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Message-ID: <424107F9.5070807@gmail.com>
Date: Wed, 23 Mar 2005 15:08:57 +0900
From: Tejun Heo <htejun@gmail.com>
User-Agent: Debian Thunderbird 1.0 (X11/20050118)
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To: James Bottomley <James.Bottomley@SteelEye.com>
Cc: Jens Axboe <axboe@suse.de>, SCSI Mailing List <linux-scsi@vger.kernel.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH scsi-misc-2.6 01/08] scsi: remove unused bounce-buffer
 release path
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  Hello, James.

James Bottomley wrote:
> On Wed, 2005-03-23 at 11:14 +0900, Tejun Heo wrote:
> 
>>01_scsi_remove_scsi_release_buffers.patch
>>
>>	Buffer bouncing hasn't been done inside the scsi midlayer for
>>	quite sometime now, but bounce-buffer release paths are still
>>	around.  This patch removes these unused paths.
> 
> 
> Yes, but scsi_release_buffers isn't referring to bounce buffers anymore,
> it's simply releasing the sg buffers.
> 

  That's what I did.  Replacing scsi_release_buffers() calls with calls 
to scsi_free_sgtable().  The only logic removed is bounce-buffer 
release/copy-back.

> [...]
> 
>>-	else if (cmd->buffer != req->buffer) {
>>-		if (rq_data_dir(req) == READ) {
>>-			unsigned long flags;
>>-			char *to = bio_kmap_irq(req->bio, &flags);
>>-			memcpy(to, cmd->buffer, cmd->bufflen);
>>-			bio_kunmap_irq(to, &flags);
>>-		}
>>-		kfree(cmd->buffer);
>>-	}
> 
> 
> I'll defer to Jens here, but I don't thing you can just remove this ...
> sg_io with a misaligned buffer will fail without this.

  AFAIK, those are done by blk_rq_map_user() and blk_rq_unmap_user(), 
both of which are invoked directly by sg_io().

> That rather nasty code freeing cmd->buffer needs to be in there as
> well ... so it does make sense to keep this API

  That code is invoked only for REQ_BLOCK_PC requests without bio, and I 
digged pretty hard but, in those cases, AFAICT, the callers are 
responsible for supplying dma-able buffers and nothing seems to alter 
cmd->buffer after the cmd gets initialized, but I might be missing 
things here.  If so, please point out.

  Thanks.

-- 
tejun

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