Re: [PATCH v2] serial: amba-pl011: don't wait for BUSY after every earlycon character

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jul 03, 2026 at 07:38:04PM +0000, Eric Curtin wrote:
> pl011_putc(), used exclusively by the pl011 earlycon (pl011_early_write()
> -> uart_console_write()), waits for UART01x_FR_TXFF to clear before
> writing a character (correct: don't overrun the TX FIFO) and then *also*
> busy-waited for UART01x_FR_BUSY to clear before returning, i.e. it waited
> for the character to be fully shifted out on the wire before the next
> character in the string could even be considered.
> 
> Waiting for BUSY per character defeats the TX FIFO: instead of letting
> the UART buffer several queued bytes and transmit them back to back,
> every single character printed through earlycon was forced to wait for
> that character's own complete transmission (a full UART bit-time at the
> configured baud rate) before the driver would even look at writing the
> next one. This is wasted time on real hardware, and it is much worse
> under virtualization: each read of UARTFR and each write to UARTDR is
> an MMIO access that traps to the hypervisor, so every extra poll is a
> full VM-exit/entry round trip.
> 
> git blame on this function is unhelpful (this tree's history stops at a
> shallow-clone boundary), but the equivalent history is available from
> the upstream patch that added the QDF2400 erratum 44 workaround:

So this was generated by a LLM, right?  Otherwise why can't you find
where this changed by NOT doing a shallow clone?

When using a LLM, you must document that.

thanks,

greg k-h