linux-kernel - Re: [RFC 2/2] serial/8250.c: Use self-adjusting list for port poll order.

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Message-ID: <20081115001013.GM25117@mit.edu>
Date:	Fri, 14 Nov 2008 19:10:13 -0500
From:	Theodore Tso <tytso@....edu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	George Spelvin <linux@...izon.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/2] serial/8250.c: Use self-adjusting list for port poll
	order.

On Fri, Nov 14, 2008 at 09:47:11PM +0000, Alan Cox wrote:
> > This code does that by using the previous poll cycles as a hint.
> > If a port is idle, it will migrate to the end of the list and
> > only have to be checked once.
> > 
> > Part 1 changed the list to singly-linked to make the list shuffling easier.
> > 
> > Comments?  Please?
> 
> Is it really worth the complexity
> 
> - PCI ports are shared IRQ always
> - Legacy ports are almost never shared IRQ on the LPC bus (and are
> increasingly going away)

It's worth the complexity only *if* you have enough ports shared on a
single IRQ and simultaneously such that there a risk that if you don't
poll them quickly enough, characters will actually get dropped from
the UART's FIFO.  The question is whether that is likely to happen on
modern CPU's.  I worred about such things when I tried to make 16
115kbps serial ports work at full-speed using relatively primitive
16550A UART's with 16 character FIFO's on a 40 MHz 386.  But (a)
UART's generally have deeper FIFO's these days, and (b) CPU's have
gotten a wee bit faster since 1992.

So color me dubious that this is actually necessary....

							- Ted
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