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Re: [PATCH] MIPS: mm: Move some checks out of 'for' loop in DMA operatio

To: Jayachandran C <jchandra@broadcom.com>
Subject: Re: [PATCH] MIPS: mm: Move some checks out of 'for' loop in DMA operations
From: Markos Chandras <Markos.Chandras@imgtec.com>
Date: Wed, 25 Sep 2013 14:19:11 +0100
Cc: <ralf@linux-mips.org>, <linux-mips@linux-mips.org>
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On 09/25/13 14:01, Jayachandran C wrote:
The check cpu_needs_post_dma_flush() in mips_dma_sync_sg_for_cpu() and
the check !plat_device_is_coherent() in mips_dma_sync_sg_for_device()
can be moved outside the for loop.

As a side effect, this also avoids a GCC bug that caused kernel compile
to fail with the error:

arch/mips/mm/dma-default.c: In function 'mips_dma_sync_sg_for_cpu':
arch/mips/mm/dma-default.c:316:1: internal compiler error: in add_insn_before, 
at emit-rtl.c:3852

This gcc failure is seen in Code Sourcery toolchains [e.g. gcc version
4.7.2 (Sourcery CodeBench Lite 2012.09-99)] after commit "MIPS: Optimize
current_cpu_type() for better code."
---
  arch/mips/mm/dma-default.c |   12 ++++--------
  1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/arch/mips/mm/dma-default.c b/arch/mips/mm/dma-default.c
index e45e4b0..2e94185 100644
--- a/arch/mips/mm/dma-default.c
+++ b/arch/mips/mm/dma-default.c
@@ -307,12 +307,10 @@ static void mips_dma_sync_sg_for_cpu(struct device *dev,
  {
        int i;

-       /* Make sure that gcc doesn't leave the empty loop body.  */
-       for (i = 0; i < nelems; i++, sg++) {
-               if (cpu_needs_post_dma_flush(dev))
+       if (cpu_needs_post_dma_flush(dev))
+               for (i = 0; i < nelems; i++, sg++)
                        __dma_sync(sg_page(sg), sg->offset, sg->length,
                                   direction);
-       }
  }

  static void mips_dma_sync_sg_for_device(struct device *dev,
@@ -320,12 +318,10 @@ static void mips_dma_sync_sg_for_device(struct device 
*dev,
  {
        int i;

-       /* Make sure that gcc doesn't leave the empty loop body.  */
-       for (i = 0; i < nelems; i++, sg++) {
-               if (!plat_device_is_coherent(dev))
+       if (!plat_device_is_coherent(dev))
+               for (i = 0; i < nelems; i++, sg++)
                        __dma_sync(sg_page(sg), sg->offset, sg->length,
                                   direction);
-       }
  }

  int mips_dma_mapping_error(struct device *dev, dma_addr_t dma_addr)


Hi,

The patch looks good to me and it fixes the gcc problem indeed (the bug is also present in gcc-4.7.3 from the 2013.05 release)

Reviewed-by/Tested-by: Markos Chandras <markos.chandras@imgtec.com>


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