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

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Date:	Thu, 23 Apr 2015 20:37:36 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.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] staging: fbtft: Disable DMA support if DMA is not available

On Thu, Apr 23, 2015 at 8:27 PM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Thu, Apr 23, 2015 at 08:06:52PM +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.
>>
>> Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> ---
>>  drivers/staging/fbtft/fbtft-core.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/staging/fbtft/fbtft-core.c b/drivers/staging/fbtft/fbtft-core.c
>> index 53b748be27124793..88bf4705ebadc590 100644
>> --- a/drivers/staging/fbtft/fbtft-core.c
>> +++ b/drivers/staging/fbtft/fbtft-core.c
>> @@ -47,9 +47,13 @@ static unsigned long debug;
>>  module_param(debug, ulong, 0);
>>  MODULE_PARM_DESC(debug, "override device debug level");
>>
>> +#ifdef CONFIG_HAS_DMA
>>  static bool dma = true;
>>  module_param(dma, bool, 0);
>>  MODULE_PARM_DESC(dma, "Use DMA buffer");
>> +#else
>> +static const bool dma;
>> +#endif
>
> I don't see how this change can fix the build error you show above.
> What am I missing?

You're missing that gcc is optimized optimize away the branch that calls
dmam_alloc_coherent():

                if (dma) {
                        dev->coherent_dma_mask = ~0;
                        txbuf = dmam_alloc_coherent(dev, txbuflen,
&par->txbuf.dma, GFP_DMA);
                } else {
                        ...
                }

HOWEVER, please drop this patch.

Originally I had "static const bool dma = false;", but checkpatch rightfully
complained about the initialization of a static, so I dropped it.
Guess what, gcc 4.1.2 no longer optimizes away the branch :-(

Will send v2 in a few minutes.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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