linux-kernel - Re: [PATCH][resend][SCSI] Reduce number of sequential pointer derefs in scsi

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Date:	Thu, 18 Nov 2010 22:21:24 +0100 (CET)
From:	Jesper Juhl <jj@...osbits.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	linux-scsi@...r.kernel.org,
	"James E.J. Bottomley" <James.Bottomley@...e.de>,
	linux-kernel@...r.kernel.org, Eric Youngdale <eric@...ante.org>,
	"David S. Miller" <davem@...emloft.net>,
	Mike Anderson <andmike@...ibm.com>,
	Russell King <rmk@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH][resend][SCSI] Reduce number of sequential pointer derefs
 in scsi_error.c and reduce size as well

On Thu, 18 Nov 2010, Linus Torvalds wrote:

> On Thu, Nov 18, 2010 at 12:49 PM, Jesper Juhl <jj@...osbits.net> wrote:
> >
> > Fair enough. Seeing your version of this and looking a second time at mine
> > I have to agree completely.
> > You are absolutely right in stating that the compiler will handle this
> > just fine, so it's only a readabillity issue and your version is a *lot*
> > more readable than what I came up with.
> 
> Btw, one thing to look out for is all those function calls: it really
> looks like many of them would be better off not having to dereference
> the thing inside each helper function, but just have it dereferenced
> in the caller.
> 
> You might trying passing in "struct Scsi_Host *host" to a lot of those
> helper functions in addition to the 'scmd' part. There's a lot of
> "scmd->device->host" going on, and even if you remove some of them
> _within_ a function, if you really want to get rid of them you should
> probably do one of them in the caller.
> 
> That's why the queuecommand() function was changed to take
> 
>     struct Scsi_Host *h, struct scsi_cmnd *
> 
> as its arguments, because that host is used so commonly. And passing
> two arguments is usually free (ie almost all architectures pass it in
> registers), and that host variable almost always already exists in the
> caller because the caller already needed it.
> 
> So if you changed the functions that only take "scsi_cmnd *" as an
> argument to match the new queuecommand() interface, I bet you'd get
> more cleanups. And the interfaces would match.
> 

That's a good point.

I don't have time to look at that tonight, but I'll (hopefully) get around 
to it over the weekend. I'll submit a new patch (or at least email) during 
the weekend (or early next week) when I've had some time to look into 
that.

Thanks.

-- 
Jesper Juhl <jj@...osbits.net>            http://www.chaosbits.net/
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