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

[Vinod Koul <vinod.koul-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

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.

> > On Sat, Jul 19, 2014 at 12:48:51PM +0200, Guennadi Liakhovetski wrote:
> > > This patch adds a driver for NBPF DMAC IP cores from Renesas, designed for
> > > the AMBA AXI bus.
> > > 
> > 
> > > +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!

> > > +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
> 
> > 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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