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

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Date:	Thu, 23 Apr 2015 20:47:15 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Noralf Trønnes <noralf@...nnes.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: [PATCH v2] staging: fbtft: Disable DMA support if DMA is not available

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().

Signed-off-by: Geert Uytterhoeven <geert@...ux-m68k.org>
---
v2:
  - Explain better how it works,
  - Keep the "= false" to avoid breakage (seen not only with gcc 4.1.2,
    but also with 4.6.3 and 4.9.0).
---
 drivers/staging/fbtft/fbtft-core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/staging/fbtft/fbtft-core.c b/drivers/staging/fbtft/fbtft-core.c
index 53b748be27124793..600dbdcaa716cd76 100644
--- a/drivers/staging/fbtft/fbtft-core.c
+++ b/drivers/staging/fbtft/fbtft-core.c
@@ -47,9 +47,14 @@ 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
+/* Do not remove the "= false", gcc needs it to optimize away the DMA calls */
+static const bool dma = false;
+#endif
 
 
 void fbtft_dbg_hex(const struct device *dev, int groupsize,
-- 
1.9.1

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