[bug report] drm/i915: Allow compaction upto SWIOTLB max segment size

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Chris Wilson,

The patch 871dfbd67d4e: "drm/i915: Allow compaction upto SWIOTLB max
segment size" from Oct 11, 2016, leads to the following static
checker warning:

	drivers/gpu/drm/i915/i915_gem.c:2357 i915_gem_object_get_pages_gtt()
	error: we previously assumed 'sg' could be null (see line 2341)

drivers/gpu/drm/i915/i915_gem.c
  2339                  /* Check that the i965g/gm workaround works. */
  2340                  WARN_ON((gfp & __GFP_DMA32) && (last_pfn >= 0x00100000UL));
  2341          }
  2342          if (sg) /* loop terminated early; short sg table */

We added a new check for NULL.

  2343                  sg_mark_end(sg);
  2344  
  2345          /* Trim unused sg entries to avoid wasting memory. */
  2346          i915_sg_trim(st);
  2347  
  2348          ret = i915_gem_gtt_prepare_pages(obj, st);
  2349          if (ret)
  2350                  goto err_pages;

but we hit this goto

  2351  
  2352          if (i915_gem_object_needs_bit17_swizzle(obj))
  2353                  i915_gem_object_do_bit_17_swizzle(obj, st);
  2354  
  2355          return st;
  2356  
  2357  err_pages:
  2358          sg_mark_end(sg);

Then don't check here.  Also do we really need to sg_mark_end() twice?

  2359          for_each_sgt_page(page, sgt_iter, st)
  2360                  put_page(page);
  2361          sg_free_table(st);
  2362          kfree(st);

regards,
dan carpenter
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