Re: [PATCH v9 5/8] serial: core: allow drivers to reserve flushed transmit storage

[Andy Shevchenko]

On Mon, Nov 02, 2026 at 04:14:32PM -0700, James Hilliard wrote:
> uart_flush_buffer() resets the transmit FIFO before calling the driver's
> flush callback. An asynchronous DMA transfer can still access that storage
> after the callback returns, while a new write can immediately reuse it.
> 
> Add uart_xmit_block() and uart_xmit_unblock() so a driver can reserve the
> flushed storage until its transfer and completion callbacks have retired.
> Serialize the reservation with FIFO producers using the port lock.
> 
> While the storage is reserved, write_room() reports no space and write()
> and put_char() accept no data. Defer write-ready notifications until the
> reservation is released, preserving one-shot TTY wakeups. Keep the flush's
> write-ready notification under the port lock so it cannot race another
> reservation.
> 
> Wake tty->write_wait after flushing even when writes remain blocked.
> tty_wait_until_sent() waits for the logical FIFO to empty, independently
> of whether its storage can be reused. Notify these drain waiters without
> invoking the line discipline's write_wakeup() callback, so a reservation
> does not prevent a completed logical flush from waking a drain waiter or
> consume a one-shot write-ready notification prematurely.
> 
> Suppress ordinary TX starts while the FIFO storage is blocked. Preserve
> high-priority flow-control characters, which use separate storage.
> Releasing the reservation wakes writers through the existing TTY wakeup
> path. This preserves immediate logical flushing while applying write
> backpressure until the storage is reusable.
> 
> Track port readiness separately from DMA ownership. Retirement releases the
> reservation even during startup or shutdown, while writes and wakeups stay
> gated until successful startup or resume. Reopening or restarting a port
> wakes deferred writers with either a new or retained FIFO. Restart queued
> output when reusing a FIFO after suspension. Keep its write-ready
> notification pending until the retained output drains below the normal
> wakeup threshold.
> 
> Publish resume readiness under the same port lock as the TX restart,
> without clearing any reservation. A concurrent flush can then reserve a
> new transfer without resume releasing its storage prematurely.
> 
> Assert the port-lock requirement at uart_write_wakeup(), alongside the
> reservation helpers, so callers cannot inspect readiness or release a
> one-shot notification concurrently with a reservation change.

...

> +	/* Retained output must drain before consuming a write-ready notification. */
> +	if (kfifo_len(&state->port.xmit_fifo) < WAKEUP_CHARS)
> +		uart_write_wakeup(port);

I believe I have seen this already a few times. Perhaps a helper with
documented idea behind it?

-- 
With Best Regards,
Andy Shevchenko