[PATCH RFC 6/6] arm64: wait for transmit completion before next character transmission

[Geoff Levand]

On Thu, 2015-04-16 at 22:17 +0530, Pratyush Anand wrote:
> Previous transmission must be completed before next character to be
> transmitted, otherwise TX buffer may saturate and we will not see all
> the characters on screen.
> 
> @@ -34,6 +38,8 @@ static const char arm64_opts_usage[] __attribute__ ((unused)) =
>  "     --lite                Fast reboot, no memory integrity checks.\n"
>  "     --page-offset         Kernel page-offset for binary image load.\n"
>  "     --port=ADDRESS        Purgatory output to port ADDRESS.\n"
> +"     --port-lsr=ADDRESS    Purgatory output port line status ADDRESS.\n"
> +"     --port-lsr-val=VALUE  Purgatory output port Line status expected SET value when TX empty.\n"
>  "     --ramdisk=FILE        Use FILE as the kernel initial ramdisk.\n"
>  "     --reuse-cmdline       Use command line arg of primary kernel.\n";

We just need to put some chars to the screen, so I want to avoid a lot
of parameters.

Usually the status register is at a fixed offset from the TX port.  Is
that not the case for the ARM uarts?

Can't we just poll port-lsr until empty then start writing again?  That
will at least eliminate port-lsr-val.

-Geoff