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

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Date:	Wed, 29 Apr 2009 09:41:54 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	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 Wed, 2009-04-29 at 18:13 +0900, Tejun Heo wrote:
> rq->data_len served two purposes - the length of data buffer on issue
> and the residual count on completion.  This duality creates some
> headaches.
> 
> First of all, block layer and low level drivers can't really determine
> what rq->data_len contains while a request is executing.  It could be
> the total request length or it coulde be anything else one of the
> lower layers is using to keep track of residual count.  This
> complicates things because blk_rq_bytes() and thus
> [__]blk_end_request_all() relies on rq->data_len for PC commands.
> Drivers which want to report residual count should first cache the
> total request length, update rq->data_len and then complete the
> request with the cached data length.
> 
> Secondly, it makes requests default to reporting full residual count,
> ie. reporting that no data transfer occurred.  The residual count is
> an exception not the norm; however, the driver should clear
> rq->data_len to zero to signify the normal cases while leaving it
> alone means no data transfer occurred at all.  This reverse default
> behavior complicates code unnecessarily and renders block PC on some
> drivers (ide-tape/floppy) unuseable.
> 
> This patch adds rq->resid_len which is used only for residual count.
> 
> While at it, remove now unnecessasry blk_rq_bytes() caching in
> ide_pc_intr() as rq->data_len is not changed anymore.
> 
> [ Impact: cleanup residual count handling, report 0 resid by default ]

This looks good (although I'd like to test it first).

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.

James


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