
• RTS5249 (unverified)
• RTL8402 (unverified)
• RTL8411 (unverified)

the three marked (unverified) are literally the original page saying "unverified" — I'm just the messenger, bestie 💅

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🔗 SEE ALSO 🔗

• mmc(4)
• mmcsd(4)
• SD Specifications, Part 2, SD Host Controller, Simplified Specification
• SanDisk Secure Digital Card

like, go read those if you want the full lore 🧚

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🦋 HISTORY 🦋

the rtsx driver appeared in FreeBSD 13.0 and was ported from OpenBSD with modifications found in Linux and NetBSD. so it's basically a collab across four operating systems — iconic behavior honestly 🌈

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💜 AUTHORS 💜

shoutout to the people who actually built this:

• Henri Hennebert <hlh@restart.be>
• Gary Jennejohn <gj@freebsd.org>
• Jesper Schmitz Mouridsen <jsm@FreeBSD.org>

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🫶 CONTRIBUTORS 🫶

and one more legend:

• Lutz Bichler <Lutz.Bichler@gmail.com>

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🔥 DEBUGGING INFORMATION 🔥

so if something's acting cursed, dev.rtsx.0.debug_mask can be set with the following masks — pick your fighter:

• 0x01 — to show the basic flow of the driver,
• 0x02 — to trace the SD commands,
• 0x04 — to trace the tuning phase.

ngl this is genuinely such a cute little feature, iykyk 🍓

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💀 BUGS 💀

okay. this is the part where you sit down and take notes, because some of this is actually scary 😭

• RTS522A on Lenovo T470p: card detection and read-only switch are reversed. this is solved by adding in loader.conf(5):

dev.rtsx.0.inversion=1

the driver tries to automate those exceptions. if this automation is wrong, it can be avoided by adding in loader.conf(5):

dev.rtsx.0.inversion=0

• Mounting a filesystem with write access on a card that's write protected may involve a kernel crash. yeah. a crash. like actually. be careful 🚨
• Suspend/Resume do not work under MMCCAM. not great, not terrible, but definitely not great.
• For some chips (e.g. RTS5260), after devctl disable/enable or kldunload/kldload, the driver can't detect a card correctly. so, something something, it gets confused. 🙈

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and that's the whole page, bestie!! no skipped flags, no skipped warnings, no skipped emails — we kept it all 💖

this iconic little writeup came from the Debian edition of the manual, dated May 26, 2025, and it's filed under RTSX(4) in the Kernel Interfaces Manual — go forth and read your SD cards 🎀✨