linux-kernel - Re: [PATCH v2] staging: fbtft: Disable DMA support if DMA is not available

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Message-ID: <20150424071744.GA6146@kroah.com>
Date:	Fri, 24 Apr 2015 09:17:44 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Noralf Trønnes <noralf@...nnes.org>,
	driverdevel <devel@...verdev.osuosl.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] staging: fbtft: Disable DMA support if DMA is not
 available

On Fri, Apr 24, 2015 at 09:15:00AM +0200, Geert Uytterhoeven wrote:
> On Thu, Apr 23, 2015 at 9:17 PM, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> > On Thu, Apr 23, 2015 at 08:47:15PM +0200, Geert Uytterhoeven wrote:
> >> If NO_DMA=y:
> >>
> >>     drivers/built-in.o: In function `fbtft_framebuffer_alloc':
> >>     (.text+0xb53cae): undefined reference to `dmam_alloc_coherent'
> >>
> >> As DMA support is already optional, make it depend on HAS_DMA.
> >>
> >> If !HAS_DMA, "dma" will always be false, and the compiler will optimize
> >> away the call to dmam_alloc_coherent().
> >
> > Oh that's "sneaky" and not good, let's just make this driver be
> > dependant on DMA and be done with it.  I don't like to rely on the
> > optimizer to get things to work properly.
> 
> Alternatively, I can stick in one more #ifdef, to no longer rely on the compiler
> optimization?

That would be better.

thanks,

greg k-h
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