Date: Fri, 18 May 2012 10:21:21 +0200 (CEST)
From: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
To: Vinod Koul <vinod.koul@linux.intel.com>
cc: linux-kernel@vger.kernel.org, Magnus Damm <magnus.damm@gmail.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>,
        Paul Mundt <lethal@linux-sh.org>, linux-sh@vger.kernel.org,
        Sascha Hauer <s.hauer@pengutronix.de>
Subject: Re: [PATCH 1/9 v3] dmaengine: add an shdma-base library
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Hi Vinod

Thanks for your review.

On Tue, 15 May 2012, Vinod Koul wrote:

> On Wed, 2012-05-09 at 17:09 +0200, Guennadi Liakhovetski wrote: 
> > This patch extracts code from shdma.c, that does not directly deal with
> > hardware implementation details and can be re-used with diverse DMA
> > controller variants, found on SH-based SoCs.
> Have you see recent Russell's virtual dma patches [1]? I like the idea
> and simplifies things for implementation in each driver.
> Said that, do we need this libarry. Can you see if you can use/enhance
> that one.

No, I haven't seen it. I had a look at it, and it doesn't seem to provide 
an immediate solution to our problem. It can be used as a part of one, but 
a significant amount of work would still be required to convert shdma to a 
virtual-channel based design.

Let me try to describe, what such a virtual channel solution would look 
like:

1. One (virtual) DMA channel is allocated per client
2. As clients issue dma_request_channel(), the correct channel is selected
3. The client configures the channel with a call to 
   dmaengine_slave_config()
4. As transfers are prepared and issued they are queued in per-virtual 
   channel lists
5. Each virtual channel links to a list of suitable DMA devices and 
   channels
6. As a new transfer request on a specific virtual channel has to be 
   processed, a suitable free physical channel is picked up, configured 
   and the request is sent out

If this my understanding is right, this should be doable, but not within 
the current 3.5 time-frame. Further, if we first do this conversion, we'll 
still have to split the driver similar to what is proposed by this patch 
series, but then it would be largely a new effort, most of the work 
invested into this so far will be lost. OTOH, if we now push the shdma 
driver split, we enable the merge of the SUDMAC support and porting to the 
virtual channel design should also become easier, because the hardware and 
the software parts of the driver will already be separated and presumably 
the hardware part will not need to be changed.

One more thing, before going into detail and replying to your comments: 
this split doesn't change driver's functionality, it simply splits it into 
two parts. So, most questions, that you ask in this your review would also 
apply to the current driver. This patch just extracts hardware-neutral 
code from the shdma driver into a separate file. I think, any functional 
changes should be done separately from this driver-split series.

[snip]

> > +static dma_cookie_t shdma_tx_submit(struct dma_async_tx_descriptor *tx)
> > +{
> > +	struct shdma_desc *chunk, *c, *desc =
> > +		container_of(tx, struct shdma_desc, async_tx),
> > +		*last = desc;
> > +	struct shdma_chan *schan = to_shdma_chan(tx->chan);
> > +	struct shdma_slave *slave = tx->chan->private;
> > +	dma_async_tx_callback callback = tx->callback;
> > +	dma_cookie_t cookie;
> > +	bool power_up;
> > +
> > +	spin_lock_irq(&schan->chan_lock);
> > +
> > +	power_up = list_empty(&schan->ld_queue);
> > +
> > +	cookie = dma_cookie_assign(tx);
> > +
> > +	/* Mark all chunks of this descriptor as submitted, move to the queue */
> > +	list_for_each_entry_safe(chunk, c, desc->node.prev, node) {
> > +		/*
> > +		 * All chunks are on the global ld_free, so, we have to find
> > +		 * the end of the chain ourselves
> > +		 */
> > +		if (chunk != desc && (chunk->mark == DESC_IDLE ||
> > +				      chunk->async_tx.cookie > 0 ||
> > +				      chunk->async_tx.cookie == -EBUSY ||
> > +				      &chunk->node == &schan->ld_free))
> > +			break;
> > +		chunk->mark = DESC_SUBMITTED;
> > +		/* Callback goes to the last chunk */
> > +		chunk->async_tx.callback = NULL;
> > +		chunk->cookie = cookie;
> > +		list_move_tail(&chunk->node, &schan->ld_queue);
> > +		last = chunk;
> > +
> > +		dev_dbg(schan->dev, "submit #%d@%p on %d\n",
> > +			tx->cookie, &last->async_tx, schan->id);
> > +	}
> > +
> > +	last->async_tx.callback = callback;
> > +	last->async_tx.callback_param = tx->callback_param;
> > +
> > +	if (power_up) {
> > +		int ret;
> > +		schan->pm_state = SHDMA_PM_BUSY;
> > +
> > +		ret = pm_runtime_get(schan->dev);
> any reason why this is here and not in issue pending?

Sorry, we have discussed this multiple times already. See, for example,

http://thread.gmane.org/gmane.linux.kernel/1181989/focus=12187

[snip]

> > +/*
> > + * Drivers, using this library are expected to embed struct shdma_dev,
> > + * struct shdma_chan, struct shdma_desc, and struct shdma_slave
> > + * in their respective device, channel, descriptor and slave objects.
> > + */
> > +
> > +struct shdma_slave {
> > +	unsigned int slave_id;
> this should be moved to struct dma_slave_config 

Again, this would be a functional change. And struct dma_slave_config 
doesn't have a slave-ID field in it, which is the only thing we need for 
now. Instead it has a bunch of other fields, of which none is so far used 
by this driver.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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