linux-kernel - Re: [RFC v2 0/5] dmaengine: Slave DMA interface and example users

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Date:	Mon, 4 Feb 2008 16:32:00 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	"Dan Williams" <dan.j.williams@...el.com>
Cc:	"David Brownell" <david-b@...bell.net>,
	linux-kernel@...r.kernel.org,
	"Shannon Nelson" <shannon.nelson@...el.com>, kernel@...32linux.org,
	"Francis Moreau" <francis.moro@...il.com>,
	"Paul Mundt" <lethal@...ux-sh.org>,
	"Vladimir A. Barinov" <vbarinov@...mvista.com>,
	"Pierre Ossman" <drzeus-list@...eus.cx>
Subject: Re: [RFC v2 0/5] dmaengine: Slave DMA interface and example users

On Wed, 30 Jan 2008 10:39:47 -0700
"Dan Williams" <dan.j.williams@...el.com> wrote:

> Agreed, the issue is how to do this without requiring an
> interrupt+callback sequence for each transaction or requiring the
> client to carry per transaction unmap-data.  For example NET_DMA never
> sees a dma_addr_t and assumes that all it needs to care about is the
> last transaction in a sequence.  Since it is alive for the duration of
> a transaction, we could put unmap data in dma_async_tx_descriptor
> along with an unmap function pointer since dma_unmap* routines have an
> equal number of parameters.  But I just got through making this
> structure smaller so maybe there is a better way.

I have to say I'm not crazy about the idea of adding more callbacks to
the descriptor...

The client must somehow know when the transfer is complete -- after
all, it has to call async_tx_ack() at some point. So additional
callbacks shouldn't be needed.

How about adding more variants of the "ack" function -- one for each
kind of transfer? For example, after an async_memcpy() transaction is
complete, the client must call async_memcpy_ack(), which could be an
inline function containing something along the lines of

static inline void async_memcpy_ack(struct dma_async_tx_descriptor *tx)
{
	struct dma_device *dma = tx->chan->device;

	dma_unmap_page(dma->dev, tx->src_phys, tx->len, DMA_TO_DEVICE);
	dma_unmap_page(dma->dev, tx->dst_phys, tx->len, DMA_FROM_DEVICE);
	async_tx_ack(tx);
}

which would evaluate to just async_tx_ack(tx) in most cases, since
dma_unmap_page() usually doesn't actually do anything.

This requires three additional fields in the dma_async_tx_descriptor
structure, but in many cases the driver needs these fields in its own
private descriptor wrapper anyway.

Haavard
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