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[PATCH 2/3] drm/i915: split out check for noncontiguous pfn range

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  [PATCH 2/3] drm/i915: split out check for noncontiguous pfn range Arnd Bergmann <arnd@arndb.de> - 2017-03-20 10:50 +0100
    Re: [PATCH 2/3] drm/i915: split out check for noncontiguous pfn range Chris Wilson <chris@chris-wilson.co.uk> - 2017-03-21 11:20 +0100
      Re: [Intel-gfx] [PATCH 2/3] drm/i915: split out check for  noncontiguous pfn range Chris Wilson <chris@chris-wilson.co.uk> - 2017-03-21 11:30 +0100

#1604295 — [PATCH 2/3] drm/i915: split out check for noncontiguous pfn range

We get a warning with gcc-7 about a pointless comparison when
using a linear memmap:

drivers/gpu/drm/i915/selftests/scatterlist.c: In function 'alloc_table':
drivers/gpu/drm/i915/selftests/scatterlist.c:219:66: error: self-comparison always evaluates to false [-Werror=tautological-compare]

Splitting out the comparison into a separate function avoids the warning
and makes it slightly more obvious what happens.

Fixes: 935a2f776aa5 ("drm/i915: Add some selftests for sg_table manipulation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/gpu/drm/i915/selftests/scatterlist.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/selftests/scatterlist.c b/drivers/gpu/drm/i915/selftests/scatterlist.c
index eb2cda8e2b9f..c072b0f9180b 100644
--- a/drivers/gpu/drm/i915/selftests/scatterlist.c
+++ b/drivers/gpu/drm/i915/selftests/scatterlist.c
@@ -189,6 +189,11 @@ static unsigned int random(unsigned long n,
 	return 1 + (prandom_u32_state(rnd) % 1024);
 }
 
+static inline bool page_contiguous(struct page *first, struct page *last,  unsigned long npages)
+{
+	return first + npages == last;
+}
+
 static int alloc_table(struct pfn_table *pt,
 		       unsigned long count, unsigned long max,
 		       npages_fn_t npages_fn,
@@ -216,7 +221,8 @@ static int alloc_table(struct pfn_table *pt,
 		unsigned long npages = npages_fn(n, count, rnd);
 
 		/* Nobody expects the Sparse Memmap! */
-		if (pfn_to_page(pfn + npages) != pfn_to_page(pfn) + npages) {
+		if (!page_contiguous(pfn_to_page(pfn),
+				     pfn_to_page(pfn + npages), npages)) {
 			sg_free_table(&pt->st);
 			return -ENOSPC;
 		}
-- 
2.9.0

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#1605481

On Mon, Mar 20, 2017 at 10:40:28AM +0100, Arnd Bergmann wrote:
> We get a warning with gcc-7 about a pointless comparison when
> using a linear memmap:
> 
> drivers/gpu/drm/i915/selftests/scatterlist.c: In function 'alloc_table':
> drivers/gpu/drm/i915/selftests/scatterlist.c:219:66: error: self-comparison always evaluates to false [-Werror=tautological-compare]
> 
> Splitting out the comparison into a separate function avoids the warning
> and makes it slightly more obvious what happens.

I worried whether gcc will simply get smarter and apply its warning more
consistently, but I agree the helper is good documentation.
 
> Fixes: 935a2f776aa5 ("drm/i915: Add some selftests for sg_table manipulation")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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#1605490 — Re: [Intel-gfx] [PATCH 2/3] drm/i915: split out check for noncontiguous pfn range

On Tue, Mar 21, 2017 at 09:56:52AM +0000, Chris Wilson wrote:
> On Mon, Mar 20, 2017 at 10:40:28AM +0100, Arnd Bergmann wrote:
> > We get a warning with gcc-7 about a pointless comparison when
> > using a linear memmap:
> > 
> > drivers/gpu/drm/i915/selftests/scatterlist.c: In function 'alloc_table':
> > drivers/gpu/drm/i915/selftests/scatterlist.c:219:66: error: self-comparison always evaluates to false [-Werror=tautological-compare]
> > 
> > Splitting out the comparison into a separate function avoids the warning
> > and makes it slightly more obvious what happens.
> 
> I worried whether gcc will simply get smarter and apply its warning more
> consistently, but I agree the helper is good documentation.
>  
> > Fixes: 935a2f776aa5 ("drm/i915: Add some selftests for sg_table manipulation")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

And applied.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre

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