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

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Message-ID: <498F72C8.8020204@ru.mvista.com>
Date:	Mon, 09 Feb 2009 03:03:20 +0300
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Mikael Pettersson <mikpe@...uu.se>
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

Hello

Mikael Pettersson wrote:
> 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.
>   

   Clearly?! :-O
   Do you really think that the transfers having lengths non-divisible 
by 4 make any *significant* percentage even on the ATAPI devices? I 
think it's you who is really wrong.

MBR, Sergei


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