lan78xx_con..._flowcontrol(9)

💅 lan78xx_configure_flowcontrol — the Kernel API Manual glow-up 🎀

okay so bestie, welcome to the LAN78xx flow control page ✨ this is from the Kernel API Manual and it's literally all about telling the chip when to send pause frames and when to chill. stay with me, it's giving hardware tea ☕

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✨ NAME ✨

lan78xx_configure_flowcontrol — set MAC and FIFO flow control configuration 🦋

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🌸 SYNOPSIS 🌸

int lan78xx_configure_flowcontrol (struct lan78xx_net *dev , bool
tx_pause , bool rx_pause );

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🧚 ARGUMENTS 🧚

• dev — pointer to the LAN78xx device structure. that's your main character right there 👑
• tx_pause — enable transmission of pause frames (yes, we're sending them 📤)
• rx_pause — enable reception of pause frames (and yes, we're receiving them 📥)

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🔥 DESCRIPTION 🔥

So this function configures the LAN78xx flow control settings by writing to the FLOW and FCT_FLOW registers. no cap, that's the whole job 💅

The pause time is set to the maximum allowed value (65535 quanta). FIFO thresholds are selected based on USB speed. slay.

The Pause Time field is measured in units of 512-bit times (quanta):

• At 1 Gbps: 1 quanta = 512 ns → max ~33.6 ms pause ⚡
• At 100 Mbps: 1 quanta = 5.12 µs → max ~335 ms pause 🌸
• At 10 Mbps: 1 quanta = 51.2 µs → max ~3.3 s pause 🐢

Flow control thresholds (FCT_FLOW) are used to trigger pause/resume:

• RXUSED is the number of bytes used in the RX FIFO
• Flow is turned ON when RXUSED ≥ FLOW_ON threshold 🟢
• Flow is turned OFF when RXUSED ≤ FLOW_OFF threshold 🔴
• Both thresholds are encoded in units of 512 bytes (rounded up) 🧮

Thresholds differ by USB speed because available USB bandwidth affects how fast packets can be drained from the RX FIFO:

• USB 3.x (SuperSpeed): FLOW_ON = 9216 bytes → 18 units, FLOW_OFF = 4096 bytes → 8 units 🚀
• USB 2.0 (High-Speed): FLOW_ON = 8704 bytes → 17 units, FLOW_OFF = 1024 bytes → 2 units 🐌

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💀 NOTE 💀

The FCT_FLOW register must be configured before enabling TX pause (i.e., before setting FLOW_CR_TX_FCEN_), as required by the hardware. like, the hardware said what it said and we are NOT arguing 🙈 do it in that order, fr fr.

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⭐ RETURN ⭐

0 on success or a negative error code on failure. that's it, that's the vibe check ✅

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🌈 SEE ALSO 🌈

Kernel file ./drivers/net/usb/lan78xx.c, plus these icons:

• lan78xx_phy_int_ack(9)
• lan78xx_configure_usb(9)
• lan78xx_mac_eee_enable(9)
• lan78xx_set_fixed_link(9)
• lan78xx_get_phy(9)
• lan78xx_mac_prepare_for_phy(9)
• lan78xx_configure_leds_from_dt(9)

go read them, iykyk 🫶

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and that's the whole page, bestie 💜 this lil' icon lives in ./drivers/net/usb, dated October 2026 — go off, periodt 💅✨