linux-kernel - Re: [PATCH 0/4 v6] i.MX31: dmaengine and framebuffer drivers

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Date:	Tue, 6 Jan 2009 11:19:29 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	linux-kernel@...r.kernel.org,
	linux-fbdev-devel@...ts.sourceforge.net, adaplas@...il.com,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	Dan Williams <dan.j.williams@...el.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH 0/4 v6] i.MX31: dmaengine and framebuffer drivers

On Tue, Jan 06, 2009 at 10:51:37AM +0100, Guennadi Liakhovetski wrote:
> Hi Sascha
> 
> On Tue, 6 Jan 2009, Sascha Hauer wrote:
> 
> > Hi Guennadi,
> > 
> > On Fri, Dec 26, 2008 at 06:11:13PM +0100, Guennadi Liakhovetski wrote:
> > > Hi,
> > > 
> > > This is version 6 of dmaengine and framebuffer drivers for i.MX31.
> > > 
> > > Changes since version 5: as requested by Sascha Hauer switched to dynamic 
> > > IPU IRQ mapping.
> > 
> > I tried to express that it's really odd that you demux your _internal_
> > interrupts as chained handlers.
> 
> Your last comment was:
> 
> > Preferably a way which does not involve introducing 100+ unused
> > interrupts.
> 
> which I have done. Now we're back to demux / not demux...

Can *please* anybody else say something to this issue to calm this down?
To make it easier for you please sign:

[ ] The code is fine the way it is. Sascha go home.
[ ] The code could be better, but hey, it works and the world is not
    perfect.
[ ] No, we shouldn't abuse chained interrupts for internal irq
    dispatching.

> 
> > Consider a network driver which has a
> > rx, tx and an error status bit, all of them can trigger an interrupt. A
> > you aware of a single driver that uses chained interrupts for this
> > case?
> > No, they don't have to, because all this happens inside one driver
> > and this can easily be dispatched in one interrupt handler. IMHO chained
> > interrupts only make sense when you have an interrupt source which
> > leaves your driver code and you don't know who might be interested in,
> > like the assorted non channel interrupts the IPU also provides.
> > 
> > I bet you'd never have the idea for such a code design without the
> > Freescale code as a sample.
> 
> Think about other IPU interrupts. For now I removed the CSI interrupt, but 
> it may well be needed at some point.

That's one of the use cases for chained interrupts, yes.

> Or any of overflow interrupts?

The overflow interrupts are channel interrupts and should be handled by
the idmac code. Who else should know how to recover from a channel
overflow?

> What 
> do we do then? Why shall we build in limitations from the beginning, when 
> there is an implementation already there, which doesn't have them? Sorry, 
> do not understand.
> 
> I know you proposed to dispatch all DMA EOF interrupts to one IRQ number 
> and (optionally / as required) others to single IRQs. But I like to see 
> single DMA sources as separate entries in /proc/interrupts, even if only 
> for debugging.

Then I propose to change all drivers this way. I want to see how
many interrupts in my network driver I get for errors, tx and so on, even
if only for debugging.

> 
> And no, it is not thanks to Freescals code, that I'm doing that, it's more 
> contrary to their code - they do not implement an irq_chip API and instead 
> add an API of their own.

Yes, I know, but it was their idea to treat the channel status bits as
interrupts, even if they invented their own API instead of using the
existing one.

Sascha

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