Re: [PATCH v4 2/2] dmaengine: add a driver for AMBA AXI NBPF DMAC IP cores

On Fri, Jul 25, 2014 at 05:00:53PM +0200, Guennadi Liakhovetski wrote:

Hi Vinod,

On Fri, 25 Jul 2014, Vinod Koul wrote:

Thanks for your review. However, I find the following a bit odd. As you 
remember, you already reviewed v2 of this driver:

http://www.spinics.net/lists/dmaengine/msg00880.html

and provided your comments to it, which I addressed in versions 3 and 4. 
Code, that you're commenting on here, hasn't (significantly) changed since 
v1. During your v2 review it didn't seem offending to you, now it does. 
Does this mean, that if / once I fix these issues, your next review might 
find some more _old_ code snippets, that you decide aren't appropriate?

This isn't the first time such a thing is happening with various reviewers 
and patch submitters. I think a reasonable approach is to "trust your own 
review." Once I've reviewed a piece of code and found it ok, I _normally_ 
won't ask a patch author to modify code, that didn't change since my 
previous review. Simple. Of course, sometimes it does happen, that the 
first review skips some important flaws, but then I consider that my 
fault, if I didn't find them during the first round and try to find a 
solution to minimise the damage to the author. Now, to specific comments.

I agree you can blame me for not spotting them earlier and yes criticism is
fair. But then overall goal is to ensure that code is better, so even if
comment comes late we should accept it.

quoted

On Sat, Jul 19, 2014 at 12:48:51PM +0200, Guennadi Liakhovetski wrote:

quoted

This patch adds a driver for NBPF DMAC IP cores from Renesas, designed for
the AMBA AXI bus.

quoted

+struct nbpf_desc {
+	struct dma_async_tx_descriptor async_tx;
+	bool user_wait;

sounds odd?

Maybe it's not the best name, I can gladly try to improve it, but I'm sure 
I'm not the best "namer," so, the same can be said about more or less 
every identifier in all my code - it could be improved. However, I don't 
think it's critical enough to delay mainlining until the next kernel 
version?

No it is not critical at all!

quoted

quoted

+static int nbpf_chan_probe(struct nbpf_device *nbpf, int n)
+{
+	struct dma_device *dma_dev = &nbpf->dma_dev;
+	struct nbpf_channel *chan = nbpf->chan + n;
+	int ret;
+
+	chan->nbpf = nbpf;
+	chan->base = nbpf->base + NBPF_REG_CHAN_OFFSET + NBPF_REG_CHAN_SIZE * n;
+	INIT_LIST_HEAD(&chan->desc_page);
+	spin_lock_init(&chan->lock);
+	chan->dma_chan.device = dma_dev;
+	dma_cookie_init(&chan->dma_chan);
+	nbpf_chan_prepare_default(chan);
+
+	dev_dbg(dma_dev->dev, "%s(): channel %d: -> %p\n", __func__, n, chan->base);
+
+	snprintf(chan->name, sizeof(chan->name), "nbpf %d", n);
+
+	ret = devm_request_threaded_irq(dma_dev->dev, chan->irq,
+			nbpf_chan_irq, nbpf_chan_irqt, IRQF_SHARED,
+			chan->name, chan);

devm_request_irq and friends arent apt here. You are in .remove and get an
irq, what prevents that race?

How is this your comment different for DMA drivers from any other drivers? 
Of course you have to make sure no more interrupts are arriving once in 
.remove() you lost the ability to handle IRQs. So, AFAIU, it's always a 
per-case study: you have to look at .remove() and consider, what happens 
if IRQ hits at any point inside it - if at all possible. Besides, DMA 
engine drivers are additionally protected by incremented module 
use-counts, once a channel is in use? As soon as a channel is released 
your driver has to make sure no more IRQs are occurring. Besides, there's 
a dmaengine_get()... So, I think, we really need to first find a race 
possibility, and then fix it. Besides, you could try

grep -rl devm_request_irq drivers/dma/*.c

And please also see there were regression on few of them, recently dw was
fixed for this case. Similarly few other drivers were fixed for their
behaviour in tasklet kill, syncronizing the irq and freeing up behaviour.

So now back to this driver, since you are using devm_ the irq is not freed,
so how does it prevent race. If we do ensure that, then yes driver is fine
and can be merged, otherwise needs a fix

quoted

You need to run free_irq and synchronize_irq()
to ensure everything is proper before freeing up.

Also why threaded_irq when dmaengine API mandates you to use tasklet? The
callback is supposed to be invoked from a tasklet.

As for this one. Yes, I'm aware, that most other dmaengine drivers use a 
tasklet for bottom half IRQ processing. But is there a real requirement 
for that? This driver has been written in the end of last year and has 
been in use since then - no problems. As long as proper locking is used, 
an IRQ thread seems to work just fine here. And allows us to leverage the 
threaded-IRQ API instead of inventing own bottom-half processing.

But if there's a _real_ essential reason to use a tasklet here, I can 
switch, np. Does it really have to be done now or would an incremental 
patch suffice?

The API mandates this for a reason. I think we can add this later as it
doesn't break the functionality.

-- 
~Vinod
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