[patch 6/6] dma: edma: Provide granular accounting

[Thomas Gleixner]

On Thu, 17 Apr 2014, Joel Fernandes wrote:
> On 04/17/2014 09:40 AM, Thomas Gleixner wrote:
> > +	u32				residue_stat;
> > +	int				slot0;
> 
> Instead of slot0, perhaps storing a pointer to the echan would be great
> incase in the future we need to access something else in the channel.
> Then you can just do edesc->echan->slot[0].

Sure.
 
> > @@ -645,7 +652,17 @@ static void edma_callback(unsigned ch_nu
> >  				vchan_cookie_complete(&edesc->vdesc);
> >  				edma_execute(echan);
> >  			} else {
> > +				int i, n;
> > +
> >  				dev_dbg(dev, "Intermediate transfer complete on channel %d\n", ch_num);
> > +
> > +				/* Update statistics for tx_status */
> > +				n = edesc->processed;
> > +				for (i = edesc->processed_stat; i < n; i++)
> > +					edesc->residue -= edesc->pset[i].len;
> > +				edesc->processed_stat = n;
> > +				edesc->residue_stat = edesc->residue;
> > +
> 
> This is a bit of a NAK since it is O(n) on every intermediate transfer
> completion. I'm not sure of a better way to do it but I certainly don't
> want the IRQ slowed down anymore... Is there a way to pre-calculate the
> size of the intermediate transfer and store it somewhere for accounting?

Yeah, I can do that. Adds an extra field to edma_pset, but that
shouldnt hurt.

> Also another thing is I don't think processed_stat is required at all,
> because I think you introduced it for the possibility that there might
> be delays in processing interrupts for intermediate transfers. That is

No, I introduced it to not walk the full list of psets in every
tx_status() call. It's keeping track of the already finished slots.

> actually an impossibility because such a scenario would result in
> catastrophic failure anyway. Because for intermediate transfer
> interrupts shouldn't be missed, so in this case you can just do:
> 
> for (i = 1; i <= MAX_NR_SG; i++) {
> 	edesc->residue -= edesc->pset[edesc->processed - i].len;
> }
> 
> I think that'll work.

No. The point is:

submit()

tx_status()
	
  We dont want to iterate through all psets when we are polling in a
  loop. So we progress processed_stat when we see a finished slot.
  And thereby we adjust edesc->residue.

Now in the intermediate interrupt we need to take processed_stat into
account when we update residue.

But when we store the sums in the psets then this should become O(1)
and avoid the loop business.

> > +	if (ret)
> > +		return edesc->residue_stat;
> > +
> 
> This check could be dropped following the discussion in earlier patch
> and also residue_stat could be dropped if there's no other use for it.

Right.
 
> > +
> > +		/* Otherwise mark it done and update residue[_stat]. */
> > +		edesc->processed_stat++;
> > +		edesc->residue -= pset->len;
> > +		edesc->residue_stat = edesc->residue;
> > +	}
> 
> 
> Also, just curious - doesn't calling status too aggressively result in
> any slowness for you? Since vchan lock spinlock will be held during the
> duration of this processing, and interrupts would be disabled causing
> edma_callback interrupt delays possibly. I'm not saying you introduced
> the lock or anything because status would previously also hold the lock.
> I just haven't played status enough to know how it will behave when
> happening concurrently with DMA transfers.

Well, in the case I'm looking at (cyclic transfer) I do not care about
the interrupt at all. I get a notification for the first incoming
packet via the peripheral and go from there.

But yes, it might cause slight delays for the interrupt in the SG
case. Though the irq disabled region is small if you actively poll as
we keep track of the processed slots and therefor only have one or two
readouts to process.

But as Russell pointed out, this should be rewritten by updating the
chain on the fly and not waiting until MAX_SG has been processed.

Thanks,

	tglx