serial8250_em485_start_tx(9)

📦 version 7.3.1, in nixpkgs unstable

✨ serial8250_em485_start_tx(9) ✨

okay bestie, welcome to the Kernel API Manual 💖 this one's a lil function called serial8250_em485_start_tx and honestly? she's a whole vibe. let's get into it 🫶

🌸 NAME 🌸

serial8250_em485_start_tx — the generic ->rs485_start_tx() callback 🎀 no cap, that's literally her whole job description.

🦋 SYNOPSIS 🦋

void serial8250_em485_start_tx (struct uart_8250_port *up , bool
toggle_ier );

like yes, she returns void (nothing back, she's not about that return-value life 💅) and she takes two args, we'll get to those 👇

⭐ ARGUMENTS ⭐

  • up — the uart 8250 port 🎀 that's your port struct, the whole point of the operation.
  • toggle_ier — a bool; pass true if you're cool with receive interrupts getting disabled 🙈 like that's the permission slip, bestie.

💜 DESCRIPTION 💜

so this is the generic callback that 8250 uart drivers can use to kick off rs485 transmission 🔥. it's giving reusable, it's giving icon, it's giving "here, let me handle the start-of-tx part for you."

but listen — she makes some assumptions, and you gotta know them before you commit, fr fr:

  • she assumes that setting the RTS bit in the MCR register means RTS is high 🫶 that's the whole semantic model here. heads up though: some chips use inverse semantics, so if yours does, this assumption is not your girl 😭
  • she further assumes that reception is stoppable by disabling the UART_IER_RDI interrupt ✨ cute and clean in theory. except… some chips go ahead and set the UART_LSR_DR bit even when UART_IER_RDI is disabled, which straight-up foils this approach 💀 so yeah, chip-dependent, check your datasheet bestie.
it's a handy little helper, but she's built on those two assumptions — if your hardware doesn't play by those rules, she can't slay for you 🧚

🌈 SEE ALSO 🌈

like the whole em485 friend group, honestly — init, destroy, config, and stop_tx are all in the group chat 💬

aaaand that's the page!! 🌸 this lil manual page lives over in ./drivers/tty/serial/8250 and it's dated October 2026 — so go forth, start that transmission, and remember: assumptions are real, chips are chaotic, and you're doing amazing sweetie 💖🎀🦋⭐💅

📦 where this page comes from