SMB(4)

💅 SMB(4) — the Kernel Interfaces Manual girlies know

okay so bestie, this is the smb page from the Kernel Interfaces Manual, and it's literally all about the SMB generic I/O device driver. strap in 🎀

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✨ NAME ✨

smb — SMB generic I/O device driver

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🌸 SYNOPSIS 🌸

device smb

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🦋 DESCRIPTION 🦋

The smb character device driver provides generic I/O to any smbus(4) instance. no cap, any of them. 💜

To control SMB devices, you use /dev/smb? with the ioctls described below. Every single one of these ioctl commands takes a pointer to struct smbcmd as its argument. yes, all of them, it's giving consistent 💅

#include <sys/types.h>

struct smbcmd {
        u_char cmd;
        u_char reserved;
        u_short op;
        union {
                char    byte;
                char    buf[2];
                short   word;
        } wdata;
        union {
                char    byte;
                char    buf[2];
                short   word;
        } rdata;
        int  slave;
        char *wbuf;     /* use wdata if NULL */
        int  wcount;
        char *rbuf;     /* use rdata if NULL */
        int  rcount;
};

The slave field is always used — like, every time, it's not optional, it's the main character. It gives you the address of the SMBus slave device. 🫶

⚠️ the slave address is specified in the seven most significant bits (i.e., "left-justified"), and the least significant bit of the slave address must be zero. periodt.

okay now the actual menu of ioctls, bestie. here's what each one does 👑

• SMB_QUICK_WRITE — QuickWrite does not transfer any data. it just issues the device address with write intent to the bus. that's it, that's the whole job 💀
• SMB_QUICK_READ — QuickRead does not transfer any data either. it just issues the device address with read intent to the bus. twin behavior, different intent ✨
• SMB_SENDB — SendByte sends the byte provided in cmd to the device.
• SMB_RECVB — ReceiveByte reads a single byte from the device, and that byte is returned in cmd.
• SMB_WRITEB — WriteByte first sends the byte from cmd to the device, followed by the byte given in wdata.byte.
• SMB_WRITEW — WriteWord first sends the byte from cmd to the device, followed by the word given in wdata.word. note that the SMBus byte-order is little-endian by definition — that's not a choice, that's just the rules 🎀
• SMB_READB — ReadByte first sends the byte from cmd to the device, then reads one byte of data from the device. the returned data is stored in rdata.byte.
• SMB_READW — ReadWord first sends the byte from cmd to the device, then reads one word of data from the device. the returned data is stored in rdata.word.
• SMB_PCALL — ProcedureCall first sends the byte from cmd to the device, followed by the word provided in wdata.word. then it reads one word of data from the device and returns it in rdata.word. it's giving full round trip 🔥
• SMB_BWRITE — BlockWrite first sends the byte from cmd to the device, then the byte from wcount, followed by wcount bytes of data taken from the buffer pointed to by wbuf.

heads up bestie: the SMBus specification mandates that no more than 32 bytes of data can be transferred in a single block read or write command. you can read that value from the constant SMB_MAXBLOCKSIZE. don't push it 💀

• SMB_BREAD — BlockRead first sends the byte from cmd to the device, then reads a count of data bytes that the device is going to provide, and then reads that many bytes. the count is returned in rcount. the data is returned in the buffer pointed to by rbuf.

real talk: the read(2) and write(2) system calls are not implemented by this driver. like, at all. don't try it 🙈

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😭 ERRORS 😭

The ioctl(2) commands can cause the following driver-specific errors — these are the specific ones this driver can throw at you:

• [ENXIO] — Device did not respond to selection. it's ghosting you 💀
• [EBUSY] — Device still in use.
• [ENODEV] — Operation not supported by device (not supposed to happen).
• [EINVAL] — General argument error.
• [EWOULDBLOCK] — SMBus transaction timed out.

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👑 SEE ALSO 👑

ioctl(2), smbus(4)

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🌈 HISTORY 🌈

The smb manual page first appeared in FreeBSD 3.0. iconic from day one ✨

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🫶 AUTHORS 🫶

This manual page was written by Nicolas Souchu and extended by Michael Gmelin <freebsd@grem.de>. give them their flowers 💐

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and that's the whole page, bestie!! this one came from Debian, dated April 25, 2015 — so yeah, vintage but still serving. mwah 💋