Re: [PATCH v4] serial: amba-pl011: don't wait for BUSY after every earlycon character

[Greg KH]

On Mon, Jul 20, 2026 at 12:26:46PM +0000, Eric Curtin wrote:
> An AI coding tool (OpenCode CLI, using Claude as the backing model) was
> used in preparing this patch. It was given the observation that
> earlycon output was slower than expected under virtualization and
> asked to locate the cause and propose a fix; it identified the
> per-character BUSY wait in pl011_putc() and, after being pointed at
> pl011_console_write_atomic()/_thread() as the existing precedent for a
> single final drain, produced the code that moves the wait into
> pl011_early_write(). It also helped research the QDF2400 erratum 44
> history cited above and draft this changelog. All of the above was
> reviewed by hand against the driver's other console write paths to
> confirm correctness. The boot-timing measurements and the line-for-line
> console diff were run and captured by hand on the VMM described above;
> they were not produced by the tool.

This means an Assisted-by line should be added.

Also, your changelog should be a lot smaller, LLMs love to talk a lot...


> 
> Signed-off-by: Eric Curtin <ericcurtin17@xxxxxxxxx>
> ---
> v4: v3 mistakenly replaced this patch's actual fix (moving the BUSY
>     wait to run once in pl011_early_write(), preserving the "fully
>     drained on return" guarantee) with a plain deletion of the wait,
>     which would have been a functional regression (e.g. for panic
>     output immediately followed by reboot/poweroff). This reverts to
>     the v2 fix, keeps the real, measured boot-timing numbers, and adds
>     the explicit disclosure of AI-tool assistance that was missing from
>     v2, per Documentation/process/generated-content.rst.
> v3: (erroneous, superseded by the above; sent by mistake)
> v2: Rather than simply deleting the wait, move it out of the
>     per-character pl011_putc() and into pl011_early_write(), done once
>     after the whole buffer is written (mirroring the existing pattern in
>     pl011_console_write_atomic()/_thread()), so earlycon keeps its
>     "fully transmitted by the time this call returns" guarantee. Also
>     added the QDF2400 erratum 44 history as context for why the BUSY
>     wait existed, and re-measured with the revised patch (numbers
>     updated accordingly, same conclusion).
> 
>  drivers/tty/serial/amba-pl011.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 8ed91e1da22b..05a783dda4c4 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
> @@ -2741,8 +2741,6 @@ static void pl011_putc(struct uart_port *port, unsigned char c)
>  		writel(c, port->membase + UART01x_DR);
>  	else
>  		writeb(c, port->membase + UART01x_DR);
> -	while (readl(port->membase + UART01x_FR) & UART01x_FR_BUSY)
> -		cpu_relax();
>  }
>  
>  static void pl011_early_write(struct console *con, const char *s, unsigned int n)
> @@ -2750,6 +2748,20 @@ static void pl011_early_write(struct console *con, const char *s, unsigned int n
>  	struct earlycon_device *dev = con->data;
>  
>  	uart_console_write(&dev->port, s, n, pl011_putc);
> +
> +	/*
> +	 * Wait for the last character to be fully transmitted before
> +	 * returning, same as pl011_console_write_atomic()/_thread() do for
> +	 * the non-early console. There is no need to do this after every
> +	 * character in pl011_putc(): checking TXFF there already prevents
> +	 * overrunning the FIFO, and waiting for BUSY per character forces
> +	 * the UART to be drained serially instead of letting it buffer
> +	 * queued bytes, which is needlessly slow, especially so under
> +	 * virtualization where each poll of UARTFR/UARTDR is a trapped MMIO
> +	 * access.
> +	 */

And this comment is also really not needed, right?

thanks,

greg k-h