linux-kernel - Re: [PATCH 03/10] block: add rq->resid

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Message-ID: <20090430064549.GC6725@liondog.tnic>
Date:	Thu, 30 Apr 2009 08:45:49 +0200
From:	Borislav Petkov <petkovbb@...glemail.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	axboe@...nel.dk, linux-kernel@...r.kernel.org, jeff@...zik.org,
	linux-ide@...r.kernel.org, linux-scsi@...r.kernel.org,
	bzolnier@...il.com, petkovbb@...glemail.com,
	sshtylyov@...mvista.com, mike.miller@...com,
	chirag.kantharia@...com, Eric.Moore@....com,
	stern@...land.harvard.edu, fujita.tomonori@....ntt.co.jp,
	zaitcev@...hat.com, Geert.Uytterhoeven@...ycom.com,
	sfr@...b.auug.org.au, grant.likely@...retlab.ca,
	paul.clements@...eleye.com, jesper.juhl@...il.com,
	tim@...erelk.net, jeremy@...source.com, adrian@...en.demon.co.uk,
	oakad@...oo.com, dwmw2@...radead.org, schwidefsky@...ibm.com,
	ballabio_dario@....com, davem@...emloft.net, rusty@...tcorp.com.au,
	Markus.Lidel@...dowconnect.com, bharrosh@...asas.com,
	Doug Gilbert <dgilbert@...erlog.com>,
	"Darrick J. Wong" <djwong@...ibm.com>
Subject: Re: [PATCH 03/10] block: add rq->resid_len

On Thu, Apr 30, 2009 at 10:59:10AM +0900, Tejun Heo wrote:
> Hello, James.
> 
> James Bottomley wrote:
> > This looks good (although I'd like to test it first).
> 
> Yeah, this will need quite a bit of testing.
> 
> > Might it not be better to have an accessor setting resid_len?  All
> > the other patches in the series insulate users from the actual
> > members of struct request by accessors, so this is a bit the odd man
> > out.
> 
> I actually think it's better to expose resid_len in this case as the
> semantics of the field is - initialized to zero on issue, contains
> residual count on completion and whatever it contains inbetween is
> upto the low level driver.  Request position or length are different
> as they must contain well defined values throughout request processing
> and both block layer and low level driver should agree on what they
> mean.
> 
> Fancy words aside, it basically boils down to allowing llds to do
> either "rq->resid_len = blk_rq_bytes() - xferred" on completion or
> "rq->resid_len = blk_rq_bytes()" on issue and "rq->resid_len -=
> increments" while processing.

Actually, the second one sounds more natural: resid_len == data_len on
issue and decrementing while travelling through block layer and LLDD,
while resid_len == 0 in issue might get confused somewhere.

And I like it too, we've been coming up with all sorts of hacks in
ide-atapi wrt to residual completion and accounting of what got xferred
already and rq->resid_len is much more cleaner, IMHO.

/me testing...

-- 
Regards/Gruss,
    Boris.
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