linux-kernel - Re: [RFC][PATCH] add dma_reserve_coherent_memory()/dma_free_reserved

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Date:	Fri, 27 Aug 2010 14:57:59 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	u.kleine-koenig@...gutronix.de
Cc:	fujita.tomonori@....ntt.co.jp, g.liakhovetski@....de,
	mitov@...p.bas.bg, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, akpm@...ux-foundation.org,
	linux-arm-kernel@...ts.infradead.org, linux-sh@...r.kernel.org,
	philippe.retornaz@...l.ch, gregkh@...e.de, jkrzyszt@....icnet.pl
Subject: Re: [RFC][PATCH] add
	dma_reserve_coherent_memory()/dma_free_reserved_memory() API

On Fri, 27 Aug 2010 07:19:07 +0200
Uwe Kleine-K.$(D+S.(Bnig <u.kleine-koenig@...gutronix.de> wrote:

> Hey,
> 
> On Fri, Aug 27, 2010 at 02:00:17PM +0900, FUJITA Tomonori wrote:
> > On Fri, 27 Aug 2010 06:41:42 +0200
> > Uwe Kleine-K.$(D+S.(Bnig <u.kleine-koenig@...gutronix.de> wrote:
> > > On Thu, Aug 26, 2010 at 07:00:24PM +0900, FUJITA Tomonori wrote:
> > > > On Thu, 26 Aug 2010 11:53:11 +0200
> > > > Uwe Kleine-K.$(D+S.(Bnig <u.kleine-koenig@...gutronix.de> wrote:
> > > > 
> > > > > > > We have currently a number of boards broken in the mainline. They must be 
> > > > > > > fixed for 2.6.36. I don't think the mentioned API will do this for us. So, 
> > > > > > > as I suggested earlier, we need either this or my patch series
> > > > > > > 
> > > > > > > http://thread.gmane.org/gmane.linux.ports.sh.devel/8595
> > > > > > > 
> > > > > > > for 2.6.36.
> > > > > > 
> > > > > > Why can't you revert a commit that causes the regression?
> > > > > > 
> > > > > > The related DMA API wasn't changed in 2.6.36-rc1. The DMA API is not
> > > > > > responsible for the regression. And the patchset even exnteds the
> > > > > > definition of the DMA API (dma_declare_coherent_memory). Such change
> > > > > > shouldn't applied after rc1. I think that DMA-API.txt says that
> > > > > > dma_declare_coherent_memory() handles coherent memory for a particular
> > > > > > device. It's not for the API that reserves coherent memory that can be
> > > > > > used for any device for a single device.
> > > > > The patch that made the problem obvious for ARM is
> > > > > 309caa9cc6ff39d261264ec4ff10e29489afc8f8 aka v2.6.36-rc1~591^2~2^4~12.
> > > > > So this went in before v2.6.36-rc1.  One of the "architectures which
> > > > > similar restrictions" is x86 BTW.
> > > > > 
> > > > > And no, we won't revert 309caa9cc6ff39d261264ec4ff10e29489afc8f8 as it
> > > > > addresses a hardware restriction.
> > > > 
> > > > How these drivers were able to work without hitting the hardware restriction?
> > > In my case the machine in question is an ARMv5, the hardware restriction
> > > is on ARMv6+ only.  You could argue that so the breaking patch for arm
> > > should only break ARMv6, but I don't think this is sensible from a
> > > maintainers POV.  We need an API that works independant of the machine
> > > that runs the code.
> > 
> > Agreed. But insisting that the DMA API needs to be extended wrongly
> > after rc2 to fix the regression is not sensible too. The related DMA
> > API wasn't changed in 2.6.36-rc1. The API isn't responsible for the
> > regression at all.
> I think this isn't about "responsiblity".  Someone in arm-land found
> that the way dma memory allocation worked for some time doesn't work
> anymore on new generation chips.  As pointing out this problem was
> expected to find some matches it was merged in the merge window.  One
> such match is the current usage of the DMA API that doesn't currently
> offer a way to do it right, so it needs a patch, no?

No, I don't think so. We are talking about a regression, right?

On new generation chips, something often doesn't work (which have
worked on old chips for some time). It's not a regresiion. I don't
think that it's sensible to make large change (especially after rc1)
to fix such issue. If you say that the DMA API doesn't work on new
chips and proposes a patch for the next merge window, it's sensible, I
suppose.

Btw, the patch isn't a fix for the DMA API. It tries to extend the DMA
API (and IMO in the wrong way). In addition, the patch might break the
current code. I really don't think that applying such patch after rc1
is senseble.
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