Re: [PATCH 05/21] powerpc: Avoid comparison of unsigned long >= 0 in pfn_valid

Le 04/03/2018 à 11:55, Michael Ellerman a écrit :

Mathieu Malaterre [off-list ref] writes:

quoted

Rewrite comparison since all values compared are of type `unsigned long`.

Fix a warning (treated as error in W=1):

   CC      arch/powerpc/kernel/irq.o
In file included from ./include/linux/bug.h:5:0,
                  from ./include/linux/cpumask.h:13,
                  from ./include/linux/smp.h:13,
                  from ./include/linux/kernel_stat.h:5,
                  from arch/powerpc/kernel/irq.c:35:
./include/linux/dma-mapping.h: In function ‘dma_map_resource’:
./arch/powerpc/include/asm/page.h:129:32: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
  #define pfn_valid(pfn)  ((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
                                 ^
Suggested-by: Segher Boessenkool <redacted>
Signed-off-by: Mathieu Malaterre <redacted>
---
  arch/powerpc/include/asm/page.h | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
index 8da5d4c1cab2..19dea64e7ed2 100644
--- a/arch/powerpc/include/asm/page.h
+++ b/arch/powerpc/include/asm/page.h

@@ -126,7 +126,8 @@ extern long long virt_phys_offset;
  
  #ifdef CONFIG_FLATMEM
  #define ARCH_PFN_OFFSET		((unsigned long)(MEMORY_START >> PAGE_SHIFT))
-#define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
+#define pfn_valid(pfn) \
+		(((pfn) - ARCH_PFN_OFFSET) < (max_mapnr - ARCH_PFN_OFFSET))

I'm not a big fan of this one, because the original code is *far* more
obvious as to what it's doing.

I'm not sure if we can make this one a static inline, or whether that
would help, but it would be worth investigating.

The following seems to give a good result:

diff --git a/arch/powerpc/include/asm/page.h 

b/arch/powerpc/include/asm/page.h
index 8da5d4c1cab2..6f74938483b7 100644

--- a/arch/powerpc/include/asm/page.h
+++ b/arch/powerpc/include/asm/page.h

@@ -126,7 +126,15 @@ extern long long virt_phys_offset;

  #ifdef CONFIG_FLATMEM
  #define ARCH_PFN_OFFSET		((unsigned long)(MEMORY_START >> PAGE_SHIFT))
-#define pfn_valid(pfn)		((pfn) >= ARCH_PFN_OFFSET && (pfn) < max_mapnr)
+#ifndef __ASSEMBLY__
+extern unsigned long max_mapnr;
+static inline bool pfn_valid(unsigned long pfn)
+{
+	unsigned long min_pfn = ARCH_PFN_OFFSET;
+
+	return pfn >= min_pfn && pfn < max_mapnr;
+}
+#endif
  #endif

  #define virt_to_pfn(kaddr)	(__pa(kaddr) >> PAGE_SHIFT)

Christophe

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