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From: Paul Brook <paul@codesourcery.com>
Date: Wed, 5 May 2010 15:10:48 +0100
References: <1273009170-17530-1-git-send-email-amit.shah@redhat.com>
	<201005051434.20020.paul@codesourcery.com>
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Subject: [Qemu-devel] Re: [PATCH v7 4/6] char: Add qemu_chr_write_nb() for
	nonblocking writes
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Amit Shah <amit.shah@redhat.com>, Juan Quintela <quintela@redhat.com>, qemu list <qemu-devel@nongnu.org>, Gerd Hoffmann <kraxel@redhat.com>

> On 05/05/2010 08:34 AM, Paul Brook wrote:
> >> I really dislike the idea of adding another function for this.  Can you
> >> explain why you need this functionality for virtio-console and why this
> >> functionality isn't needed for everything else?
> > 
> > This functionality should (in principle) be used by all serial port
> > implementations.
> > 
> > Physical serial ports are sufficiently crufty and low-performance that
> > noone actually uses them nowadays.  I expect that the only significant
> > real-world use is for serial consoles, which never send enough data to
> > care that writes stall the whole machine.
> 
> We don't implement control flow in the character driver layer today.
> Different backends use different policies.  Some drop data (like pty)
> while other block (like tcp).

Really? I thought data was only dropped when no client was connected, and that 
there was a user visible option to control this.  Either way, I agree that 
this should be done consistently.
 
> This patch adds optional control flow in a pretty crufty way to *some*
> backends but not all.  This just adds a bunch of complexity that will
> certainly introduce bugs.

I admit I've only really looked at the device emulation side of the interface, 
not the chardev backend implementation.

Paul