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Subject: Re: Changes to UART ns8250
From: Ian Lepore <ian@FreeBSD.org>
To: Adrian Chadd <adrian@FreeBSD.org>
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Date: Mon, 07 Oct 2013 14:26:23 -0600
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Cc: Zbigniew Bodek <zbb@FreeBSD.org>,
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On Mon, 2013-10-07 at 13:15 -0700, Adrian Chadd wrote:
> Hi,
> 
> You should add:
> 
> *[snip]
> * a DELAY(1) or something.
> 

Why?  Why oh why do people write 
  
 while (!read_some_status_register())
    DELAY(n);

when DELAY() is implemented as, roughly:

  while (read_some_counter_register() < something)
     continue;

The whole point of DELAY() is to busy-wait.  

What might be nice is some function that we can call in such a loop such
as cpu_busywait(), so that when running in an emulated or virtualized
environment the emulator or hypervisor could use that as a hint to do
something smart.

-- Ian