Re: [PATCH 2/2] dmaengine: Add DW AXI DMAC driver

[Eugeniy Paltsev]

Thanks for response.
My comments are given inline below.

On Wed, 2017-01-25 at 19:25 +0200, Andy Shevchenko wrote:
> On Wed, 2017-01-25 at 18:34 +0300, Eugeniy Paltsev wrote:
> >Â
> > This patch adds support for the DW AXI DMAC controller.
> >Â
> > DW AXI DMAC is a part of upcoming development board from Synopsys.
> >Â
> > In this driver implementation only DMA_MEMCPY and DMA_SG transfers
> > are supported.
> >Â
> Few more comments on top of not addressed/answered yet.
>Â
> >Â
> > +static inline void axi_chan_disable(struct axi_dma_chan *chan)
> > +{
> > +	u32 val;
> > +
> > +	val = axi_dma_ioread32(chan->chip, DMAC_CHEN);
> > +	val &= ~(BIT(chan->id) << DMAC_CHAN_EN_SHIFT);
> > +	val |=ÂÂ(BIT(chan->id) << DMAC_CHAN_EN_WE_SHIFT);
> > +	axi_dma_iowrite32(chan->chip, DMAC_CHEN, val);
> > +}
> > +
> > +static inline void axi_chan_enable(struct axi_dma_chan *chan)
> > +{
> > +	u32 val;
> > +
> > +	val = axi_dma_ioread32(chan->chip, DMAC_CHEN);
> >Â
> > +	val |= (BIT(chan->id) << DMAC_CHAN_EN_SHIFT |
> > +		BIT(chan->id) << DMAC_CHAN_EN_WE_SHIFT);
> >Â
> Redundant parens.
Will be fixed.

> >Â
> > +	axi_dma_iowrite32(chan->chip, DMAC_CHEN, val);
> > +}
>Â
> >Â
> > +static void axi_desc_put(struct axi_dma_desc *desc)
> > +{
> > +	struct axi_dma_chan *chan = desc->chan;
> > +	struct dw_axi_dma *dw = chan->chip->dw;
> > +	struct axi_dma_desc *child, *_next;
> > +	unsigned int descs_put = 0;
> > +
> >Â
> > +	if (unlikely(!desc))
> > +		return;
> Would it be the case?
Yes, I checked the code - this NULL check is unnecessary, so I'll
remove it.

Also about your previous question about likely/unlikely:
I checked disassembly - gcc generates different code when I use likely
and unlikely. So, I guess they are useful.Â

> >Â
> > +static void dma_chan_issue_pending(struct dma_chan *dchan)
> > +{
> > +	struct axi_dma_chan *chan = dchan_to_axi_dma_chan(dchan);
> > +	unsigned long flags;
> > +
> >Â
> > +	dev_dbg(dchan2dev(dchan), "%s: %s\n", __func__,
> > axi_chan_name(chan));
> Messages like this kinda redundant.
> Either you use function tracer and see them anyway, or you are using
> Dynamic Debug, which may include function name.
>Â
> Basically you wrote an equivalent to something like
>Â
> dev_dbg(dev, "%s\n", channame);
Agreed. I'll remove dev_dbg from here because I also have it inÂ
axi_chan_start_first_queued.

> >Â
> > +
> > +	spin_lock_irqsave(&chan->vc.lock, flags);
> > +	if (vchan_issue_pending(&chan->vc))
> > +		axi_chan_start_first_queued(chan);
> > +	spin_unlock_irqrestore(&chan->vc.lock, flags);
> ...and taking into account the function itself one might expect
> something useful printed in _start_first_queued().
>Â
> For some cases there is also dev_vdbg().
>Â
> >Â
> > +}
> > +
> > +static int dma_chan_alloc_chan_resources(struct dma_chan *dchan)
> > +{
> > +	struct axi_dma_chan *chan = dchan_to_axi_dma_chan(dchan);
> > +
> > +	/* ASSERT: channel is idle */
> > +	if (axi_chan_is_hw_enable(chan)) {
> > +		dev_err(chan2dev(chan), "%s is non-idle!\n",
> > +			axi_chan_name(chan));
> > +		return -EBUSY;
> > +	}
> > +
> >Â
> > +	dev_dbg(dchan2dev(dchan), "%s: allocating\n",
> > axi_chan_name(chan));
> Can you try to enable DMADEVICES_DEBUG and VDEBUG and see what is
> useful
> and what is not?
>Â
> Give a chance to function tracer as well.
Yep, this dev_dbg is redundant, so I'll remove it.

> >Â
> > +static void dma_chan_free_chan_resources(struct dma_chan *dchan)
> > +{
> > +	struct axi_dma_chan *chan = dchan_to_axi_dma_chan(dchan);
> > +
> >Â
> >Â
> > +	/* ASSERT: channel is idle */
> > +	if (axi_chan_is_hw_enable(chan))
> > +		dev_err(dchan2dev(dchan), "%s is non-idle!\n",
> > +			axi_chan_name(chan));
> Yeah, as I said, dw_dmac is not a good example. And this one can be
> managed by runtime PM I suppose.
See comment below.

> >> > +static inline bool axi_dma_is_sw_enable(struct axi_dma_chip
*chip)
> >> > +{
> >> > +ÂÂÂstruct dw_axi_dma *dw = chip->dw;
> >> > +ÂÂÂu32 i;
> >> > +
> >> > +ÂÂÂfor (i = 0; i < dw->hdata->nr_channels; i++) {
> >> > +ÂÂÂÂÂÂÂÂÂÂÂif (dw->chan[i].in_use)
> >> Hmm... I know why we have such flag in dw_dmac, but I doubt it's
> >> needed
> >> in this driver. Just double check the need of it.
> > I use this flag to check state of channel (used now/unused) to
disable
> > dmac if all channels are unused for now.
>Â
> Okay, but wouldn't be easier to use runtime PM for that? You will not
> need any special counter and runtime PM will take case of
> enabling/disabling the device.

Now in_use variable has several purposes - it is also used to mask
interrupts from channels, which are not used - that is the reason I
prefer leave it untouched.
Also all existing SoCs with this DMAC don't support power management -
so there is no really profit from implementing PM.

> >Â
> > +
> > +/* TODO: REVISIT: how we should choose AXI master for mem-to-mem
> > transfer? */
> Read datasheet for a SoC/platform? For dw_dmac is chosen with
> accordance
> to hardware topology.
Yep, for existing SoCs there is no difference - both masters have
access to memory. But it isn't necessarily true for future SoCs.
But possibly I shouldn't think about it now.

> >Â
> > +	while (true) {
> Usually it makes readability harder and rises a red flag to the code.
I can write something like next code. Not sure is it looks better.
----------->8------------------
while ((dst_len || dst_sg && dst_nents) &&
   Â(src_len || src_sg && src_nents)) {
...
----------->8------------------

> >Â
> >Â		/* Manage transfer list (xfer_list) */
> > +		if (!first) {
> > +			first = desc;
> > +		} else {
> > +			list_add_tail(&desc->xfer_list, &first-
> > >Â
> > > xfer_list);
> > +			write_desc_llp(prev, desc->vd.tx.phys |
> > lms);
> > +		}
> > +		prev = desc;
> > +
> > +		/* update the lengths and addresses for the next
> > loop
> > cycle */
> > +		dst_len -= xfer_len;
> > +		src_len -= xfer_len;
> > +		dst_adr += xfer_len;
> > +		src_adr += xfer_len;
> > +
> > +		total_len += xfer_len;
> I would suggest to leave this on caller.
I don't think it is a good idea. Caller doesn't know value of internal
parameters like max data width and max block size, but we know these
values and can organize LLI the most optimal way.
And if the caller has already prepared suitable SG list there will be
really little overhead, so I prefer leave this code unchanged.

> At some point, if no one
> else
> do this faster than me, I would like to introduce something like
> struct
> dma_parms per DMA channel to allow caller prepare SG list suitable
> for the DMA device.
In my opinion it is driver responsibility, not the caller.

> >Â
> > +
> > +static struct dma_async_tx_descriptor *
> > +dma_chan_prep_dma_memcpy(struct dma_chan *dchan, dma_addr_t dest,
> > +			Âdma_addr_t src, size_t len, unsigned long
> > flags)
> > +{
> Do you indeed have a use case for this except debugging and testing?
At least for IO coherency engine testing.

> >Â
> > +	unsigned int nents = 1;
> > +	struct scatterlist dst_sg;
> > +	struct scatterlist src_sg;
> > +
> > +	sg_init_table(&dst_sg, nents);
> > +	sg_init_table(&src_sg, nents);
> > +
> > +	sg_dma_address(&dst_sg) = dest;
> > +	sg_dma_address(&src_sg) = src;
> > +
> > +	sg_dma_len(&dst_sg) = len;
> > +	sg_dma_len(&src_sg) = len;
> > +
> > +	/* Implement memcpy transfer as sg transfer with single
> > list
> > */
> >Â
> > +	return dma_chan_prep_dma_sg(dchan, &dst_sg, nents,
> > +				ÂÂÂÂ&src_sg, nents, flags);
> One line?
It will be more than 80 character.

> >Â
> > +}
> >Â
> > +
> > +static void axi_chan_dump_lli(struct axi_dma_chan *chan,
> > +			ÂÂÂÂÂÂstruct axi_dma_desc *desc)
> > +{
> > +	dev_err(dchan2dev(&chan->vc.chan),
> > +		"SAR: 0x%x DAR: 0x%x LLP: 0x%x BTS 0x%x CTL:
> > 0x%x:%08x",
> > +		le32_to_cpu(desc->lli.sar_lo),
> > +		le32_to_cpu(desc->lli.dar_lo),
> > +		le32_to_cpu(desc->lli.llp_lo),
> > +		le32_to_cpu(desc->lli.block_ts_lo),
> > +		le32_to_cpu(desc->lli.ctl_hi),
> > +		le32_to_cpu(desc->lli.ctl_lo));
> I hope at some point ARC (and other architectures which will use this
> IP) can implement MMIO tracer.
But we had to use such code until it happens.

> >Â
> > +}
> >Â
> > +		axi_chan_dump_lli(chan, desc);
> > +}
> > +
> > +
> Extra line.
I guess we don't need any extra line here:
----------->8------------------
axi_chan_dump_lli(chan, desc_head);
list_for_each_entry(desc, &desc_head->xfer_list, xfer_list)
	axi_chan_dump_lli(chan, desc);
----------->8------------------
	
> >Â
> > +static void axi_chan_handle_err(struct axi_dma_chan *chan, u32
> > status)
> > +{
> >Â
> > +
> > +static int dma_chan_pause(struct dma_chan *dchan)
> > +{
> > +	struct axi_dma_chan *chan = dchan_to_axi_dma_chan(dchan);
> > +	unsigned long flags;
> > +	unsigned int timeout = 20; /* timeout iterations */
> > +	int ret = -EAGAIN;
> > +	u32 val;
> > +
> > +	spin_lock_irqsave(&chan->vc.lock, flags);
> > +
> > +	val = axi_dma_ioread32(chan->chip, DMAC_CHEN);
> > +	val |= (BIT(chan->id) << DMAC_CHAN_SUSP_SHIFT |
> > +		BIT(chan->id) << DMAC_CHAN_SUSP_WE_SHIFT);
> Redundant parens.
Will be fixed.

> >Â
> > +	axi_dma_iowrite32(chan->chip, DMAC_CHEN, val);
> > +
> > +	while (timeout--) {
> >Â
> > +		if (axi_chan_irq_read(chan) &
> > DWAXIDMAC_IRQ_SUSPENDED) {
> >Â
> > +			axi_chan_irq_clear(chan,
> > DWAXIDMAC_IRQ_SUSPENDED);
> You may move this invariant out from the loop. Makes code cleaner.
Agreed.

> >Â
> > +			ret = 0;
> > +			break;
> > +		}
> >Â
> > +		udelay(2);
>Â
> >Â
> > +	}
> > +
> > +	chan->is_paused = true;
> This is indeed property of channel.
> That said, you may do a trick and use descriptor status for it.
> You channel and driver, I'm sure, can't serve in interleaved mode
> with
> descriptors. So, that makes channel(paused) == active
> descriptor(paused).
>Â
> The trick allows to make code cleaner.

I don't understand how we can use use descriptor status for it.
I determine descriptor status basing on the is_paused value.
Look at the code:
----------->8------------------
ret = dma_cookie_status(dchan, cookie, txstate);
if (chan->is_paused && ret == DMA_IN_PROGRESS)
	return DMA_PAUSED;
----------->8------------------

> >Â
> > +	ret = device_property_read_u32_array(dev, "priority",
> > carr,
> I'm not sure you will have a use case for that. Have you?
I guess we don't have use case for priority managment for mem-to-mem
transfers.
But we need priority management for slave dma (I'll implement slave dma
api soon)

> >Â
> > +	ret = devm_request_irq(chip->dev, chip->irq,
> > dw_axi_dma_intretupt,
> > +			ÂÂÂÂÂÂÂIRQF_SHARED, DRV_NAME, chip);
> > +	if (ret)
> > +		return ret;
> You can't do this unless you are using threaded IRQ handler without
> any
> tasklets involved.
>Â
> There was a nice mail by Thomas who explained what the problem you
> have
> there.
>Â
> It's a bug in your code.
Yep, thanks a lot.
Looks like I have problem with driver remove function.

I guess I can solve this problem by freeing irq manually before tasklet
kill:
----------->8------------------
static int dw_remove(...)
{
	devm_free_irq(chip->dev, chip->irq, chip);
	tasklet_kill(&dw->tasklet);
	...
}
----------->8------------------

> >Â
> > +static int __init dw_init(void)
> > +{
> > +	return platform_driver_register(&dw_driver);
> > +}
> > +subsys_initcall(dw_init);
> Hmm... Why it can't be just a platform driver? You describe the
> dependency in Device Tree, if you have something happened, you may
> utilize -EPROBE_DEFER.
>Â
Will be fixed.


> >ÂÂ* Add DT bindings
> >Â
> > Eugeniy Paltsev (2):
> >ÂÂÂdt-bindings: Document the Synopsys DW AXI DMA bindings
> >ÂÂÂdmaengine: Add DW AXI DMAC driver
> >Â
> >ÂÂ.../devicetree/bindings/dma/snps,axi-dw-dmac.txtÂÂÂ|ÂÂÂ33 +
> >ÂÂdrivers/dma/KconfigÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ|ÂÂÂÂ8 +
> >ÂÂdrivers/dma/MakefileÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ|ÂÂÂÂ1 +
> >Â
> >ÂÂdrivers/dma/axi_dma_platform.cÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ| 1060
> > ++++++++++++++++++++
> This surprises me. I would expect more then 100+ LOC reduction when
> switched to virt-dma API. Can you double check that you are using it
> effectively?
There is a simple explanation: I switched to virt-dma API (and it cause
LOC reduction) and I implemented some TODOs (and it cause LOC
increase).

Also about previous comment, which I missed before:
> > +static u32 axi_chan_get_xfer_width(struct axi_dma_chan *chan,
> >ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂdma_addr_t src, dma_addr_t dst, size_t len)
> > +{
> > +	u32 width;
> > +	dma_addr_t sdl = (src | dst | len);
> > +	u32 max_width = chan->chip->dw->hdata->m_data_width;
>Â
> Use reverse tree.
>Â
> dma_addr_t use above is wrong. (size_t might be bigger in some cases)

What do you mean by the phrase "Use reverse tree" ?

--Â
ÂEugeniy Paltsev