linux-kernel - Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense buffer

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Message-Id: <20090527103637W.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 27 May 2009 10:36:40 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	bharrosh@...asas.com
Cc:	fujita.tomonori@....ntt.co.jp,
	James.Bottomley@...senPartnership.com, jens.axboe@...cle.com,
	rdreier@...co.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, chris.mason@...cle.com,
	david@...morbit.com, hch@...radead.org, akpm@...ux-foundation.org,
	jack@...e.cz, yanmin_zhang@...ux.intel.com,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH 03/13] scsi: unify allocation of scsi command and sense
 buffer

On Tue, 26 May 2009 19:05:05 +0300
Boaz Harrosh <bharrosh@...asas.com> wrote:

> On 05/26/2009 06:31 PM, FUJITA Tomonori wrote:
> > 
> > Can we just fix some drivers not to do the DMA with the sense buffer in
> > scsi_cmnd? IIRC, there are only five or six drivers that do such.
> 
> This is not so.
> All drivers that go through scsi_eh_prep_cmnd() will eventually DMA through
> the regular read path. Including all the drivers that do nothing and let
> scsi-ml do the REQUEST_SENSE
> 
> Actually I have exact numbers, from the last time I did all that

Hmm, we discussed this before, I think.

scsi-ml uses scsi_eh_prep_cmnd only via scsi_send_eh_cmnd(). There are
some users of scsi_send_eh_cmnd in scsi-ml but only scsi_request_sense
does the DMA in the sense_buffer of scsi_cmnd.

Only scsi_error_handler() uses scsi_request_sense() and
scsi_send_eh_cmnd() works synchronously. So scsi-ml can easily avoid
the the DMA in the sense_buffer of scsi_cmnd if we have one sense
buffer per scsi_host.
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