Re: [PATCH v3 1/8] dma: sun4i: Add support for the DMA engine on sun[457]i SoCs

[Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

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On Thu, Aug 07, 2014 at 04:37:16PM -0300, Emilio López wrote:
> Hi,
> 
> El 05/08/14 a las 17:00, Maxime Ripard escibió:
> >Hi,
> >
> >Overall it looks very nice, thanks.
> >
> >On Mon, Aug 04, 2014 at 05:09:55PM -0300, Emilio López wrote:
> >>+static struct sun4i_dma_pchan *find_and_use_pchan(struct sun4i_dma_dev *priv,
> >>+                                                 struct sun4i_dma_vchan *vchan)
> >>+{
> >>+       struct sun4i_dma_pchan *pchan = NULL, *pchans = priv->pchans;
> >>+       unsigned long flags;
> >>+       int i, max;
> >>+
> >>+       /*
> >>+        * pchans 0-NDMA_NR_MAX_CHANNELS are normal, and
> >>+        * NDMA_NR_MAX_CHANNELS+ are dedicated ones
> >>+        */
> >>+       if (vchan->is_dedicated) {
> >>+               i = NDMA_NR_MAX_CHANNELS;
> >>+               max = DMA_NR_MAX_CHANNELS;
> >>+       } else {
> >>+               i = 0;
> >>+               max = NDMA_NR_MAX_CHANNELS;
> >>+       }
> >>+
> >>+       spin_lock_irqsave(&priv->lock, flags);
> >>+       for_each_clear_bit_from(i, &priv->pchans_used, max) {
> >>+               pchan = &pchans[i];
> >>+               pchan->vchan = vchan;
> >>+               set_bit(i, priv->pchans_used);
> >>+               break;
> >
> >ffz instead?
> 
> I think it'd look way more messy with ffz, as it only works on
> unsigned longs and it's undefined on the all-1s case.
> 
> We could have something like this, but I don't see much point in
> what's basically expanding the macro
> 
> 	spin_lock_irqsave(&priv->lock, flags);
> 	i = find_next_zero_bit(&priv->pchans_used, max, i);
> 	if (i < max) {
> 		pchan = &pchans[i];
> 		pchan->vchan = vchan;
> 		set_bit(i, priv->pchans_used);
> 	}
> 	spin_unlock_irqrestore(&priv->lock, flags);

What about 

spin_lock_irqsave(&priv->lock, flags);
i = ffz(&priv->pchans_used);
if (!i)
   return NULL;

pchan = &pchans[i];
pchan->vchan = vchan;
set_bit(i, priv->pchans_used);

spin_unlock_irqrestore(&priv->lock, flags);

It looks quite clean to me.

> >>+static irqreturn_t sun4i_dma_submit_work(int irq, void *dev_id)
> >>+{
> >>+	struct sun4i_dma_dev *priv = dev_id;
> >>+	struct sun4i_dma_vchan *vchan;
> >>+	unsigned long flags;
> >>+	int i;
> >>+
> >>+	for (i = 0; i < DMA_NR_MAX_VCHANS; i++) {
> >>+		vchan = &priv->vchans[i];
> >>+		spin_lock_irqsave(&vchan->vc.lock, flags);
> >>+		execute_vchan_pending(priv, vchan);
> >>+		spin_unlock_irqrestore(&vchan->vc.lock, flags);
> >>+	}
> >>+
> >>+	return IRQ_HANDLED;
> >>+}
> >
> >Judging from Russell's comment here, that should be in the interrupt
> >handler
> >
> >http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/277737.html
> 
> Wouldn't that make the interrupt handler quite big time wise? I was
> under the impression they should remain as short as possible. LDD3
> says "The programmer should be careful to write a routine that
> executes in a minimum amount of time (...)" on chapter 10

Yeah, but if that involves delaying a lot a task that should have been
performed asap, it would make sense.

Plus, besides the loop in the contract's demands,
execute_vchan_pending is actually rather small. Did you timed it to
have such concerns?

> I did try something mid-way a bit ago; I added code to see if we
> could issue the next DMA operation from the current set, but it was
> just extra complexity for a noop in performance, as they're sets of
> one on all cases I tried.
> 
> It's worth noting that the very latency sensitive DMA users we will
> have (ie, audio) are already completely managed in the IRQ handler
> thanks to the less flexible concept of cyclic transfers.

It's not just a matter of latency, but also of throughput. If you wait
for the tasklet softirq to be executed to schedule a new vchan, you
aren't going to do anything during that time, while you could have.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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