linux-kernel - Re: [PATCH] dmaengine: clean up and abstract function types (was Re: [PATCH 08/19] dmaengine: enable multiple clients and operations)

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Date:	Tue, 19 Sep 2006 12:20:09 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Olof Johansson <olof@...om.net>
Cc:	Dan Williams <dan.j.williams@...il.com>,
	christopher.leech@...el.com, Jeff Garzik <jeff@...zik.org>,
	neilb@...e.de, linux-raid@...r.kernel.org, akpm@...l.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: clean up and abstract function types (was
	Re: [PATCH 08/19] dmaengine: enable multiple clients and operations)

Ar Llu, 2006-09-18 am 20:05 -0500, ysgrifennodd Olof Johansson:
> On Mon, 18 Sep 2006 15:56:37 -0700 "Dan Williams" <dan.j.williams@...il.com> wrote:

> op.src_type = PG; op.src = pg;
> op.dest_type = BUF; op.dest = buf;
> op.len = len;
> dma_async_commit(chan, op);

At OLS Linus suggested it should distinguish between sync and async
events for locking reasons.

	if(dma_async_commit(foo) == SYNC_COMPLETE) {
		finalise_stuff();
	}

	else		/* will call foo->callback(foo->dev_id) */

because otherwise you have locking complexities - the callback wants to
take locks to guard the object it works on but if it is called
synchronously - eg if hardware is busy and we fall back - it might
deadlock with the caller of dmaa_async_foo() who also needs to hold the
lock.

Alan

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