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Date: Sun, 3 Nov 2002 14:48:09 -0800 (PST)
From: Linus Torvalds <torvalds@transmeta.com>
To: Pavel Machek <pavel@ucw.cz>
cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, <benh@kernel.crashing.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: swsusp: don't eat ide disks
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On Sun, 3 Nov 2002, Pavel Machek wrote:
> 
> How do I quiesce a queue? Is it ll_rw_blk stuff?

Just send a request down the request list, and make sure that

 - the command is marked as being non-mergeable or re-orderable by 
   software (as all special commands are)

 - the command is not re-orderable / mergeable by hardware (and since the
   command in question would be something like "flush" or "spindown",
   hardware really would be quite broken if it re-ordered it ;)

and then just wait for its completion.

The code is not that complicated, it looks roughly something like

	struct request *rq;

	rq = blk_get_request(q, WRITE, __GFP_WAIT);
	rq->flags = REQ_BLOCK_PC;
	rq->data = NULL;
	rq->data_len = 0;
	rq->timeout = 5*HZ; /* Or whatever */
	memset(rq->cmd, 0, sizeof(rq->cmd));
	rq->cmd[0] = SYNCHRONIZE_CACHE;
	.. fill in whatever bytes the SYNCHRONIZE_CACHE cmd needs ..
	rq->cmd_len = 10;
	err = blk_do_rq(q, bdev, rq);
	blk_put_request(rq);

and you're done. The above should work pretty much on all block drivers
out there, btw: the ones that don't understand SCSI commands should just
ignore requests that aren't the regular REQ_CMD commands.

See drivers/block/scsi_ioctl.c for other examples of sending down commands 
to block devices.

		Linus

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