Message-ID: <20131113131812.GX8834@intel.com>
Date:	Wed, 13 Nov 2013 18:48:12 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Andy Gross <agross@...eaurora.org>
Cc:	devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Bryan Huntsman <bryanh@...eaurora.org>,
	Dan Williams <dan.j.williams@...el.com>,
	David Brown <davidb@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] dmaengine: add msm bam dma driver

On Thu, Nov 07, 2013 at 05:03:17PM -0600, Andy Gross wrote:
> On Thu, Oct 31, 2013 at 04:46:21PM -0500, Andy Gross wrote:
> > On Thu, Oct 31, 2013 at 10:29:48PM +0530, Vinod Koul wrote:
> > > On Fri, Oct 25, 2013 at 03:24:02PM -0500, Andy Gross wrote:
> > > 
> > > This should be sent to dmaengine@...r.kernel.org
> >  
> > I'll add the list when I send the second iteration or should I send it over mid
> > stream?
either ways is okay, but pls make sure the next rev is sent on list.

> > 
> > > > +	/* reset channel */
> > > > +	iowrite32(1, bdev->regs + BAM_P_RST(bchan->id));
> > > > +	iowrite32(0, bdev->regs + BAM_P_RST(bchan->id));
> > > > +
> > > > +	dma_free_coherent(bdev->dev, BAM_DESC_FIFO_SIZE, bchan->fifo_virt,
> > > > +				bchan->fifo_phys);
> > > > +
> > > > +	/* mask irq for pipe/channel */
> > > > +	val = ioread32(bdev->regs + BAM_IRQ_SRCS_MSK_EE(bchan->ee));
> > > > +	val &= ~(1 << bchan->id);
> > > > +	iowrite32(val, bdev->regs + BAM_IRQ_SRCS_MSK_EE(bchan->ee));
> > > > +
> > > > +	/* disable irq */
> > > > +	iowrite32(0, bdev->regs + BAM_P_IRQ_EN(bchan->id));
> > > > +
> > > > +	clear_bit(bchan->ee, &bdev->enabled_ees);
> > > > +}
> > > > +
> > > > +/*
> > > > + * bam_slave_config - set slave configuration for channel
> > > > + * @chan: dma channel
> > > > + * @cfg: slave configuration
> > > > + *
> > > > + * Sets slave configuration for channel
> > > > + * Only allow setting direction once.  BAM channels are unidirectional
> > > > + * and the direction is set in hardware.
> > > > + *
> > > > + */
> > > > +static void bam_slave_config(struct bam_chan *bchan,
> > > > +		struct bam_dma_slave_config *bcfg)
> > > 
> > > > +{
> > > > +	struct bam_device *bdev = bchan->device;
> > > > +
> > > > +	bchan->bam_slave.desc_threshold = bcfg->desc_threshold;
> > > what does the desc_threshold mean?
> >  
> > The desc threshhold determines the minimum number of bytes of descriptor that
> > causes a write event to be communicated to the peripheral.  I default to 4 bytes
> > (1 descriptor), but this is configurable through the DMA_SLAVE_CONFIG interface
> > using the extended slave_config structure.
sounds like src/dst_maxburst?

> > > > + * bam_tx_submit - Adds transaction to channel pending queue
> > > > + * @tx: transaction to queue
> > > > + *
> > > > + * Adds dma transaction to pending queue for channel
> > > > + *
> > > > +*/
> > > > +static dma_cookie_t bam_tx_submit(struct dma_async_tx_descriptor *tx)
> > > > +{
> > > > +	struct bam_chan *bchan = to_bam_chan(tx->chan);
> > > > +	struct bam_async_desc *desc = to_bam_async_desc(tx);
> > > > +	dma_cookie_t cookie;
> > > > +
> > > > +	spin_lock_bh(&bchan->lock);
> > > > +
> > > > +	cookie = dma_cookie_assign(tx);
> > > > +	list_add_tail(&desc->node, &bchan->pending);
> > > > +
> > > > +	spin_unlock_bh(&bchan->lock);
> > > > +
> > > > +	return cookie;
> > > > +}
> > > Okay you are *NOT* using virt-dma layer, all this can be removed if you do that,
> > > this is similar to what vchan_tx_submit() does!
> > >
> > 
> > I'll look into reworking and utilizing the virt-dma layer.
> >  
> 
> Is it acceptable to pick and choose the pieces of the virt-dma layer that
> benefit me? Or do I have to absorb all of it.  The smaller helper structs and
> functions are fine, but in some cases they force a certain implementation.
and that implementation is the right one and the what we expect from users!

> The bam_tx_submit and perhaps the cleanup functions are about the only pieces
> I'd be able to leverage from the virt-dma, aside from the structures.
> 
> The main issue with the rest of the code is that it doesn't fit the behavior of
> my dma controller.  Because i have a fixed sized circular buffer for my
> descriptor FIFO, I have to copy in the new descriptors before I start up the
> dma channel.
the virt-dma is for managing the descriptors and the lists for managing the
descriptors. Using this is right way and is based on dmaengine APIs and not on
dma controllers, so I see no reason why you cant use this!

> Using the vchan_cookie_complete() forces me to start the next transaction within
> the interrupt, or schedule another tasklet to do that work for me.  The overhead
> for copying what could be a large number of descriptors into the FIFO might
> introduce too much latency inside the IRQ handler  (especially since this is done
> to non-cached memory).  Using another tasklet for just restarting the dma
> controller seems klunky to me.
That is the expectation from API. Once a txn is complete, you need to quickly
start the next one in the completion.

> I would rather start the next transaction within the tasklet queued from the IRQ
> (vchan_cookie_complete), but because it uses it's own tasklet, I wouldn't be
> able to leverage that.
why dont you call the vchan_cookie_complete from the irq. That will trigger the
virt-tasklet to complete the current one, then you schedule your tasklet to
program the next one.

> The vchan_cookie_complete() usage within the IRQ handler also implys that only 1
> dma transaction is completed per IRQ.  That might not be the case for me,
> because I can advance the DMA FIFO offset to tell the controller to keep going
> to the next transaction.  By the time I get to servicing the IRQ, I might have
> completed more than 1 transaction.   I suppose you could call
> vchan_cookie_complete() multiple times, but that seems wrong to me due to the
> multiple calls to tasklet_schedule.
I think you are mistaken here. If you have issued 3 descriptors to your HW, then
assuming you got single completion (which IMO is wrong and you should get
interrupt for every completion), then you mark all three as completed, the
completion would move all the completed descriptors

> > > > +static int bam_control(struct dma_chan *chan, enum dma_ctrl_cmd cmd,
> > > > +	unsigned long arg)
> > > > +{
> > > > +	struct bam_chan *bchan = to_bam_chan(chan);
> > > > +	struct bam_device *bdev = bchan->device;
> > > > +	struct bam_dma_slave_config *bconfig;
> > > > +	int ret = 0;
> > > > +
> > > > +	switch (cmd) {
> > > > +	case DMA_PAUSE:
> > > > +		spin_lock_bh(&bchan->lock);
> > > > +		iowrite32(1, bdev->regs + BAM_P_HALT(bchan->id));
> > > > +		spin_unlock_bh(&bchan->lock);
> > > > +		break;
> > > > +	case DMA_RESUME:
> > > > +		spin_lock_bh(&bchan->lock);
> > > > +		iowrite32(0, bdev->regs + BAM_P_HALT(bchan->id));
> > > > +		spin_unlock_bh(&bchan->lock);
> > > > +		break;
> > > > +	case DMA_TERMINATE_ALL:
> > > > +		bam_dma_terminate_all(chan);
> > > > +		break;
> > > > +	case DMA_SLAVE_CONFIG:
> > > > +		bconfig = (struct bam_dma_slave_config *)arg;
> > > > +		bam_slave_config(bchan, bconfig);
> > > DMA_SLAVE_CONFIG expects arg as struct dma_slave_config, not this. Pls dont
> > > voilate APIs
> > 
> > So for extended slave_config structures, the caller needs to send in a ptr to
> > the dma_slave_config and then I can determine the bam_dma_slave_config.  Will
> > fix.
What are the additional parameters you need to "extended" config?

> > 
> > > > +		break;
> > > > +	default:
> > > > +		ret = -EIO;
> > > I would expect -ENXIO here!
> > > 
> > 
> > OK.
> > 
> > > > +		break;
> > > > +	}
> > > > +
> > > > +	return ret;
> > > > +}
> > > > +
> > > > +/*
> > > > + * bam_tx_status - returns status of transaction
> > > > + * @chan: dma channel
> > > > + * @cookie: transaction cookie
> > > > + * @txstate: DMA transaction state
> > > > + *
> > > > + * Return status of dma transaction
> > > > + */
> > > > +static enum dma_status bam_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
> > > > +		struct dma_tx_state *txstate)
> > > > +{
> > > > +	return dma_cookie_status(chan, cookie, txstate);
> > > hmmm, this wont work in many cases for slave. For example if you try to get this
> > > working with audio you need to provide the residue values.
> > > The results you are providing here will not be useful, so I would recommedn
> > > fixing it
> > > 
> > 
> > Ok so in this case I'd need to read where I am in the descriptor chain and
> > calculate the residual.  That shouldn't be a problem.
Yup!

-- 
~Vinod
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