linux-kernel - Re: [PATCH] libata-sff: fix 32-bit PIO regression

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Date:	Mon, 9 Feb 2009 00:48:07 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Sergei Shtylyov <sshtylyov@...mvista.com>
Cc:	Hugh Dickins <hugh@...itas.com>, jgarzik@...ox.com,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	alan@...rguk.ukuu.org.uk, rjw@...k.pl
Subject: Re: [PATCH] libata-sff: fix 32-bit PIO regression

Sergei Shtylyov writes:
 > Hello.
 > 
 > Hugh Dickins wrote:
 > 
 > >> Commit 871af1210f13966ab911ed2166e4ab2ce775b99d (libata: Add 32bit PIO support)
 > >> caused all kind of errors on the ATAPI devices, so it's been empirically proven
 > >> that one shouldn't read/write an extra data word when a device isn't expecting
 > >> it already. "Don't do it then"; however still taking a chance to use 32-bit I/O
 > >> one last  time when there are exactly 3 trailing bytes.  Oh, and stop pointless
 > >> swapping bytes to and fro as well by using io*_rep() which shouldn't byte-swap.
 > >>
 > >> This should fix the kernel.org bug #12609.
 > >>
 > >> ---
 > >> This is hopefully better replacement for Hugh Dickins most recent patch
 > >> (http://marc.info/?l=linux-ide&m=123352294619281)...
 > >>     
 > >
 > > Yes, looks nice, and works for me.  My only criticism would be,
 > > minor issue unchanged by your patch, that actually "slop" isn't
 > > unlikely enough to deserve an "unlikely" - slop of 1 or 3 is
 > > unlikely, but slop of 2 is quite common.
 > >   
 > 
 >    I guess that's the case only for the ATAPI devices, so I'm going to 
 > keep it.

I really don't think it's a good idea to play micro-optimisation
unlikely() tricks when the likelihood depends on what kind of device
the driver is talking to. In this case unlikely() is clearly wrong.
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