linux-kernel - Re: [Linaro-mm-sig] [RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

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Date:	Mon, 12 Dec 2011 14:44:09 -0800
From:	Robert Morell <rmorell@...dia.com>
To:	Mauro Carvalho Chehab <mchehab@...hat.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Sumit Semwal <sumit.semwal@...com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"jesse.barker@...aro.org" <jesse.barker@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"daniel@...ll.ch" <daniel@...ll.ch>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [RFC v2 1/2] dma-buf: Introduce dma buffer
 sharing mechanism

On Sat, Dec 10, 2011 at 03:13:06AM -0800, Mauro Carvalho Chehab wrote:
> On 09-12-2011 20:50, Robert Morell wrote:
> > On Mon, Dec 05, 2011 at 09:18:48AM -0800, Arnd Bergmann wrote:
> >> On Friday 02 December 2011, Sumit Semwal wrote:
> >>> This is the first step in defining a dma buffer sharing mechanism.
> >>
> > [...]
> >>
> >>> +	return dmabuf;
> >>> +}
> >>> +EXPORT_SYMBOL(dma_buf_export);
> >>
> >> I agree with Konrad, this should definitely be EXPORT_SYMBOL_GPL,
> >> because it's really a low-level function that I would expect
> >> to get used by in-kernel subsystems providing the feature to
> >> users and having back-end drivers, but it's not the kind of thing
> >> we want out-of-tree drivers to mess with.
> >
> > Is this really necessary?  If this is intended to be a
> > lowest-common-denominator between many drivers to allow buffer sharing,
> > it seems like it needs to be able to be usable by all drivers.
> >
> > If the interface is not accessible then I fear many drivers will be
> > forced to continue to roll their own buffer sharing mechanisms (which is
> > exactly what we're trying to avoid here, needless to say).
> 
> Doing a buffer sharing with something that is not GPL is not fun, as, if any
> issue rises there, it would be impossible to discover if the problem is either
> at the closed-source driver or at the open source one. At the time I was using
> the Nvidia proprietary driver, it was very common to have unexplained issues
> caused likely by bad code there at the buffer management code, causing X
> applications and extensions (like xv) to break.
>
> We should really make this EXPORT_SYMBOL_GPL(), in order to be able to latter
> debug future share buffer issues, when needed.

Sorry, I don't buy this argument.  Making these exports GPL-only is not
likely to cause anybody to open-source their driver, but will rather
just cause them to use yet more closed-source code that is even less
debuggable than this would be, to those without access to the source.

Thanks,
Robert

> Regards,
> Mauro
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